ScienceWeek

SCIENCE-WEEK

A Weekly Email Digest of the News of Science

A journal devoted to the improvement of communication
between the scientific disciplines, and between scientists,
science educators, and science policy makers.

January 14, 2000 -- Vol. 4 Number 2

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A hypothesis or theory is clear, decisive, and positive,
but it is believed by no one but the man who created it.
Experimental findings, on the other hand, are messy,
inexact things, which are believed by everyone except
the man who did that work.
-- Harlow Shapley (1885-1972)

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Contents of This Issue:

1. Science and Society: A Call for Activism Against Creationism
2. Cell Biology: On Prokaryotes
3. Animal Biology: Comparative Mammalian Genomics
4. Medical Biology: On Mutations and Cancer
5. Astronomy: On the Effects of Asteroid-Earth Impacts
6. Physics: On the Standard Model and a Unified Physics
 
In Focus: On Mitochondria, DNA, and Sperm Cells

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1. SCIENCE AND SOCIETY: A CALL FOR ACTIVISM AGAINST CREATIONISM
One result of the decision last summer by education authorities
in Kansas (US) to reduce the teaching of Earth science and
evolution in Kansas public schools may be to galvanize the US
scientific community to activism against creationism. Certainly,
as evidenced by letters and editorials in the scientific media
during the past months, this issue has provoked responses among
scientists ranging from concern to outrage. Now, at a recent
meeting (December 1999), the American Geophysical Union (AGU) has
denounced the teaching of creationism and called for scientists
to become politically involved in promoting the teaching of
evolution. The AGU council states as follows: "The American
Geophysical Union affirms the central importance of scientific
theories of Earth history and organic evolution in science
education... Creationism is not science and does not have a
legitimate place in any science curriculum." In an unsigned
editorial commenting on the AGU statement, the journal _Nature_
states that the resurgence of creationism and the exploitation of
creationism by politicians "pose challenges to scientists that
cannot be ignored. More resolute activism is required if a decent
scientific education is not to be denied to some young
Americans." The editorial points out that most scientists tend to
avoid public confrontation. "But the key to the battle for the
teaching of good science in the presence of fundamentalism is to
be more resolute in informing the public of the important role of
science and actively oppose the use of distorted perceptions of
science as a political vehicle... Scientists need to reach out,
to become involved in local school-board issues and to seek
election to ensure appropriate scientific curricula." In
conclusion, the editorial states: "It will take coordination,
coherence and some powerful advocacy drawn from the ranks of many
independently minded scientists to carry the day."
-----------
Nature: Scientists rally to defend schools against creationists.
(Nature 23/30 Dec 99 402:847)
-----------
Nature: Combating the exploiters of creationism.
(Nature 23/30 Dec 99 402:843)
QY: feedback@nature.com
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 14Jan00


