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.

February 4, 2000 -- Vol. 4 Number 5

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A star is simpler than an insect.
-- Martin Rees

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Contents of This Issue:

1. Quantum Physics: A New Framework for Ionization Problems
2. Materials Science: On Block Polymer Self-Assembly
3. Earth Science: Ozone Depletion and Plant DNA Damage
4. Anthropology: Recombination in Hominid Mitochondrial DNA
5. Molecular Biology: Carbonic Anhydrase -- An Ancient Enzyme
6. Biotechnology: On the Future of Cloning
 
In Focus: On the Formation of Raindrops

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1. QUANTUM PHYSICS: A NEW FRAMEWORK FOR IONIZATION PROBLEMS
In general, the term "ionization" refers to the process of
producing ions. Electrolytes (e.g., salts and acids) ionize
spontaneously in solution. In certain reactions, ionization may
be produced by electron transfer in the course of the reaction.
Ions may also be formed when an atom or molecule loses one or
more electrons as a result of energy gained in a collision with
another particle or with a photon. Although electron-impact
ionization of atoms and molecules is a phenomenon intensively
studied and used in physics and applied physics, there has not
yet been a complete quantitative description of the phenomenon
derived from quantum mechanics.
... ... T.N. Rescigno et al (4 authors 3 installations, US)
present a theoretical framework which the authors suggest
provides a basis for developing practical quantitative methods to
treat ionizing collisions of electrons with atoms and molecules.
The authors make the following points:
     1) Electron-impact ionization of atoms and molecules is one
of the most basic phenomena in low-energy collision physics, and
is one of the most efficient means for exciting and ionizing
atoms and molecules. It is the fundamental mechanism for ion
formation in mass spectroscopy and is responsible for forming and
sustaining low-temperature ionized gases (low-temperature
plasmas) that are used in applications ranging from fluorescent
lighting to the processing of silicon chips.
     2) Since the invention of quantum mechanics, even the
simplest example of the electron-impact collisional breakup of a
system of charged particles, e(-) + H --> H(+) + e(-) + e(-)
(where e is an electron and H is hydrogen), has resisted solution
and is now one of the last unsolved fundamental problems in
atomic physics. A complete solution requires calculation of the
energies and directions for a final state in which all three
particles are moving away from each other. Even with
supercomputers, the theoretically correct mathematical
description has proved difficult to achieve.
     3) The authors suggest that by using a mathematical
transformation of the Schroedinger equation that makes the final
state tractable, they have devised a framework for solving
ionization problems in many areas of chemistry and physics, and
have provided the key to a numerical solution of this problem
that reveals the full dynamics of the phenomenon.
     4) The method of the authors involves transformation of the
appropriate system of quantum mechanical differential equations
into large systems of complex linear equations represented on a
5-million by 5-million 2-dimensional numerical grid. Such
calculations require special techniques and are only practical to
carry out on massively parallel supercomputers.
     5) The authors conclude: "At a time when large-scale
computers are generally thought to be necessary to investigate
the "complexity" of the physical world in the very different
sense of treating increasingly larger systems, it is noteworthy
that the same computing tools are needed to answer a basic
physics question for one of the simplest systems imaginable in
physics and chemistry."
-----------
T.N. Rescigno et al: Collisional breakup in a quantum system of
three charged particles.
(Science 24 Dec 99 286:2474)
QY: T.N. Rescigno, Lawrence Livermore National Laboratory,
Livermore, CA 94551 US.
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 4Feb00
[For more information: http://scienceweek.com/search/search.htm]


2. MATERIALS SCIENCE: ON BLOCK POLYMER SELF-ASSEMBLY
     The term "polymer" derives from the Greek "polymeros" and
means "many parts". The individual units of polymers are called
"monomers", and most common polymers are composed of regular
repetitions of one or more monomers. There is no formal
restriction on the composition of a polymer: e.g., asbestos is an
inorganic polymer. Although polymeric substances have been known
and used for thousands of years, the first polymeric materials to
attract recorded scientific interest were silk and cobwebs: in
1665 Robert Hooke (1635-1703) suggested that the products of the
silkworm and spider could be imitated by drawing a suitable glue-
like substance out into a thread. This is essentially the process
used today in industry to manufacture synthetic polymer fibers
such as rayon.
     The explosive growth of the plastics industry in the 19th
and 20th centuries was based on two developments: the elucidation
of the principles of polymer chemistry and the extraction of the
monomers required for polymer synthesis as products of petroleum
refining. It was not until the 1920s and the work of Hermann
Staudinger (1881-1965) that it was understood that the various
plastics being produced in industry consisted of linear molecular
chains rather than disorderly conglomerates of small molecules.