2. CELL BIOLOGY: ON PROKARYOTES
One of the important concepts in current biology is the division
of living cells into prokaryotes and eukaryotes. In the 19th
century, it was already recognized that some types of cells
contain a nucleus and other distinctive structures, while other
cell types do not, but it was only in the 20th century,
especially after the advent of the electron microscope, that the
fundamental structural distinctions between prokaryotes and 
eukaryotes became apparent. In general, eukaryotic cells are
defined as cells in which the bulk of genetic information is
contained in a nucleus surrounded by a double-membrane envelope,
while the genetic material of prokaryotic cells is not membrane-
enclosed. Prokaryotic cells include all forms of bacteria, while
eukaryotic cells comprise single-celled organisms other than
bacteria (e.g., algae, fungi, and protozoa), as well as the cells
of multicellular plants and animals. In addition to the absence
of a nucleus, also absent in prokaryotes (but generally present
in eukaryotes) are the following intracellular structures:
*endoplasmic reticulum, *Golgi complex, *lysosomes, *peroxisomes,
*nucleolus, *mitochondria, *chloroplasts, etc., all of which are
termed "organelles" of eukaryotic cells. There are other
structural distinctions, but these are the main differences
establishing the two types of living cells, prokaryotes and
eukaryotes.
... ... M. Hoppert and F. Mayer (2 installations, DE) present a
review of the internal organization of prokaryote cells, the
authors making the following points:
     1) Throughout this century, cell biologists have refined
their understanding of the various compartments in eukaryotes.
Each such compartment is effectively a subcellular organ (an
"organelle") housing all the elements necessary to perform a
specific metabolic function. For example, mitochondria generate
chemical energy with which to power all the other activities of
the cell. Lysosomes and peroxisomes contain enzymes that degrade
molecules. Each organelle operates efficiently because it bundles
together all the biomolecules required to perform a particular
task. In contrast, prokaryotes lack membranous organelles and a
typical eukaryotic *cytoskeleton. Therefore, biomolecules in
prokaryotes are commonly believed to be scattered randomly
throughout the cytoplasm, with prokaryotic metabolism extremely
chaotic and inefficient. Actually, this common belief is false:
prokaryotic cells perform their biological functions with great
efficiency, and it appears that even without membrane-bound
compartments and a eukaryotic cytoskeleton, in prokaryotes the
molecules required for a particular metabolic activity are
grouped together into areas that are essentially "functional
compartments".
     2) Prokaryotes such as bacteria are often described as "bags
of enzymes", but closer study reveals several levels of
subcellular organization. For example, in Escherichia coli, a
typical bacterium, there are several well-defined intracellular
compartments: a) the cell is surrounded by two membranes
enclosing a periplasmic compartment used for capturing and
sorting nutrients and wastes; b) at the center of the cell,
densely packed DNA strands are folded into a compact "nucleoid",
forming a loosely defined compartment devoted to the storage and
use of genetic information; c) the cytoplasm occupies the
remaining portions of the cell and is filled with *ribosomes and
many different enzymes. Multiprotein complexes are engaged in
many tasks: a) a *motor protein apparatus turns a long helical
flagellum to propel the cell through its environment; b)
*chaperonins and *proteasomes oversee the folding of new proteins
and the disposal of obsolete proteins, respectively; c) *DNA
polymerase copies the genetic information; d) various other
enzyme complexes perform sequential reactions with a number of
pathways linked together.
     3) A striking example of functional compartmentalization is
the development by several forms of bacteria of enzymes that
attack and degrade cellulose, a tough and nearly indigestible
glucose polymer. In the bacterium Clostridium thermocellum, these
cellulose-degrading enzymes form a huge complex called the
"cellulosome" that is tethered to the surface of the cell, and in
the cellulosome a large "*scaffolding protein" organizes the
arrangement of the many digestive cellulase proteins.
     4) In certain prokaryotes, entities known as "inclusion
bodies" have been recognized. These appear to be storage granules
enclosing large aggregates of water-insoluble materials such as
fats and starches. Materials inside the inclusion bodies are
degraded into smaller fragments that are released into the
cytoplasm when they are required to fuel a metabolic activity.
     5) The authors suggest that when considering prokaryote
intracellular organization, it is important to include possible
microenvironments within the cytoplasm, one form of which may be
brought about by differential structures of water. The authors
propose that a boundary of *structured water approximately 2 to
10 nanometers thick can be found abutting a cell membrane and
other macromolecular cell structures. The authors suggest that
such intracellular layers of structured water in prokaryotes are
functional compartments that must be considered with other
functional compartments such as inclusion bodies, the nucleoid,
and other specialized macromolecular structures.
-----------
M. Hoppert and F. Mayer: Prokaryotes
(American Scientist Nov/Dec 1999 87:518)
QY: Michael Hoppert [mhopper@gwdg.de]
-----------
Text Notes:
... ... *endoplasmic reticulum: A complex system of flattened
sacs in eukaryotic cells, the site of many important syntheses,
apparently including the production of new surface membrane.
... ... *Golgi complex: The Golgi apparatus (Golgi complex) is a
collection of organelles (Golgi bodies) in eukaryotic cells that
essentially function as a collecting and packaging center for
substances that the cell manufactures for export.
... ... *lysosomes: A lysosome is a cytoplasmic membrane-bound
vesicle 5 to 8 nanometers in diameter and containing a variety of
glycoprotein hydrolytic enzymes used to digest foreign material
or defective organelles.
... ... *peroxisomes: A peroxisome is an organelle rich in
enzymes that act on or generate hydrogen peroxide.
... ... *nucleolus: A small dense body, rich in RNA and protein,
present in the nucleus of eukaryotes.
... ... *mitochondria: Organelles of the cell cytoplasm,
mitochondria are the principal energy source of the cell,
containing various enzymes involved in electron transport and
metabolic cycles.
... ... *chloroplasts: Cell organelles involved in
photosynthesis; they are found in all photosynthetic plant cells.
... ... *cytoskeleton: The quasi-rigid matrix that among other
things determines cell shape.
... ... *ribosomes: A ribosome (not to be confused with riboZYME)
is a small particle, a complex of various ribonucleic acid
component subunits and proteins that functions as the site of
protein synthesis.
... ... *motor protein: "Motor proteins" are mechanico-chemical
enzymes involved in locomotion or transport.
... ... *chaperonins: Chaperonins are a subclass of molecular
chaperones, proteins that assist in the correct folding of other
proteins without becoming actual components of the folding
proteins.
... ... *proteasomes: Large multi-subunit protease complexes that
selectively degrade intracellular proteins.
... ... *DNA polymerase: An enzyme that catalyzes the formation
of DNA polymers.
... ... *scaffolding protein: Any of a group of proteins that
have specific binding sites and are instrumental in determining
the structure and assembly of large 3-dimensional structures.
... ... *structured water: When ice melts to form liquid water,
the tetrahedral structure breaks down, but some hydrogen bonds
apparently continue to exist, and liquid water consists of groups
of associated water molecules mixed with some monomers and
dimers. That is one form of structured water. Another form of
structured water is the structure created by both the hydrogen
bonding and dipole properties of the water molecule when water
molecules are in an electric field produced by a macromolecule.
The current view is that water in association with charged
macromolecules (such as biomolecules) is in an ordered state. In
the context of this report, one would certainly expect various
intracellular macromolecular structures to have an organizing
effect on nearby water molecules, but it is not yet clear what is
happening in such regions and how it might effect biological
processes.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 14Jan00
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
IN FOCUS: ON PROKARYOTIC AND EUKARYOTIC CELLS
"The oldest fossils known on Earth, found in South Africa,
Australia, and Siberia, indicate that the earliest forms of life
were individual (not colonial), single-celled, prokaryotic (pro
-- before; karyo -- kernel, a reference to the nucleus, or
central "kernel" of the cell) organisms. Prokaryotic cells are
small, have no nucleus or other internal partitions, and are the
type of cell found in bacteria. Today, along with prokaryotes,
however, there is a second very different kind of cell on Earth:
eukaryotes (eu -- true). Eukaryotic cells are larger and more
complex, and they have a nucleus and a variety of internal
chambers called 'organelles', each with specific functions. All
single-celled organisms [other than prokaryotes], as well as
multicellular life, are eukaryotic. The evolution of eukaryotes
from prokaryotes, arguably the single most significant event in
the history of life, probably occurred about 1.5 billion years
ago. An evolutionary step of this magnitude is hard to explain,
but one very plausible scenario, proposed in the late 1960s by L.
Margulis of the University of Massachusetts, suggests that
eukaryotic cells are in fact the products of prokaryotic cells
initially having been swallowed by other prokaryotes. This
theory, called the 'endosymbiotic theory' of the origin of
eukaryotes (endo -- inside; symbios -- living together), proposes
that the ingested prokaryotes continued to live with within their
hosts in a mutually beneficial arrangement, adopting specialized
functions as organelles. A large body of evidence supports this
theory. For example, the DNA and RNA of certain organelles is
like that of prokaryotic cells and different from that of the
nucleus; certain organelles have a separate cell membrane; and
organelles have separate reproductive mechanisms. Moreover,
chemical reactions within the tissue of eukaryotic cells are not
like those of certain organelles, whose own chemical reactions
more closely resemble those found in prokaryotes. Clearly,
evolution does not necessarily proceed in tiny mutation-based
steps occurring over millions of years!"
-----------
D.E. Fastovsky and D.B. Weishampel: _The Evolution and Extinction
of the Dinosaurs_.
(Cambridge University Press, New York 1996, p.67)
(Science-Week 19 Nov 99)
-------------------
Related Background:
ON PROKARYOTES IN THE BIOSPHERE
Prokaryotes (bacteria) are unicellular life forms lacking a
membrane-bound nucleus, structured chromosomes, and complex
internal organization. They are invisible to the naked eye, but
they are an essential component of the Earth's biota. They
catalyze unique and indispensable transformations in the
biogeochemical cycles of the biosphere, produce important
components of the Earth's atmosphere, and represent a large
portion of life's genetic diversity. Although the abundance of
prokaryotes has been estimated indirectly, the actual number of
prokaryotes and the total amount of their cellular carbon on
Earth has never been directly assessed. ... ... Whitman et al (3
authors at University of Georgia, US) present an array of
calculations based on analysis of various habitats, and make the
following points: 1) The number of prokaryotes and the total
amount of their cellular carbon on Earth are estimated to be 4-6
x 10^(30) cells and 350-550 x 10^(15) grams of carbon,
respectively. The total amount of prokaryotic carbon is thus 60
to 100 percent of the estimated total carbon in plants, and
inclusion of prokaryotic carbon in global models will almost
double estimates of the amount of carbon stored in living
organisms. 2) In addition, the Earth's prokaryotes contain 85-130
x 10^(15) grams of nitrogen, and 9-14 x 10^(15) grams of
phosphorus, or about 10-fold more of these nutrients than do
plants, and represent the largest pool of these nutrients in
living organisms. 3) Most of the Earth's prokaryotes occur in the
open ocean, in soil, and in oceanic and terrestrial subsurfaces,
where the numbers of cells is of the order of 10^(29) to 10^(30).
4) The numbers of heterotrophic prokaryotes (bacteria that feed
on organic material) in the upper 200 meters of open ocean, the
ocean below 200 meters, and soil are consistent with average
turnover times of 6-25 days, 0.8 years, 2.5 years, respectively.
An uncertain estimate for the average turnover time of
prokaryotes in the subsurface is of the order of 1000 to 2000
years. 5) The cellular production rate for all prokaryotes on
Earth is estimated at 1.7 x 10^(30) cells per year and is highest
in the open ocean. The authors suggest that the large population
size and rapid growth of prokaryotes provides and enormous
capacity for genetic diversity, and that given the numerical
abundance and importance of prokaryotes in biogeochemical
transformations, the absence of detailed knowledge of prokaryotic
diversity is a major omission in our knowledge of life on Earth.
QY: William B. Whitman [whitman@uga.cc.uga.edu]
(Proc. Natl. Acad. Sci. US 9 Jun 98 95:6578)
(Science-Week 3 Jul 98)


3. ANIMAL BIOLOGY: COMPARATIVE MAMMALIAN GENOMICS
Mammals comprise a vertebrate group with certain specific
characteristics: a) They are all four-footed (all tetrapods); b)
they maintain a relatively constant body temperature (they are
homeothermic); c) they have at least some hair; d) they secrete
milk with which they nurse their young; e) they have a double
blood circulation: oxygenated and deoxygenated blood are
separated in a 4-chambered heart; f) the brain is relatively
large. Based on the current fossil record, mammals originated
approximately 165 million years ago, remained a minor group of
vertebrates for approximately 100 million years, and then
suddenly flowered as a group after the extinction of the
dinosaurs approximately 65 million years ago.
... ... S.J. O'Brien et al (10 authors at 4 installations, US AU)
present a review of current research in the comparative genomics
of mammals, the authors making the following points:
     1) Since the first appearance of mammals, tens of thousands
of mammalian species have emerged, diverged, and disappeared, and
the 4600 to 4800 existing species comprise approximately 28
orders, including the primitive egg-laying mammals (Monotremata
platypus and echidna), 7 marsupial orders, and 20 placental
(eutherian) orders. Encrypted in the genomes of surviving
mammalian species are novel genes, lost genes, modified genes,
and reordered genes. These blueprints for species adaptation and
distinction are vestiges of pivotal changes that discriminated a
whale from a bat, a dog from a cat, or a chimpanzee from a human.
The ongoing molecular deciphering of the genomes of living
species, whether focusing on homologous gene sequences, gene
segments, chromosomes, or entire genomes, provides a new vision
of important evolutionary questions about natural history,
species origins and survival, and adaptation to occupy ecological
niches. The comparative genomics approach is already revealing
valuable insights into developmental functions, reproductive
enhancements, inborn errors, and disease defense mechanisms that
have protected our ancestors and ourselves from extinction.
     2) All mammals contain between 70,000 and 100,000 genes
arranged in linear order along their chromosomes, with a total
length of approximately 3.2 billion nucleotide pairs in each
genome. Chromosome numbers range from a low of 3 pairs (2N = 6 in
the Indian muntjac, Muntiacus muntjak, a small deer) to a high of
67 pairs (2N = 134 in the black rhinoceros, Diceros bicornis).
     3) Gene maps have been constructed in human, mouse, and
approximately 30 other mammalian species for two general reasons:
a) as a resource for locating the genetic determinants of
heritable characteristics, behaviors, and phenotypes; and b) as a
template for resolving and interpreting patterns of evolving
genome organization in the ancestries of the various species.
     4) Until recently, comparative genomics was a minor research
discipline overshadowed by genetic advances in human and model
organisms, but now improved technologies and the potential for
valuable applications have put the prospect for dense gene maps
of domesticated livestock and companion animal species within our
reach. Some immediate practical applications of these maps
include:
... ... a) supplying animal models for human genetic diseases
based on explicit gene homology as monitors for pathogenesis and
therapy;
... ... b) an opportunity to identify candidate polygenes that
affect human and veterinary disease.
... ... c) assessing multifactorial pathologies and characters;
... ... d) as a prelude to gene therapy, the discovery of evolved
adaptations in mammal species that ameliorate maladies homologous
to human hereditary and infectious diseases;
... ... e) developing treatments for veterinary pathologies based
on already accomplished human trials for homologous gene defects;
... ... f) the prospect of building fatter pigs, finer wool,
leaner beef, more tasty chickens, etc. Hope for each of these
applications continues to grow.
     5) The authors conclude: "The promise of comparative
genomics for mammals extends further than we can imagine, as few
biological disciplines will not be enhanced by knowledge of the
natural history of the genes that make up living forms."
-----------
S.J. O'Brien: The promise of comparative genomics in mammals.
(Science 15 Oct 99 286:458)
QY: Stephen J. O'Brien, National Cancer Institute, Frederick, MD 
21702-1201 US.
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 14Jan00
[For more information: http://scienceweek.com/search/search.htm]