     A simple linear polymer is a chain molecule composed of
monomers with two reactive sites (bifunctional monomers), with
monofunctional terminal units. If more than one bifunctional
monomer is present, the chain is known as a "copolymer". A
copolymer in which a number of units of the same monomer are
located adjacent to one another (in "blocks" of monomers) is
called a "block copolymer". A "diblock copolymer" is composed of
two types of monomers (e.g., A and B), and may be depicted thus:
AAAAAABBBBBAAAAAABBBBBAAAAAAA.
... ... Edwin L. Thomas (Massachusetts Institute of Technology,
US) presents a review of current research on the self-assembly of
polymers, the author making the following points:
     1) The structure of biological macromolecules is typically
determined by sequences of different monomer units (e.g., a
variety of amino acids in proteins). In contrast, polymer
chemists have long used large blocks of identical monomer units
to design the microstructure of polymer materials. Recent work on
such self-assembled block copolymers has produced materials
exhibiting intricate 3-dimensional periodic structures, and these
novel interconnected nanoscale patterns are providing
opportunities in a variety of technological areas from separation
membranes to photonic crystals.
     2) The simplest block copolymer consists of two chemically
dissimilar monomers, A and B, which form a single macromolecule
consisting of linear groups of identical monomers. Such AB
diblock copolymers can be thought of as giant *amphiphiles whose
components segregate into domains to avoid unfavorable contact
with each other, with complete phase separation prevented by the
covalent linkage between the components. The spatially periodic
domain structure, whose domain size and spacing are typically on
the 10- to 50-nanometer scale, must then minimize unfavorable
interfaces and at the same time avoid overstretching the polymer
blocks. Depending on the length of the A and B segments,
spherical, cylindrical, or lamellar domains may form. Higher
structural complexity is observed in 3-component systems, and in
recent years ABC triblock copolymers have proven a fertile source
of new microdomain patterns.
     3) The synthesis of polymers that can form topologically
connected periodic patterns can result in useful materials such
as a superefficient membrane of selective permeability (perm-
selective membrane). Also, periodic dielectrics made from such
polymers will be useful in certain optical applications, and
self-assembled optical components based on block copolymers are
now being realized.
     4) The author concludes: "The huge ABC polymer parameter
space available to synthetic chemists, and the rich set of
resultant self-assembled morphologies, suggest that many more
opportunities lie ahead."
-----------
Edwin L. Thomas: The ABCs of self-assembly.
(Science 12 Nov 99 286:1307)
QY: Edwin L. Thomas [elt@mit.edu]
-----------
Text Notes:
... ... *amphiphiles: In general, a molecule with parts (groups)
having diverse affinities for different solvents. For example,
polar groups have an affinity for water, while hydrocarbon groups
have an affinity for oils. Most detergents are amphiphiles,
molecules with a polar head and a long hydrocarbon tail. In this
context, however, possible solvent interactions are only one
aspect of amphiphilic character. The important consideration is
that in large polymeric amphiphiles, the molecule as a system of
interacting parts will attempt to minimize free energy by
essentially "self-solubilizing" itself through the formation of
stable domains of noncovalent interactions. The main point of the
report is that by manipulating the organization and extents of
monomer blocks in a triblock copolymer, one can achieve
structural periodicities that may have important applications.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 4Feb00
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
CONTROL OF POLYMER SHAPE IN SYNTHETIC SELF-ASSEMBLY SYSTEMS
In general, the term "polymer conformation" refers to the higher
order structure of polymers, and one goal of synthetic polymer
chemistry is to define the conditions producing various types of
conformations, since in many cases both the physical properties
and reactivity of polymers is conformation dependent. In polymer
chemistry, "quasi-equivalent" building blocks are chemically
identical subunits that self-control their shape by switching
between different conformational states during the process of
self-assembly. The significance of quasi-equivalent units is
perhaps most apparent in the comparative supramolecular
structures of icosahedral-shaped versus rod-shaped viruses.
... ... Percec et al (6 authors at 3 installations, US UK DE) now
report a general method for the control of polymer conformation
through the self-assembly of quasi-equivalent monodendritic
(branched) side-groups attached to flexible backbones, with the
production of either spherical or cylindrical polymers controlled
by the degree of polymerization. The authors suggest their
strategy will provide new approaches to the rational design of
organized supramolecular materials, expanding the synthetic and
technological uses of dendritic building blocks.
QY: V. Percec [vxp5@po.cwru.edu]
(Nature 8 Jan 98) (Science-Week 23 Jan 98)
-------------------
Related Background:
ON ANALYSIS OF SELF-ORGANIZING POLYMERIC MATERIALS
Self-organizing polymeric domains are of considerable interest in
materials science, and are essential for the existence of
biological systems. Polymer chemists and biochemists categorize
the types of forces that may be involved in self-organizing
domains as 1) hydrophobic and hydrophilic effects; 2) hydrogen
bonding; 3) coulombic interactions; and 4) van der Waals forces.