4. MEDICAL BIOLOGY: ON MUTATIONS AND CANCER
The term "cancer", which means "crab" in Latin, was introduced by
Hippocrates (460-370 B.C.) to describe diseases in which tissues
grow and spread unrestrained throughout the body, eventually
causing death. Cancers can originate in almost any tissue of the
body, including nerve, muscle, blood, connective tissue, etc.
Depending on the cell type involved, cancers are grouped into 3
main categories: a) carcinomas, the most common types of cancer,
arise from the *epithelial cells that cover external and internal
body surfaces, with lung, breast, and colon cancers the most
frequent cancers of this type; b) sarcomas originate in
supporting tissues of *mesodermal origin, such as bone,
cartilage, fat, connective tissue, and muscle; c) lymphomas and
leukemias arise from cells of blood and *lymphatic origin, the
term "leukemia" used when such cancer cells circulate in large
numbers in the bloodstream rather than growing mainly as solid
masses of tissue. Cancer is a disease of the genomic apparatus of
the cell, in particular of the growth-regulation apparatus, and
considering the vast number of activities that must be
coordinated and regulated by the genomic apparatus during the
lifetime of each cell, it is not surprising that malfunctions
arise. In general, cancer is the most prominent of the many
diseases arising from aberrations in cell function, with more
than 25 percent of people in the US now expected to develop
cancer in their lifetime.
... ... C.R. Boland and L. Ricciardiello (2 installations, US)
present a review of current research on the genomic basis of
cancer, the authors making the following points:
     1) It has been known during most of this century that cancer
is often associated with visible derangements in the nucleus of
the cell. The cells of solid tumors commonly exhibit chromosome
duplications, deletions, and rearrangements, but before the
organization of the human cell nucleus was understood, these
chromosome aberrations were difficult to categorize and were of
little help in understanding the biological basis of cancer.
     2) Within a few decades after the discovery of the structure
of DNA, cancer-related genes (oncogenes) were isolated, and these
were frequently found to be mutant versions of normal cellular
genes in which an activating *point mutation or an aberrant
*genetic amplification process resulted in a gain of function for
that gene product, and a growth advantage for that aberrant cell.
But as more and more oncogenes were identified, researchers
realized that tumor growth was also associated with loss of
function of certain "tumor suppressor genes". These tumor
suppressor genes were often inactivated by their deletion from
the nucleus, and the phrase "loss of heterozygosity" (LOH) was
applied to genetic loci in which both *alleles were present in
normal tissues, but one copy was lost in tumor tissue. In many
instances, tumor suppressor genes were first identified by virtue
of germ-line mutations that were present at a high frequency in a
rare tumor, e.g., retinoblastoma, but it soon became apparent to
researchers that many tumor suppressor genes were associated with
a variety of different tumors, many of which were not rare at
all.
     3) There are no oncogenes or tumor suppressor genes that are
activated or deleted in and from all cancers. Even tumors of a
single organ rarely have uniform genetic alterations, although
tumor types from one specific organ do have a tendency to share
mutations in certain genes or in different genes within a single
growth-regulatory pathway.
     4) At the present time, it is not known how many critical
mutations are required to convert a single normal cell into a
malignant cell. Human cells have been difficult to transform in
vitro, and the basis for this difficulty is not yet understood.
The simplest model of tumorigenesis is as follows:
... ... a) Human cells experience a certain number of mutations
each day as a result of exposure to carcinogens or as a result of
ordinary biological degradation, both of which can alter
nucleotide sequences. Errors will also occur during new DNA
synthesis and in the process of disentangling the chromosomes
during *mitosis. Most of these errors would be either irrelevant
to the life of the cell or deleterious because of the loss of a
gene critical for cellular viability.
... ... b) By chance, an occasional genomic mutation might create
a growth advantage for a cell, permitting increased net cellular
growth, because of increased proliferation or a reduction in
programmed cell death (reduction in apoptosis), with a resulting
*clonal expansion of that lineage. A second genomic alteration
might then occur within this expanding clone, again by chance,
providing an additional growth advantage for that cell and its
progeny. By virtue of these two advantages, the cells of this
clone would eventually overgrow neighboring cells, creating yet
another expanding clone. This scenario would repeat as a
consequence of each new mutation that provided an additional
growth advantage. The accumulation of these growth promoting
mutations is the basis of the current view of "multistep
carcinogenesis".
-----------
C.R. Boland and L. Ricciardiello: How many mutations does it take
to make a tumor?
(Proc. Natl. Acad. Sci. US 21 Dec 99 96:14675)
QY: C. Richard Boland [crboland@ucsd.edu]
-----------
Text Notes:
... ... *epithelial cells: In animals, "epithelial cells"
compose the cell layers that form the interface between a tissue
and the external environment, for example, the cells of the skin,
the lining of the intestinal tract, and the lung airway passages.
... ... *mesodermal: In the embryos of higher animals, there
occurs the transformation of a single-layer "blastula" into a
3-layered "gastrula" consisting of ectoderm (outermost layer),
mesoderm (middle layer), and endoderm (innermost layer)
surrounding a cavity with one opening. The 3 layers are called
the "germ layer", and these layers, via further cell
differentiation and proliferation, determine the development of
all the major body systems and organs.
... ... *lymphatic: The lymphatic system is a complex network
for the distribution of lymph fluid (which is similar to blood
plasma -- blood without red cells). Lymph is collected by
drainage from the tissues throughout the body, flows in the
lymphatic vessels through the lymph nodes, and is eventually
added to the venous blood circulation.
... ... *point mutation: A minor changes in the genome; a single
base-pair substitution.
... ... *genetic amplification process: The production, by
various means, of additional copies of a stretch of genomic DNA.
... ... *alleles: One of two or more forms of a given gene that
control a particular characteristic, with the alternative forms
occupying corresponding loci on homologous chromosomes. 
... ... *mitosis: Programmed division of the nucleus during cell
replication.
... ... *clonal expansion: This refers to the expansion of a
population of cells all derived from repeated replications of
progeny of a single cell.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 14Jan00
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
ON GENETICS AND HUMAN CANCERS
The current consensus is that cancer results from the
accumulation of mutations in the genes that directly control the
birth and death of biological cells. But the mechanisms through
which these mutations are generated are the subject of continuing
debate and much research. It has been argued that an underlying
genetic instability is absolutely essential for the generation of
the multiple mutations that underlie cancer. On the other hand,
it has also been suggested that normal rates of mutation, coupled
with waves of *clonal expansion, are sufficient for the cancer
process to occur in humans. ... ... C. Lengauer et al (Johns
Hopkins University, US) present a review of observations
concerning the stability of the genome of human cancer cells, the
authors making the following points:
... 1) Numerous genetic alterations that affect growth-
controlling genes have been identified in neoplastic cells over
the past 15 years, and these observations provide persuasive
evidence for the genetic basis of human cancer. The alterations
can be divided into 4 major categories:
... ... a) Subtle sequence changes: These changes involve
nucleotide base substitutions or deletions or insertions of a few
nucleotides in the genome, and unlike the alterations described
below, they cannot be detected via cytogenetic analysis. Such
mutations, for example, occur in over 80 percent of pancreatic
cancers.
... ... b) Alterations in chromosome number: Such alterations
involve losses or gains of whole chromosomes. Such changes are
found in nearly all major human tumor types.
... ... c) Chromosome translocations: These alterations can be
detected cytogenetically as fusions of different chromosomes or
of normally non-contiguous segments of a single chromosome. At
the molecular level, such translocations produce fusions between
two different genes, endowing the fused genetic entity with
tumorigenic properties. Such translocations are known to occur in
the *chronic myelogenous leukemias.
... ... d) Gene amplifications: These are seen at the cytogenetic
level as homogeneously stained regions, and at the molecular
level they involve multiple copies of a gene. An example of gene
amplification occurs in advanced *neuroblastomas.
... 2) All 4 of the alterations described above occur commonly in
specific tumor types but are rarely or never observed in normal
cells. However, the existence of genetic alterations in a tumor,
even when frequent, does not mean that the tumor is genetically
unstable. By definition, instability is a matter of rate, and the
existence of a mutation provides no information about the rate of
its occurrence. The higher prevalence of mutations in tumor cells
compared with normal cells still requires explanation.
... The authors conclude: "One can argue persuasively that all
chemotherapeutic compounds used at present are more toxic to
cancer cells than to normal cells only and specifically because
of the defective *checkpoints that occur in cancer cells. This
line of reasoning suggests that, although instability may be
essential for neoplasia to develop, it may also prove to be its
Achilles' heel when the tumor is attacked by the right agents.
Further research to define the molecular and physiological bases
of instability may, therefore, yield entirely new approaches to
treating common forms of cancer."
-----------
Editor's note: In addition to the background material appended to
this report, see report #5 in this issue.
-----------
C. Lengauer et al: Genetic instabilities in human cancers.
(Nature 17 Dec 98 396:643)
QY: Christoph Lengauer [lengauer@jhmi.edu]
-----------
Text Notes:
... ... *clonal expansion: In this context, this refers to the
expansion of a population of cells all deriving from a single
mutated cell.
... ... *chronic myelogenous leukemias: (granulocytic leukemias)
These leukemias are characterized by an uncontrolled
proliferation of myelopoietic cells (blood cells derived from
bone marrow).
... ... *neuroblastomas: Neuroblastomas are malignant neoplasms
characterized by only slightly differentiated immature nerve
cells of embryonic type.
... ... *checkpoints: In this context, the term "checkpoint"
refers to a point in the eukaryotic cell division cycle where the
cycle can be halted until conditions are suitable for the cell to
proceed to the next stage. (eukaryotic = containing membrane-
bound organelles such as a nucleus.)
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 26Mar99
-------------------
Related Background:
ANEUPLOIDY AND GENETIC INSTABILITY OF CANCER CELLS
In general, germ cells (egg cells and sperm cells) and somatic
cells (non-germ cells) carry different numbers of chromosomes,
with germ cells carrying exactly half the number (haploid number)
of somatic cell chromosomes (diploid number). The term
"aneuploidy" (heteroploidy) refers to a condition in which the
number of chromosomes in a cell is not an integer multiple of the
haploid number typical for that cell or organism. For example,
the haploid human chromosome number is 23; the normal somatic
cell contains 46 chromosomes; a somatic cell with 47 or 44
chromosomes is aneuploid. Some authors, however, use the term
"aneuploidy" to indicate merely an abnormal number of
chromosomes. In cell biology, the term "karyotype" refers to the
characteristics profile (number, size, and shape) of a set of
chromosomes of a cell or organism. In this context, the term
"phenotype" refers to the total appearance of a cell as
determined by the interaction during development between its
genetic constitution (genotype) and the cell's environment.
Genetic and phenotypic instability are hallmarks of cancer cells,
but the cause of the instability is not clear. The leading
hypothesis suggests that a poorly defined gene mutation generates
genetic instability and that one or more of the many subsequent
mutations then cause cancer [*Note #1]. ... ... P. Duesberg et al
(2 installations, DE US) report an investigation of the
hypothesis that genetic instability of cancer cells is caused by
aneuploidy, which they define as "an abnormal balance of
chromosomes". The authors point out that because symmetrical
segregation of chromosomes during mitosis depends on exactly two
copies of the genes involved in mitosis ("mitosis genes"),
aneuploidy involving chromosomes bearing mitosis genes will
destabilize the karyotype. The authors propose that the
aneuploidy hypothesis predicts that the degree of genetic
instability should be proportional to the degree of aneuploidy,
and it should thus be difficult to maintain the particular
karyotype of a highly aneuploid cancer cell on *clonal
propagation. The authors report this prediction is confirmed with
clonal cultures of chemically transformed aneuploid Chinese
hamster embryo cells. Defining the "ploidy factor" as the
quotient of the modal chromosome number divided by the normal
number of the species, it was found that the higher the ploidy
factor of a clone, the more unstable was its karyotype. The
authors point out that work by others has established an exact
correspondence between the karyotype instability of human colon
cancer cell lines and the degree of aneuploidy. The present
authors suggest that, independent of gene mutation, aneuploidy is
sufficient to explain genetic instability and the resulting
karyotypic and phenotypic heterogeneity of cancer cells. The
authors further suggest that because aneuploidy has also been
proposed to cause cancer, their hypothesis "offers a common,
unique mechanism of altering and simultaneously destabilizing
normal cellular phenotypes."
-----------
P. Duesberg et al: Genetic instability of cancer cells is
proportional to their degree of aneuploidy.
(Proc. Natl. Acad. Sci. US 10 Nov 98 95:13692)
QY: Peter Duesberg [duesberg@uclink4.berkeley.edu]
-----------
Text Notes:
... ... *Note #1: In 1976, Peter Nowell postulated that a
precancerous mutation generates exceptional "genetic instability"
or "mutability", and that the highly mutable "premalignant" cell
then suffers many further gene mutations, including those that
cause cancer (P.C. Nowell, Science 194:21 1976).
... ... *clonal propagation: In general, in this context, a
"clone" is a line of identical cells produced from one or a few
originating cells.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 22Jan99
[For more information: http://scienceweek.com/search/search.htm]


5. ASTRONOMY: ON THE EFFECTS OF ASTEROID-EARTH IMPACTS
During the past several years, there has been much media
attention devoted to the prospect of an asteroid impacting Earth.
Such impacts were apparently more common several billion years
ago than at present, but impacts are definitely possible at any
time, and the US National Aeronautics and Space Agency (NASA) has
in place a program to detect all near-Earth asteroids larger than
approximately 1 kilometer in radius. But if we do detect a large
asteroid on a collision-course with Earth, it is not yet clear
what we can do about it with our present technology except
perhaps engineer a nuclear missile hit to deflect it.
... ... Jack J. Lissauer (NASA Ames Research Center, US) presents
the following considerations concerning the expected effects of
asteroid impacts on Earth and life on Earth:
     1) The largest mass extinction of the past 200 million years
occurred 65 million years ago, when approximately half of the
genera of multicellular organisms on Earth, including all of the
dinosaurs, suddenly died off. The geological record indicates
that a layer of impact-produced minerals and the element iridium
(an element rare in the crust of the Earth but more abundant in
primitive meteorites) was deposited at the time the dinosaurs
vanished -- the so-called Cretaceous/Tertiary or K/T boundary. In
addition to this, the largest known crater on Earth to be dated
at less than 1 billion years old was apparently formed at this
time. Taken together, these data imply that the K/T mass
extinction was caused by the impact into the Yucatan peninsula of
an asteroid or comet of approximately 10 kilometers in radius.
[*Note #1].
     2) The author presents the following tabulation of the
effects of impacts of objects of various sizes on the Earth and
on life on Earth:
... ... a) Super colossal object (radius > 2000 kilometers):
Melts the planet; drives off all volatiles and wipes out life on
the planet.
... ... b) Colossal object (radius > 700 kilometers): Melts the
crust; wipes out life on planet.
... ... c) Huge object (radius > 200 kilometers): Vaporizes
oceans; life may survive below the surface.
... ... d) Extra large object (radius > 70 kilometers): Vaporizes
upper 100 meters of oceans; pressure-cooks photic zone; may wipe
out photosynthesis.
... ... e) Large object (radius > 30 kilometers): Heats
atmosphere and surface to approximately 1000 degrees kelvin;
continents cauterized.
... ... f) Medium object (radius > 10 kilometers): Fires, dust,
darkness; atmosphere/ocean chemical changes; large temperature
swings; half of living species extinct.
... ... g) Small object (radius > 1 kilometer): Global dusty
atmosphere for months; photosynthesis interrupted; individual
deaths but few species become extinct; civilization threatened.
... ... h) Very small object (radius > 100 meters): Major local
effect; minor hemispheric effects; dusty atmosphere; only minor
global effects on life.
-----------
Jack L. Lissauer: How common are habitable planets?
(Nature 2 Dec 99 402supp:C11)
QY: Jack L. Lissauer [lissauer@ringside.arc.nasa.gov]
-----------
Text Notes:
... ... *Note #1: Some authors (see below) have stated the
Yucatan impactor had a diameter of 10 kilometers rather than a
radius of 10 kilometers.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 14Jan00
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
ON THE IMPACT HAZARDS OF ASTEROIDS
G. Verschuur (University of Memphis, US) reviews the
probabilities and consequences of asteroid collisions with Earth.
Our civilization has just passed through an extraordinary era of
scientific discovery that has brought with it the awareness that
planet Earth is profoundly vulnerable to devastating cosmic
collisions. In recent years, the evidence that mass extinctions
of life on Earth can be attributed to the consequences of comet
or asteroid impacts has become overwhelming. Most famous among
such catastrophes is the Cretaceous-Tertiary impact that
apparently marked the demise of the dinosaurs about 65 million
years ago. The attention of many planetary scientists has turned
to the problem of assessing the likelihood that our civilization
may be threatened by a rogue comet or asteroid in the near
future. Asteroid hunters estimate that 9000 objects of dimensions
0.5 kilometers or larger are in near-Earth orbits, and that of
these only 350 have been identified to date. A small-to-medium
size 200 meter object smashing into a 5-km deep ocean at 50 km
per second would raise a splash 35 kilometers high in 40 seconds
and produce tsunamis that would inundate lands bordering the
ocean. Calculations indicate that the impact anywhere on Earth of
even a medium-size asteroid 0.2 to 1.0 km would be catastrophic.
The author suggest that we have been lucky to avoid a recent 
catastrophic collision with a comet or asteroid, and that
although it may not happen in the next year or the next century,
eventually the Earth *will* be hit by a sizable piece of cosmic
debris. QY: Gerrit L. Verschuur, Univ. of Memphis 901-678-2169.
(Sky & Telescope June 1998) (Science-Week 1 May 98)
-------------------
Related Background:
ISOTOPIC EVIDENCE FOR THE CRETACEOUS-TERTIARY IMPACTOR
The Cretaceous period is the geological period ranging
approximately from 146 million years ago to 65 million years ago,
and was apparently characterized towards its end by the rapid
extinction of a number of species, including the dinosaurs. There
have been five major extinctions according to the fossil record,
the Cretaceous extinction one of them, and the consensus is that
these extinctions were related to violent geophysical events,
perhaps asteroid impacts. The Chicxulub impact crater in the
Yucatan peninsula of Mexico is a large impact crater apparently
caused by a 10 kilometer-diameter asteroid, the impact area
extending at least 100 kilometers from the impact center. Using
Argon(40)/Argon(39) isotope dating methods, this impact crater
has been dated with high precision at 64.98 million years ago,
which places the impact at the end of the Cretaceous, and the
most popular current hypothesis to explain the Cretaceous
extinction is the global effect of the Chicxulub impact on the
extant life forms. This hypothesis was first proposed by Luis and
Walter Alvarez in the 1970s on the basis of non-terrestrial dust
of presumed cosmic origin in deposits at the K/T boundary, but
the Yucatan crater was unknown at that time and was not
discovered until the 1990s. But direct isotope evidence of an
impactor is still missing, and some researchers have argued that
high concentrations of iridium and other noble metals in K/T
boundary sediments, the basis for the K/T impactor hypothesis,
can be explained by enhanced volcanic activity that occurred near
the end of the Cretaceous, bringing up noble metals from Earth's
mantle, which similar to meteorites has high concentrations of
noble metals. ... ... A. Shukolyukov and G.W. Lugmair now report
a high-precision *mass spectrometric analysis of chromium in
sediment samples from the K/T boundary confirms the cosmic
origins of the K/T phenomenon. The authors report that the
isotopic composition of chromium in K/T boundary samples from
Stevns Klint, Denmark, and Caravaca, Spain, is different from
that of Earth and indicates its extraterrestrial source. The
authors suggest the chromium isotope signature is consistent with
a *carbonaceous chondrite-type impactor, and that the observed
differences in the chromium isotopic composition among the
various meteorite classes can serve as a diagnostic tool for
deciphering the nature of impactors that have collided with Earth
during its history.
-----------
A. Shukolyukov and G.W. Lugmair (Univ. of Calif. San Diego, US)
Isotopic evidence for the Cretaceous-Tertiary Impactor and its
Type.
(Science 30 Oct 98 282:927)
QY: A. Shukolyukov, Univ. of Calif. San Diego 619-534-2230.
-----------
Text Notes:
... ... *mass spectrometric analysis: The mass spectrometer is a
device in which molecules are ionized and the accelerated ions
are separated according to their mass to charge ratio. The
relative abundance of isotopes or other ionized species can thus
be determined by measuring positive or negative ion currents.
... ... *carbonaceous chondrite: "Stony" meteorites (aerolites)
are meteorites formed solely of rock-forming silicates, and
chondrites are a type of stony meteorite consisting of an
agglomeration of millimeter-sized globules (chondrules) that are
thought to be unchanged since the original condensation out of
the nebula from which the sun and solar system formed. A
carbonaceous chondrite is a chondritic meteorite that contains a
relatively large amount of carbon, with a resultant dark
appearance.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 27Nov98
-------------------
Related Background:
ON METEORITE IMPACT AND THE K/T MASS EXTINCTION
... In a short review of the meteorite impact hypothesis and the
K/T extinction, K.O Pope et al make the following points: 1)
Confirmation of the impact portion of the Alvarez hypothesis
marks a turning point in the study of the K/T mass extinction, a
turning point away from speculations about possible causes and
toward linking the extinctions to a single catastrophic event. 2)
Advances in computer modeling of the impact, coupled with
knowledge of the target rocks and their behavior under the
high-pressure shock, have shed light on what happened during the
first few seconds after impact. A key aspect of the Yucatan site
is that the upper 3 kilometers of rock were rich in water,
carbonate, and sulfate, which upon impact produced about 200
gigatons each of SO(sub2) and H(sub2)O vapor and other gases that
greatly altered the properties of the stratosphere. 3) Early work
predicted that smoke and dust from the impact plunged the Earth
into a freezing blackout. Recent computer simulations and
atmospheric models indicate that within a few weeks to months
temperatures and light levels would have begun to rebound due to
the release of heat stored in the oceans and the coagulation and
fall of the dust and soot. The major effects of the dust and soot
would last about 1 year or less, but SO(sub2) and water vapors
would remain in the stratosphere and ultimately produce sulfuric
acid aerosols. Models indicate that a global aerosol cloud would
be continuously produced for approximately 12 years, blocking out
over 50 percent of the sunlight during the first 10 years. The
authors conclude: "Now that we have a better understanding of the
dynamics of the impact, gleaned from the discovery of the crater
and the studies that followed, we can begin to address a wide
range of complex global effects. There is much work ahead, but
the course is clear."
-----------
K.O. Pope et al (3 authors at 3 installations, US)
Meteorite impact and the mass extinction of species at the
Cretaceous/Tertiary boundary.
(Proc. Natl. Acad. Sci. US 15 Sep 98 95:11028)
QY: Kevin O. Pope, Geo Eco Arc Research, 2222 Foothill Blvd., La
Canada, CA 91011 US.
-------------------
Summary by SCIENCE-WEEK  23Oct98
-------------------
Related Background:
ANALYSIS OF THE CHICXULUB IMPACT CRATER
... An important parameter of the [Chicxlulub] impact is the
total area of the impact crater, since that area would be related
to the amount of debris thrown into the atmosphere. Until now,
the usual figure for the largest dimension of the impact crater
has been approximately 300 kilometers. Morgan et al (20 authors
at 8 installations, UK US MX CA) now report an analysis of
seismic data of the Chicxulub impact, determining the diameter of
the transient cavity at about 100 kilometers. The authors suggest
this parameter is critical for constraining any proposed
impact-related effects on the Cretaceous environment, and that
the seismic data indicate the morphology of the crater is similar
to large impact structures observed on other planets such as
Venus.
QY: Mike Warner [.warner@ic.ac.uk]
(Nature 4 Dec 97) (Science-Week 26 Dec 97)