Biological materials exhibit special physical or chemical
functions as a result of special shapes or conformations that
result from self-assembly. To act as enzymes, for example,
proteins require specific amino acid sequences that result in
specific foldings and conformational arrangements, the end
product providing a "docking" site whose interaction with an
transition state entity catalyzes a particular reaction.
Biological self-organizing polymers such as proteins and nucleic
acids are much more complicated than the self-organizing polymers
known to polymer chemists outside biochemistry, and a recent
trend is for polymer chemists to look to the data on
self-organizing biological macromolecules for hints about special
synthetic innovations. One could call it payback time: In the
early years of biochemistry, in the 1930s and 1940s, biochemists
looked at polymer chemistry for ideas that would help them
understand biological macromolecules; and now a half century
later polymer chemists are in turn looking at the biochemistry of
biological macromolecules for ideas that will help them
synthesize new useful materials. M. Muthukumar et al (three
installations in the US) recently reviewed progress in our
understanding of interactions and ordering in self-organizing
polymeric materials.
QY. M. Muthukumar, Univ. Massachusetts Amherst (413) 545-0222
(Science 29 August) (Science-Week 19 Sep 97)
[For more information: http://scienceweek.com/search/search.htm]


3. EARTH SCIENCE: OZONE DEPLETION AND PLANT DNA DAMAGE
Ozone [O(sub3)] is a compound formed by several means, including
a) exposure of oxygen gas to ultraviolet radiation; and b) the
passage of electric sparks through air. Ozone is a blue gas and a
blue-black solid and liquid, melting point -193 degrees
centigrade, boiling point -112 degrees centigrade. Ozone gas is
present in only trace quantities in the atmosphere of Earth: if
all the ozone in the atmosphere were brought down to sea level,
the layer of ozone would be only approximately 4 millimeters
thick. Nevertheless, ozone in the outer atmosphere (lower
stratosphere; 15 to 30 kilometers above the surface) acts to
shield the Earth from excessive radiation, particularly
ultraviolet radiation of 280 to 315 nanometers wavelength (UV-B),
the UV band that is most dangerous to living systems. UV-B
radiation is lethal to simple unicellular organisms (algae,
bacteria, protozoa), and to the surface cells of higher plants
and animals. UV-B radiation damages DNA and is responsible for
sunburn in human skin. In addition, the incidence of skin cancer
in humans has been statistically correlated with the observed
surface intensities of the UV wavelengths between 290 and 320
nanometers which are not totally absorbed by the ozone layer.
In the lower atmosphere (*troposphere), ozone forms from
combustion gases and is a major air pollutant contributing to
*photochemical smog. Since the discovery in 1985 that an ozone
hole develops over the Antarctic in late winter and early spring,
intense research efforts have been devoted to clarifying the
roles of atmospheric transport and chemistry in stratospheric
ozone changes.
... ... M.C. Rousseaux et al (9 authors at 3 installations, AR
US) present a study of the impact of ozone depletion and UV-B
radiation on plant DNA damage in southern South America. The
authors make the following points:
     1) The most important consequence of the depletion of
stratospheric ozone is the increased transmission of solar UV-B
radiation to the Earth's surface. Present levels of stratospheric
ozone are at the lowest point since the measurement began in the
1970s. Ozone depletion is most pronounced over the Antarctic
continent, where ozone levels commonly decline by more than 70
percent during late winter and early spring. Acute effects of
ozone depletion on native organisms have been documented only for
marine ecosystems of Antarctic waters. For example, it has been
shown that increased UV-B can reduce *phytoplankton
photosynthesis in the marginal ice zone when the ozone hole is
overhead, reduce phytoplankton cell densities, and increase the
DNA damage burden in *icefish eggs. Virtually nothing is known
about the consequences of ozone depletion and increased solar UV-
B on natural ecosystems located outside Antarctica.
     2) The authors report that the temperate ecosystems of
southern South America have been subjected to increasingly high
levels of ozone depletion during the last decade. In the spring
of 1997, despite frequent cloud cover, the passages of the ozone
hole over Tierra del Fuego (latitude 55 degrees south) caused
concomitant increases in solar UV, and the enhanced ground-level 
UV led to significant increases in DNA damage in the native plant
Gunnera magellanica (a perennial herb). The fluctuations in solar
UV explained a large proportion (up to 68 percent) of the
variation in DNA damage, particularly when the solar UV was
weighted for biological effectiveness according to *action
spectra that assume a sharp decline in *quantum efficiency with
increasing wavelength from the UV-B into the UV-A regions of the
spectrum.
     3) The authors conclude: "Our data indicate that the UV
variations that take place during early spring, which to a large
extent are caused by ozone depletion, result in corresponding
changes in DNA damage density in naturally occurring individuals
of G. magellanica... The high correlation in the present case is
probably due to the fact that, as a photosynthetic organism, G.
magellanica is obligatorily exposed to sunlight, and therefore to
solar UV. Animals may afford to colonize habitats less exposed to
radiation, and mobile forms may even actively seek shelter in
response to high UV-B or to environmental variables that
correlate with UV-B radiation levels."
-----------
M.C. Rousseaux: Ozone depletion and UVB radiation: Impact on
plant DNA damage in southern South America.
(Proc. Natl. Acad. Sci. US 21 Dec 99 96:15310)
QY: Carlos L. Ballare [ballare@ifeva.edu.ar]
-----------
Text Notes:
... ... *troposphere: The term "troposphere" refers to the lowest
10 to 20 kilometers of the atmosphere (with the lower boundary
the surface of the Earth).
... ... *photochemical smog: Air pollution in the form of a brown
haze often seen over cities, occurring on sunny days in locations
with large volumes of automobile traffic. Such smog is produced
when sunlight acts on nitrogen oxides, ozone, and hydrocarbons.
Photochemical smog is a respiratory irritant in man, and can kill
or alter plant tissues.
... ... *phytoplankton photosynthesis: In general, the term
"photosynthesis" refers to the series of chemical reactions by
which plant cells transform light energy into chemical energy
through the production of various compounds and oxygen from
carbon dioxide and water. Phytoplankton (photoplankton) are
small, usually microscopic, aquatic plants capable of
photosynthesis; e.g., unicellular algae. Phytoplankton and
plankton are not equivalent. The term "plankton" is a general
designation for various drifting microscopic aquatic organisms in
the upper regions of the oceans, both photosynthetic and non-
photosynthetic.
... ... *icefish: A member of the family Salangidae; small
teleost fishes. A "teleost fish" is one of a group of bony fish,
with over 17,000 different species ranging from eels to trout.
... ... *action spectra: In general, an "action spectrum" is a
graph showing the range of wavelengths over which a photochemical
reaction occurs. The action spectrum indicates which wavelengths
of light are most effective for driving the reaction.
... ... *quantum efficiency: In the context of a radiation-
induced process, the term "quantum efficiency" refers to the
actual number of species which are decomposed or reacted per
quantum of energy absorbed.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 4Feb00
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
ON STRATOSPHERIC OZONE
... ... D.W. Fahey and A.R. Ravishankara present a review of
current research on stratospheric ozone, the authors making the
following points:
     1) Average ozone concentrations in the polar stratosphere
show a pronounced cyclical variation over the course of the year.
In winter and early spring, ozone builds up at the poles as
ozone-rich air is transported from lower latitudes toward the
polar regions. But when transport to high latitudes slows and
solar illumination increases in late spring and summer, catalytic
ozone destruction leads to a substantial decrease (approximately
30 percent).
     2) Ozone is produced via solar ultraviolet photolysis of
oxygen and destroyed through catalytic cycles involving reactive
nitrogen, halogen (chlorine and bromine), and hydrogen species.
     3) The balance between photolytic production, transport, and
chemical destruction determines the abundance of ozone at any
particular stratospheric location. This balance is also strongly
season dependent. In addition, the relative contributions of the
3 types of catalytic destruction of ozone differ between the
summer and winter-spring seasons.
     4) During the summer, large regions of the polar
stratosphere receive uninterrupted sunlight for many weeks.
Photolysis reactions, several of which are complete ozone
destruction cycles, occur continuously under these conditions.
Total ozone concentrations therefore continuously decrease
throughout high latitudes in late spring and early summer.
     5) The authors suggest that we now understand in some detail
how the combined effects of transport, chemical ozone production,
and catalytic ozone loss control ozone during the annual cycle of
stratospheric conditions. The summer ozone decreases at high
latitudes will persist in the future because natural reactive
nitrogen rather than human-induced reactive halogen species are
primarily responsible for ozone destruction in those regions. In
contrast, the winter-spring ozone destruction will gradually
lessen in the next decades as halogen emissions steadily decrease
-- barring other changes to the stratosphere such as major
cooling of this region due to greenhouse gases.
-----------
D.W. Fahey and A.R. Ravishankara: Summer in the stratosphere.
(Science 9 Jul 99 285:208)
QY: A.R. Ravishankara [ravi@al.noaa.gov]
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 27Aug99


4. ANTHROPOLOGY: RECOMBINATION IN HOMINID MITOCHONDRIAL DNA
     The origin of modern humans is an ongoing major focus of
research in anthropology and paleontology, and also a research
area that has seen its share of contentious disputes. There are
two conflicting views concerning the geographic aspects of human
origins: 1) in one view, the geographic origins of modern man are
multiple, with modern man (Homo sapiens) appearing more or less
at the same time on various continents; while in the second view
b) modern man originated in Africa approximately 200,000 years
ago, with modern humans migrating from Africa to the rest of the
globe.