6. PHYSICS: ON THE STANDARD MODEL AND A UNIFIED PHYSICS
In particle physics, the "Standard Model" is a theoretical
framework whose basic idea is that all the visible matter in the
universe can be described in terms of the elementary particles
leptons and quarks and the forces acting between them. Leptons
are a class of point-like fundamental particles showing no
internal structure and no involvement with the strong forces.
Electrons and neutrinos are among the particles classified as
leptons. The strong force (nuclear strong force) is one of the
four fundamental forces: the gravitational force, the
electromagnetic force, the nuclear strong force, and the nuclear
weak force (see below), with the strong force approximately 100
times stronger than the electromagnetic force. A quark is a
hypothetical fundamental particle, having charges whose
magnitudes are one-third or two-thirds of the electron charge,
and from which the elementary particles may in theory be
constructed. At the present time, ongoing experimental projects
in particle physics are expected to permit a completion of the
Standard Model, but a unified theory of all forces known to
physics is not yet in sight.
... ... Steven Weinberg (University of Texas Austin, US) presents
the following considerations concerning the Standard Model and
current attempts to achieve a unified physics:
     1) In physics, the greatest advances of the past have been
steps involving unification: a) the unification of terrestrial
and celestial mechanics by Isaac Newton (1642-1727) in the 17th
century; b) the unification of optics with the theories of
electricity and magnetism by James Clerk Maxwell (1831-1879) in
the 19th century; c) the unification of space-time geometry and
the theory of gravitation by Albert Einstein (1879-1955) in the
years 1905 to 1916; d) the unification of chemistry and atomic
physics by quantum mechanics in the 1920s.
     2) Our current theory of elementary particles and forces,
known as the Standard Model of particle physics, has achieved a
unification of electromagnetism with the weak interactions, the
forces responsible for the change of neutrons and protons into
each other in radioactive processes and in the stars. The
Standard Model also gives a separate by similar description of
the strong interactions, the forces that hold quarks together
inside protons and neutrons, and that hold protons and neutrons
together inside atomic nuclei. We have ideas about how the theory
of strong interactions can be unified with the theory of weak and
electromagnetic interactions, but the approach may only work if
gravity is included, and the inclusion of gravity presents
serious theoretical difficulties.
     3) The Standard Model is a quantum field theory. Its basic
ingredients are fields, including the electric and magnetic
fields of 19th century electrodynamics. Perturbations in these
fields carry energy and momentum from place to place, and quantum
mechanics indicates these perturbations come in bundles, or
quanta, which are recognized in the laboratory as elementary
particles. For example the quantum of the electromagnetic field
is the photon. The Standard Model includes a field for each type
of elementary particle that has been observed in high-energy
physics laboratories.
     4) The Standard Model is a quantum field theory of a special
kind, one that is "renormalizable". This term goes back to the
1940s, when physicists were learning how to use the first quantum
field theories to calculate small shifts of atomic energy levels.
They found that calculations using quantum field theory kept
producing infinite quantities, a situation which usually
indicates a theory is badly flawed or is being pushed beyond its
limits of validity. Eventually, physicists discovered a way to
deal with the infinite quantities by absorbing them into a
redefinition, or "renormalization", of just a few physical
constants, such as the charge and mass of the electron.
     5) Although the profoundest advances in fundamental physics
tend to occur when the principles of different types of theories
are reconciled within a single new framework, we do not yet know
what guiding principle underlies the unification of quantum field
theory, as embodied in the Standard Model, with general
relativity. The quantum nature of space and time must be dealt
with in a unified theory. At the shortest distance scales, space
may be replaced by a continually reconnecting structure of
*strings and membranes -- or by something stranger still.
     6) The author suggests it is impossible to state when these
problems will be overcome. "They may be solved in a preprint put
out tomorrow by some young theorist. They may not be solved by
2050, or even 2150. But when they are solved... we will not have
any trouble in recognizing the truth of the fundamental unified
theory. The test will be whether the theory successfully accounts
for the measured values of the physical constants of the Standard
Model, along with whatever effects beyond the Standard Model may
have been discovered by then."
-----------
Steven Weinberg: A unified physics by 2050?
(Scientific American December 1999)
QY: Steven Weinberg, University of Texas Austin 512-471-3434.
-----------
Text Notes:
... ... *strings and membranes: See background material below.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 14Jan00
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
ON FIELD THEORY IN PHYSICS
In physics, a field is an entity that acts as intermediary in
interactions between particles, and which is distributed over
part or all of space, and whose properties are functions of space
coordinates, and except for static fields, also functions of
time. There is also a quantum-mechanical analog of this entity,
in which the function of space and time is replaced by an
operator at each point in space-time. ... ... Roman Jackiw
reviews present theoretical work in the theory of elementary
particles and forces, and the author makes the following points:
1) Present-day theory for fundamental processes (i.e.,
descriptions of elementary particles and forces) is phenomenally
successful. Experimental data confirms theoretical prediction,
and where accurate calculation and experiments are attainable,
agreement is achieved to 6 or 7 figures. Two examples: a) The
helium atom ground state energy (*Rydbergs) is experimentally
measured as -5.8071394 and theoretically calculated as
-5.8071380. b) The muon magnetic dipole moment is experimentally
measured as 2.00233184600 and theoretically calculated as
2.00233183478. 2) The theoretical structure within which this
success has been achieved is *local field theory, which offers a
wide variety of applications, and which provides a model for
fundamental physical reality as described by our theories of
*strong, electroweak, and gravitational processes. No other
framework exists in which one can calculate so many phenomena
with such ease and accuracy. 3) But is spite of these successes,
today there is little confidence that field theory will advance
our understanding of nature at its fundamental workings beyond
what has already been achieved. Although in principle all
observed phenomena can be explained by present-day field theory,
these accounts are still imperfect, requiring ad hoc inputs.
Moreover, because of conceptual and technical obstacles,
classical gravity theory has not been integrated into the
*quantum field description of nongravitational forces:
*quantizing the *metric tensor of Einstein's theory produces a
quantum field theory beset by infinities that apparently cannot
be controlled. 4) These shortcomings are actually symptoms of a
deeper lack of understanding concerning *symmetry and symmetry
breaking... Physicists are happy in the belief that Nature in its
fundamental workings is essentially simple, but observed physical
phenomena rarely exhibit overwhelming regularity. Therefore, at
the very same time that we construct a physical theory with
intrinsic symmetry, we must find a way to break the symmetry in
physical consequences of the model. 5) These problems have
produced a theoretical impasse for over two decades, and in the
absence of new experiments to channel theoretical speculation,
some physicists have concluded that it will not be possible to
make progress on these questions within field theory, and they
have turned to a new structure, "*string theory". In field
theory, the quantized excitations are point particles with point
interactions, and this gives rise to the infinities. In string
theory, the excitations are extended objects -- strings -- with
nonlocal interactions; there are no infinities in string theory,
and that enormous defect of field theory is absent. 6) Yet in
spite of its positive features, until now string theory has
provided a framework rather than a definite structure, and a
precise derivation of the *Standard Model has yet to be given.
The author concludes: "On previous occasions when it appeared
that quantum field theory was incapable of advancing our
understanding of fundamental physics, new ideas and new
approaches to the subject dispelled the pessimism. Today we do
not know whether the impasse within field theory is due to a
failure of imagination or whether indeed we have to present
fundamental physical laws in a new framework, thereby replacing
the field theoretic one, which has served us well for over 100
years."
-----------
Roman Jackiw (Massachusetts Institute of Technology, US)
Field theory: Why have some physicists abandoned it?
(Proc. Natl. Acad. Sci. US 27 Oct 98 95:12776)
QY: Roman Jackiw, Mass. Inst. of Technology 617-253-1000.
-----------
Text Notes:
... ... *Rydbergs: A unit of energy used in atomic physics,
value = 13.605698 electronvolts.
... ... *local field theory: In this context, "locality" is the
condition that two events at spatially separated locations are
entirely independent of each other, provided that the time
interval between the events is less than that required for a
light signal to travel from one location to the other. For
example, the quantum mechanical wave function is a "local" field.
... ... *strong, electroweak, and gravitational processes: The
fundamental forces comprise the gravitational force, the
electromagnetic force, the nuclear strong force, and the nuclear
weak force. The "electroweak" interactions are a unification of
the electromagnetic and nuclear weak interactions, and are
described by the Weinberg-Salam theory (sometimes called "quantum
flavordynamics"; also called the Glashow-Weinberg-Salam theory).
... ... *quantum field description: In general, a quantum field
theory is a quantum mechanical theory applied to systems having
an infinite number of *degrees of freedom.
... ... *degrees of freedom: In general, the number of
independent parameters required to specify the configuration of a
system.
... ... *quantizing: In experimental physics, a quantized
variable is a variable taking only discrete multiple values of a
quantum mechanical constant. In theoretical physics, "quantizing"
means the consistent application of certain rules that lead from
classical to quantum mechanics. In general, "quantization" is a
transition from a classical theory or a classical quantity to a
quantum theory or the corresponding quantity in quantum
mechanics.
... ... *metric tensor: The mathematical statement (involving a
set of quantities) that describes the deviation of the Pythagoras