     The major evidence for the "Out of Africa" hypothesis was
published in the late 1980s by R.L. Cann et al (1987), the
evidence based primarily on analysis of mitochondrial DNA in
diverse existing human groups.
     Mitochondria are double-membrane enclosed organelles of
cells, and they are involved in 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.
     In the late 1980s, most anthropologists and paleontologists
believed that the mitochondria of sperm cells do not enter the
egg cell (or if they do, are quickly destroyed upon entry), so
that male sperm mitochondrial DNA does not mix (*recombine) with
female egg mitochondrial DNA. The idea, therefore, was that
mitochondrial DNA is of pure maternal lineage, and since analysis
of human mitochondrial DNA suggested a single origin of Homo
sapiens in Africa, the notion of an "African Eve" was quickly
publicized by the popular media [*Note #1]
     In recent years, however, the notion that mitochondria are
of pure maternal lineage has been challenged, and the dispute
among anthropologists and paleontologists concerning multiple-
origins vs. a single-origin for Homo sapiens has flared up again.
... ... P. Awadalla et al (3 authors at 2 installations, UK) now
present an analysis of possible DNA recombination in human and
chimpanzee mitochondrial genomes, the authors making the
following points:
     1) The authors point out that for many years it has been
accepted that mitochondria are inherited exclusively from the
mother in mammals, and that the inheritance of mitochondrial DNA
is therefore "clonal". This assumption has been used extensively
to date events in human prehistory, including the age of our last
common female ancestor, called "Eve", and to date the spread of
Homo sapiens in Asia and Europe. However, mitochondria do contain
the enzymes necessary for *homologous recombination, and there
are at least two routes by which the rule of strict maternal
inheritance of mitochondrial DNA could be broken: a) the entrance
of paternal mitochondria into the egg cell at fertilization
[*Note #2]; and b) the transfer of nuclear genome copies of
mitochondrial DNA sequences back to mitochondrial DNA.
     2) The authors suggest that the assumption that human
mitochondrial DNA is inherited from one parent only and therefore
does not recombine is questionable. The work of the authors
indicates that *linkage disequilibrium in human and chimpanzee
mitochondrial DNA declines as a function of the distance between
genome sites, and this pattern can be attributed to one mechanism
only: recombination.
     3) The authors conclude: "Many inferences about the pattern
and tempo of human evolution and mtDNA evolution have been based
on the assumption of clonal inheritance. These inferences will
now have to be reconsidered."
-----------
P. Awadalla et al: Linkage disequilibrium and recombination in
hominid mitochondrial DNA.
(Science 24 Dec 99 286:2524)
QY: Adam Eyre-Walker [a.c.eyre-walker@sussex.ac.uk]
-----------
Text Notes:
... ... *recombine: In this context, the term "recombination"
refers to a genome with a combination of genes other than those
that occurred in the precursor genome(s), the recombination, in
this context, produced naturally. Thus, if mitochondrial DNA has
naturally spliced into it one or more sequences of nuclear DNA or
DNA from another line of mitochondria, the mix is called
"recombination". (See note below on "homologous recombination".)
... ... *Note #1: Apart from its proposed exclusive maternal
lineage (which has now been challenged), mitochondrial DNA has a
number of research advantages: a) The complete nucleotide
sequence of human mitochondrial DNA is known, the genome
identified as a circular DNA molecule of 16,569 base pairs. b)
Since there are as much as thousands of copies of mitochondrial
DNA per cell, mitochondrial DNA can be more easily isolated from
human tissues than nuclear DNA, which has only two copies per
cell. c) It is believed that mutations occur in mitochondria 10
times more frequently than in nuclear DNA, and the consequent
rapid evolution of the mitochondrial genome enables comparisons
between groups that would be more difficult to differentiate
using slower and more complex nuclear DNA sequences.
... ... *homologous recombination: In general, the term
"homologous recombination" refers to genetic recombination that
occurs between DNAs with long stretches of homology, and which is
mediated by certain enzymes involved in DNA repair and
replication. In this context, the terms "homologous" and
"homology" refer to sequences having fundamental similarities due
to the same evolutionary origin, even if the functions of the two
sequences are quite different.
... ... *Note #2: See relevant background material below.
... ... *linkage disequilibrium: In this context, the term
"linkage" refers to gene sequences (genetic loci) that tend to be
inherited together more often than would be expected by chance.
Genetic linkage is a reflection of the physical location of the
loci on the same chromosome segment or DNA molecule. Loci which
are close together are less likely to be separated by
recombination and are therefore more likely to be inherited
together. The distance between linked loci is measured in terms
of the frequency of recombination events occurring between them.
The term "linkage disequilibrium" refers to a situation in which
a particular combination of gene variants (alleles) at two
closely linked loci appears more frequently than would be
expected by chance. The essential idea of the authors in this
report is that recombination can be detected by considering the
relation between linkage disequilibrium and gene loci distance
(genetic distance). As the distance between loci increases, the
effect of recombination should increase, and recombination should
therefore manifest itself as a significant decline in linkage
disequilibrium with distance. The study of the authors consisted
of analysis of previously published data concerning mtDNA
sequences in humans and chimpanzees (Pan troglodytes).
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 4Feb00
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
IN FOCUS: ON MITOCHONDRIA, DNA, AND SPERM CELLS
... 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).
-------------------
Related Background:
PALEOLITHIC HUMAN POPULATION EXPANSION IN AFRICA
Human populations have undergone dramatic expansions in size, but
other than the growth associated with agriculture, the dates and
magnitudes of those expansions have never been resolved. Genetic
approaches to the study of human population expansions have
focused on variation at a single genetic locus, the "control
region" of *mitochondrial DNA. But in the study of demographic
history, single-locus investigations suffer from pronounced
statistical and biological limitations. The statistical problem
is that the conclusions rely on only one particular realization
of a gene genealogy, the "tree" determining the ancestral
relationships among a set of *alleles. The biological problem is
that there are a large number of functional genes in the
mitochondrion, and due to a complete linkage, a selective sweep
for any one of the genes may lead to a spurious signal of
expansion. ... ... Reich and Goldstein (University of Oxford, UK)
present two new statistical tests for population expansion, using
variation at a number of unlinked genetic markers to study the
demographic histories of natural populations. The authors report
that analysis of genetic variation in various aboriginal
populations throughout the world reveals highly significant
evidence for a major human population expansion in Africa, but no
evidence of expansion outside of Africa. The inferred African
expansion is estimated to have occurred between 49,000 and
640,000 years ago, certainly before the Neolithic expansions, and
probably before the splitting of African and non-African
populations. The authors suggest that in showing a significant
difference between African and non-African populations, their
analysis supports the unique role of Africa in human evolutionary
history. The authors also suggest that the missing signal in non-
African populations may be the result of a population bottleneck
associated with the emergence of these populations from Africa,
as postulated in the "Out of Africa" model of modern human
origins.
QY: David B. Goldstein 
(Proc. Natl. Acad. Sci. US 7 Jul 98 95:8119)
(Science-Week 7 Aug 98)
-------------------
Related Background:
... ... *mitochondrial DNA: See main report.
... ... *alleles: An allele is 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.
-------------------
Related Background:
Y CHROMOSOME EVIDENCE INDICATES AFRICAN ORIGINS OF MAN
The Y chromosome is one of the two chromosomes that determine sex
in many animals, including humans, and it carries mostly male-
specific genes. Genetic polymorphisms are individual functional
variations of specific genes or genetic markers that occur in a
population with a significant frequency, e.g., more than 1%.
Mitochondrial DNA (sometimes denoted as mtDNA), found in the
mitochondria of all eukaryotes, is believed to evolve in parallel
with nuclear DNA and to be inherited only in the maternal lineage
in animals. Until now, it has been mitochondrial DNA that has
been greatly exploited in studies of the evolution of humans. At
a recent symposium on human evolution (Cold Spring Harbor
Laboratory, NY US), a consensus was apparently reached that
current studies of human Y chromosome polymorphisms indicate that
the major human migrations that occurred had their source in
Africa, and that a small number of present African populations,
the Ethiopians, Sudanese, and south African Khoisans, possess
markers that have been conserved since that time. The data are
considered to confirm the recent mitochondrial DNA studies which
also indicate Africa as the source of human migrations. Some
paleoanthropologists are calling the Y chromosome results an
"unquestionable major breakthrough". (Science 31 Oct 97)
(Science-Week 21 Nov 97)
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
FIRST ANALYSIS OF DNA FROM A NEANDERTHAL BONE
About 10 kilometers east of Dusseldorf in Germany, in the valley
of the Dussel, there is a little town called Neander. One hundred
and forty-one years ago, in the summer of 1856, some workmen
broke into a cave to get at the limestone inside and discovered a
set of ancient bones. Most of the bones were smashed to bits by
the workmen, but some of the bones, including part of the skull,
survived, and the skeleton was soon recognized by anthropologists
as belonging to an ancient race of men who came to be known as
the Neanderthals. A Neanderthal fossil had actually been
discovered some years earlier in Gibraltar, but not recognized as
such. Neanderthal-like fossils have also been found in France,
Spain, Italy, Yugoslavia, Iraq, China, Java, and Israel. For more
than a century, one of the central questions in paleoanthropology
has been whether modern man evolved from this race -- or was the
Neanderthal a separate branch that became extinct? Until
recently, the primary laboratory method of investigation of such
a question was analysis of the morphology of bone fragments. This
week, the field of paleoanthropology has apparently crossed an
important watershed, as M. Krings et al (University of Munich,
DE; Pennsylvania State University, US) report the first analysis
of DNA from an extinct human, in this case DNA extracted from the
actual Neanderthal skeleton found near Dusseldorf in 1856. The
key to the investigation was the analysis of mitochondrial rather
than nuclear DNA. Mitochondrial DNA is usually present in
concentrations two or three orders of magnitude greater than
nuclear DNA, and they were able to find enough of it still intact
to amplify with the PCR technique and piece together a total DNA
sequence of 379 base pairs. Comparison of this sequence with
contemporary human sequences leads to the conclusion that
Neanderthal and modern man are separate evolutionary lines, and
that the latter did not evolve from the former. The work will
have to be replicated with other Neanderthal fossils, but most
paleoanthropologists are excited by the results and expect them
to be confirmed. The technology of evolutionary paleoanthropology
has evidently now progressed from caliper measurements of bones
to measurements of bone DNA fragments.