theorem in a curved space.
... ... *symmetry and symmetry breaking: If a theory or process
does not change when certain operations are performed on it, the
theory or process is said to possess a symmetry with respect to
those operations. For example, a circle remains unchanged under
rotation or reflection, and a circle therefore has rotational and
reflection symmetry. The term "symmetry breaking" refers to the
deviation from exact symmetry exhibited by many physical systems,
and in general, symmetry breaking encompasses both "explicit"
symmetry breaking and "spontaneous" symmetry breaking. Explicit
symmetry breaking is a phenomenon in which a system is not quite,
but almost, the same for two configurations related by exact
symmetry. Spontaneous symmetry breaking refers to a situation in
which the solution of a set of physical equations fails to
exhibit a symmetry possessed by the equations themselves.
... ... *string theory: In particle physics, string theory is a
theory of elementary particles based on the idea that the
fundamental entities are not point-like particles but finite
lines (strings), or closed loops formed by strings, the strings
one-dimensional curves with zero thickness and lengths (or loop
diameters) of the order of the Planck length of 10^(-35) meters. 
... ... *Standard Model: See main report.
-------------------
Summary & Notes by SCIENCE-WEEK  4Dec98
-------------------
Related Background:
THEORETICAL PHYSICS: ON STRING THEORY
... ... B.R. Greene et al present a short review of recent
developments in string theory and make the following points: 1)
Particle physics has spent much of this century grappling with
one basic question in various forms: What are the fundamental
*degrees of freedom needed to describe nature, and what are the
laws that govern their dynamics. 2) The current "Standard Model"
of particle physics -- which is nearly 25 years old and which has
much experimental evidence in its favor -- involves 6 *quarks, 6
*leptons, 4 *forces, and the as yet unobserved *Higgs boson. But
this model contains internal indications that it too may be just
another step along the path of uncovering the truly fundamental
degrees of freedom. The Standard Model is valid to distances as
small as 10^(-16) cm, and there is some evidence that the next
level of structure will be detected only at a distance scale of
the order of 10^(-32) cm, which is far beyond our abilities to
measure in the laboratory. 3) A related important issue concerns
the unification of general relativity and quantum mechanics. A
serious problem arises when general relativity is extrapolated to
small distance scales of the order of 10^(-32) cm where quantum
effects must be taken into account: the relevant theoretical
equations produce uncontrollable divergences, and the history of
particle physics suggests this is an indication of a new physics
occurring at these distance scales. 4) String theory offers hope
of addressing both of these issues. There is only one known way
to cure the divergence problem in the quantum-mechanical
expansion of general relativity, and that is to model the
particles in the theory not as points but as one-dimensional
loops of "string". Every consistent such string model necessarily
contains a special kind of particle -- the "*graviton" --  whose
long-distance interactions are described by general relativity.
So in a sense, string theory predicts gravity. 5) An exciting new
frontier was opened during the past few years with the discovery
of "string duality", which predicts equivalences among various
different physical systems. This discovery has its roots in the
properties of "supersymmetry", a novel type of symmetry that all
consistent string theories possess. Briefly, supersymmetry
relates properties of two basic types of particles -- bosons and
*fermions -- which cannot be related by ordinary symmetry. There
is a current belief that supersymmetry will play a role in the
structure of particle physics beyond the Standard Model. One of
the important achievements of string duality has been the
determination of the behavior of the 5 consistent string theories
when interactions become strong. All the consistent string
theories are apparently related to each other, and to an
elaboration known as "membrane theory" (M-theory). String duality
has produced hope that there may be only one possible string-
theoretic model of the universe, and that it may be possible to
eventually predict such features as particle masses and
interaction strengths directly from such a theory. The authors
conclude: "Development has been rapid on many fronts since string
duality was introduced. We may be seeing glimpses of the
underlying principle manifested in these new results. The
challenging task that lies ahead is to discover that principle
and thereby find what may well be the truly fundamental degrees
of freedom in our universe."
-----------
B.R. Greene et al (3 authors at 3 installations, US)
String theory.
(Proc. Natl. Acad. Sci. US 15 Sep 98 95:11039)
QY: David R. Morrison 
-----------
Text Notes:
... ... *degrees of freedom: See previous report.
... ... *quarks: See previous report.
... ... *leptons: A class of elementary particles. Although they
are affected by electromagnetic and gravitational forces, apart
from that they are involved only with weak interactions, acted
upon by weak forces but not by strong forces, as opposed to
quarks, which are acted upon by strong forces but not by weak
forces. One further difference between leptons and quarks is that
leptons can be isolated as single particles, whereas quarks
apparently cannot. The leptons include the electron, the muon,
the tau, and their associated neutrinos. The mass of the tau is
approximately 3484 times the mass of the electron; the mass of
the muon is intermediate.
... ... *forces: See previous report (fundamental forces).
... ... *Higgs boson: Higgs fields (named after Peter W. Higgs,
University of Edinburgh, UK) constitute a set of fundamental
theoretical fields that induce spontaneous symmetry breaking. In 
general, spontaneous symmetry breaking occurs in systems whose
underlying symmetry state is unstable. A Higgs particle is
associated with a Higgs field in the same way that a photon is
associated with the electromagnetic field. Higgs bosons are
massive mesons whose existence is predicted by certain theories.
Mesons are apparently composed of quark and anti-quark pairs;
they are produced by various high-energy interactions and decay
into stable particles.
... ... *graviton: Several quantum field theories consistent with
both quantum mechanics and special relativity postulate that the
gravitational force between two quantum domain particles is
generated by the exchange of an intermediate particle called a
graviton.
... ... *fermions: Fermions (electrons, protons, neutrons) are
particles that obey the Pauli exclusion principle: i.e., no two
fermions of the same kind can occupy the same quantum state.
-------------------
Summary & Notes by SCIENCE-WEEK  16Oct98
-------------------
Related Background:
ON RECENT DEVELOPMENTS IN SUPERSTRING THEORY
Bose-Einstein statistics is the statistical mechanics of a system
of indistinguishable particles for which there is no restriction
on the number of particles that may simultaneously exist in the
same quantum energy state. Bosons are particles that obey Bose-
Einstein statistics, and they include photons, pi mesons, all
nuclei having an even number of particles, and all particles with
integer spin. Fermions (electrons, protons, neutrons) are
particles that obey the Pauli exclusion principle: i.e., no two
fermions of the same kind can occupy the same quantum state.
In particle physics, string theory is a theory of elementary
particles based on the idea that the fundamental entities are not
point-like particles but finite lines (strings), or closed loops
formed by strings, the strings one-dimensional curves with zero
thickness and lengths (or loop diameters) of the order of the
Planck length of 10^(-35) meters. The fundamental forces comprise
the gravitational force, the electromagnetic force, the nuclear
strong force, and the nuclear weak force, and the "grand unified
theories" are theories that aim to provide a mathematical frame-
work in which the electromagnetic forces, strong forces, and weak
forces emerge as parts of a single unified force, with the three
forces related by symmetry. Supersymmetry is an aspect of an
extension of the grand unified theories, an attempt to unify all
the four fundamental forces, i.e., linking gravitation to the
electromagnetic force, the strong force, and the weak force
through a supersymmetry scheme, and superstrings are strings in
this scheme that obey supersymmetry. ... ... John H. Schwarz
(California Institute of Technology, US) presents a brief
overview of some of the advances in understanding super-
string theory that have been achieved in the last few years.
String theories that have a symmetry relating bosons and
fermions, called "supersymmetry", are called "superstring"
theories. Major advances in understanding of the physical world
have been achieved during the past century by focusing on
apparent contradictions between well-established theoretical
structures. In each case the reconciliation required a better
theory, often involving radical new concepts and striking
experimental predictions. Four major advances of this type were
the discoveries of special relativity, quantum mechanics, general
relativity, and quantum field theory. This was quite an
achievement for one century, but there is one fundamental
contradiction that still needs to be resolved, namely the clash
between general relativity and quantum field theory. Many
theoretical physicists are convinced that superstring theory will
provide the answer.
QY: John H. Schwarz, Calif. Inst. of Technology 818-395-6811.
(Proc. Natl. Acad. Sci. US 17 Mar 98)
(Science-Week 10 Apr 98)
-------------------
Related Background:
ON THE EVOLUTION OF STRING THEORY TO MEMBRANE THEORY
... Membrane theory (M-theory) is a recent extension of string
theory in which the fundamental physical entities are considered
as surfaces in a many-dimensional space (membranes) rather than
as lines or loop elements (open or closed strings). Given all of
the above, some caution is necessary: the translation of a highly
abstract mathematical model of physical reality into
non-mathematical language is often an exercise of limited
usefulness, and in this case in particular, we are presenting
only the ghost of the theoretical scheme. String theory was
originally invented in the 1960s as a theory of the strong force,
became overshadowed by the strong force theory of gluons and
quarks, then had a revival in the 1980s -- but with the history
more dependent on new work than on fashion. ... ... M. Duff
(Texas A & M Univ., US), who is active in string theory and
membrane theory, in a review of various aspects of the history
and essentials of string theory and membrane theory, suggests
that future historians may judge the 20th century as "a time when
theorists were like children playing on the seashore, diverting
themselves with the smoother pebbles or prettier shells of
superstrings, while the great ocean of M-theory lay undiscovered
before them."
QY: Michael J. Duff, Texas A & M Univ. 409-847-9451.
(Scientific American February 1998) (Science-Week 23 Jan 98)
[For more information: http://scienceweek.com/search/search.htm]