(Cell 11 Jul 97) (Science-Week 18 Jul 97)
[For more information: http://scienceweek.com/search/search.htm]


5. MOLECULAR BIOLOGY: CARBONIC ANHYDRASE -- AN ANCIENT ENZYME
Carbonic anhydrase (carbonate dehydratase) is an enzyme that
catalyzes the reversible hydration of carbon dioxide to carbonic
acid (or to bicarbonate ion at certain pH values). The enzyme is
found in a wide range of living systems in various forms
(isozymes). It is an intracellular enzyme with zinc as a
cofactor. It is present in mammalian red blood cells
(erythrocytes), where it accelerates the carbon dioxide-to-
bicarbonate reaction essential in higher animals.
... ... K.S. Smith et al (4 authors at Pennsylvania State
University, US) present a study of the prevalence of carbonic
anhydrase in primitive living systems, the authors making the
following points:
     1) Since the discovery of carbonic anhydrase in bovine red
blood cells in 1933, carbonic anhydrase isozymes have been found
in virtually all mammalian tissues and cell types, where they
function in carbon dioxide transport and other physiological
processes. Carbonic anhydrases are also abundant in plants and
unicellular green algae, where they are essential for
photosynthetic carbon dioxide fixation. Although the carbonic
anhydrases are ubiquitous in highly evolved organisms from the
*Eukarya, the extent to which carbonic anhydrases occur in the
*Archaea and *Bacteria domains is unknown: since 1963, the enzyme
has been purified from only 5 *prokaryotic species.
     2) The authors report that carbonic anhydrase is widespread
in the Archaea and Bacteria domains, and is an ancient enzyme.
The occurrence in *chemilithoautotrophic species occupying deep
branches of the universal phylogenetic tree suggests a role for
this enzyme in the proposed *autotrophic origin of life. The
authors suggest that the presence of 2 of the 3 classes of
carbonic anhydrases in metabolically diverse species spanning the
Archaea and Bacteria domains demonstrates that carbon anhydrases
have a far more extensive and fundamental role in prokaryotic
biology than previously recognized.
     3) The authors report evidence for the presence of carbonic
anhydrase in a wide variety of organisms, including the following
types of living systems: freshwater, marine, *mesophilic,
thermophilic, aerobic, anaerobic, pathogenic, *symbiotic,
*methylotrophic, *methanogenic, autotrophic, *heterotrophic, and
photosynthetic.
     4) The authors conclude: "Investigations into the ways that
metabolically diverse groups in the Archaea and Bacteria domains
use carbonic anhydrase will undoubtedly reveal novel aspects of
cell physiology."
-----------
K.S. Smith et al: Carbonic anhydrase is an ancient enzyme
widespread in prokaryotes.
(Proc. Natl. Acad. Sci. US 21 Dec 99 96:15184)
QY: James G. Ferry [jgf3@psu.edu]
-----------
Text Notes:
... ... *Eukarya: (eukaryotes) The kingdom of organisms whose
cells possess a nucleus bounded by a nuclear membrane and
containing true chromosomes.
... ... *Archaea: The archaebacteria (also called the Archaea)
are considered to be ancient compared to other kingdoms, and
possibly the most ancient life forms and the ancestors of all
eukaryotes (see Eukarya note above). They typically exist in
extreme environments, and include the methane-producing bacteria
(methanogens), the "salt-loving" bacteria (halophilic bacteria),
and the sulfur-acid tolerant thermoacidophilic bacteria. There is
presently a controversy concerning whether the Archaea should be
classified as a kingdom separate from the Bacteria.
... ... *Bacteria: Organisms in this kingdom are unicellular
and without a cell nucleus.
... ... *prokaryotic: Prokaryotic cells (prokaryotes) are cells
that do not contain a nucleus or other membrane-bound organelles.