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IN FOCUS: ON MITOCHONDRIA, DNA, AND SPERM CELLS
     Mitochondria are double-membrane enclosed organelles of
cells, and they are involved with several important biochemical
pathways, including electron transport and oxidative metabolism.
Various types of eukaryotic cells (cells containing membrane-
bound organelles such as a nucleus) may contain from a few to
several thousand mitochondria in each individual cell. The
mitochondria are relatively large cylindrical structures up to 10
microns long and up to 2 microns in diameter, and they are
believed to have originated as organisms that became symbiotic
with eukaryotic cells. (In biology, "symbiosis" is an intimate
and protracted association of individuals of different species.)
Mitochondria contain their own genome, and mitochondrial DNA
(denoted as mtDNA), found in the mitochondria of all eukaryotes,
is believed to evolve in parallel with nuclear DNA.
     During the maturation of sperm cells in the human testes
(spermiogenesis), the mitochondria of sperm cells are relocated:
the mature sperm cell consists of 3 parts, the head, midpiece,
and tail (flagellum), and all the mitochondria are densely packed
into the midpiece of the mature sperm cell.
     One of the major techniques used to investigate ancient
human lineages involves the genetic analysis of mitochondrial
DNA, with such DNA considered to be primarily of maternal origin.
However, there is apparently some confusion about the reasons for
the primarily maternal origin of mitochondrial DNA. For example,
the 1998 textbook _Principles of Human Evolution_ by Roger Lewin
(Harvard University, US) [*Note #1] contains on page 414 an
illustrative drawing depicting the fate of sperm mitochondria,
the drawing showing the midpiece and tail of the sperm cell
"discarded" upon fertilization of the egg cell. The drawing has
the following caption: "Unlike nuclear DNA, for which we inherit
half from our mother and half from our father, mitochondrial DNA
is passed on only by females. When the sperm fertilizes the egg,
it leaves behind all of its mitochondria: the developing fetus
therefore inherits mitochondria only from the mother's egg."
     The above presentation by Lewin contradicts current
information in cell biology. The idea that sperm lose their
mitochondria at fertilization as a result of extracellular
"discard" of the midpiece and tail is not correct. The current
view in cell biology is that the entire human sperm cell (head,
midpiece, and tail) penetrates the egg cell during the
fertilization process. Sperm mitochondria are apparently lost
(destroyed) shortly after penetration of the egg by specific
enzymatic reactions, but the destruction of sperm mitochondria
inside the egg cell is believed to be not always complete. The
current view in cell biology is that since the sperm mitochondria
and the sperm flagellum disintegrate inside the egg, very few, if
any, sperm-derived mitochondria are found in developing or adult
organisms. In mice it is estimated that only 1 out of every
10,000 mitochondria are sperm-derived. Nevertheless, the
significance of contaminating paternal mitochondria in the use of
mitochondrial DNA to establish genetic lineages is in controversy
in the literature, and the issue is not yet resolved [*Note #2].
[The Editors wish to thank James M. Cummins, Murdoch University
(AU) for calling our attention to the question of the fate of
sperm cell mitochondria.]
-----------
By the Editors of SCIENCE-WEEK [http://scienceweek.com] 14Jan00
-----------
Text Notes:
... ... *Note #1: Roger Lewin: Principles of Human Evolution,
Blackwell Science, 1998, p.414.
... ... *Note #2: For additional material, cf. F. Ankel-Simons
and J.M. Cummins (Proc. Natl. Acad. Sci. US 1996 93:13859) and
Jim Cummins (Rev. of Reproduction 1998 3:172).

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