... ... *chemilithoautotrophic species: The term "autotroph"
(lithotroph) refers to any organism that needs only simple
inorganic substances to fulfill its nutritional requirements and
for which gaseous or dissolved carbon dioxide is the sole source
of carbon for the synthesis of cellular constituents. Autotrophs
are classified into "chemoautotrophs" (chemolithoautotrophs) and
"photoautotrophs" (photolithoautotrophs), depending on whether
the organism derives its energy from exogenous chemical sources
or from light, respectively. [Distinguish here the suffixes
"-trophic" and "-tropic". A "phototrophic" organism is an
organism that requires input of light as an energy source. A
"phototropic" organism is an organism that is attracted to light,
moving or turning towards light, but which may or may not require
light as an energy source.]
... ... *autotrophic origin of life: In general, the idea that
the first living systems on Earth were autotrophs, i.e., systems
requiring only inorganic substances or light energy to synthesize
complex organic compounds.
... ... *mesophilic: In this context, the term "mesophilic"
refers to organisms that thrive at moderate temperatures, i.e.,
between 20 and 40 degrees centigrade.
... ... *symbiotic: In biology, "symbiosis" is an intimate and
protracted association of individuals of different species
... ... *methylotrophic: Refers to organisms that can use certain
single-carbon compounds in a lower oxidation state than carbon
dioxide as the sole carbon source.
... ... *methanogenic: In general, the ability to form methane
from carbon dioxide, a property of some anaerobic bacteria.
... ... *Archaea: Until recently, living systems were classified
into 2 kingdoms, the Prokarya and the Eukarya, depending on the
absence or presence of internal membrane-bound organelles such as
a nucleus, respectively. On the basis primarily of molecular
analysis, the Prokarya (prokaryotes) were split into two
kingdoms, the Archaea and the Bacteria. The Archaea (also called
the Archaebacteria) are considered to be ancient compared to the
other two kingdoms, and possibly the most ancient life forms and
the ancestors of all eukaryotes (see Eukarya note below). They
typically exist in extreme environments, and include the
methane-producing bacteria (methanogens), the "salt-loving"
bacteria (halophilic bacteria), and the sulfur-acid tolerant
thermoacidophilic bacteria. There is presently some controversy
concerning whether the Archaea should indeed be classified as a
kingdom separate from the Bacteria.
... ... *Eukarya: (eukaryotes) Any organism whose cells possess a
nucleus bounded by a nuclear membrane and containing true
chromosomes.
... ... *Bacteria: Organisms in this kingdom are unicellular
and without a cell nucleus (i.e., "prokaryotes").
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 4Feb00
[For more information: http://scienceweek.com/search/search.htm]

     
6. BIOTECHNOLOGY: ON THE FUTURE OF CLONING
During the past several years, research on cloning has produced
much discussion in the scientific community and a whirlwind of
media attention by journalists in books and magazine articles. It
is now possible to make clones, or exact genetic copies, of
sheep, cows, goats, mice and, probably, humans. Although there
are a number of social issues concerning the possibility of human
cloning that remain unresolved, what seems certain is that in the
future the technology of cloning will improve and the
applications of mammalian cloning will be of considerable
importance.
... ... J.B. Gurdon and A. Coleman (2 installations, UK) review
the current state of the science and technology of cloning, the
authors making the following points:
     1) The authors point out that cloning techniques have been
in use for centuries. The practice of taking cuttings is
universal among gardeners, and large companies now propagate
desirable plant strains in large quantities. Lower invertebrates
can be easily cloned: for example, if one cuts an earthworm or
flatworm in half, the halves will regenerate to create two
genetically identical individuals. Although this method does not
work in vertebrates, identical twins are naturally occurring
genetic clones, and the method of nuclear transplantation, first
used 40 years ago in frogs, has been successfully used to make
clones of various mammals, and could probably be applied to
humans.
     2) Of importance in nuclear transfer techniques is the
general scheme of natural fertilization: In vertebrates,
fertilization begins with the union of the sperm cell and the egg
cell. Prior to fertilization, the egg cell has been stopped at a
certain stage of the *cell-division cycle. The sperm provides an
activation stimulus that triggers the resumption and completion
of cell division. The egg and sperm "*pronuclei" then swell,
their chromosomes unravel from the tightly packed condensed state
in which they are stored, and DNA replication can proceed. The
chromosomes then recondense, the nuclear membrane dissolves, and
the fertilized egg cell divides into two identical daughter
cells.
     2) The technique of nuclear transfer subverts fertilization
by replacing the female genetic material of an unfertilized egg
cell with the nucleus from a different cell. The general
procedure of nuclear transfer in mammals is as follows:
... ... a) The genetic material is removed from the recipient
cell (an unfertilized egg cell).
... ... b) The genetic material of this egg cell is replaced by a
nucleus from a donor cell, the donor nucleus containing donor DNA
(the donor genome).
... ... c) The egg cell, now containing the donor nucleus, begins
the series of divisions that normally follow fertilization.
... ... d) At a very early embryonic stage (*blastocyst), the
embryo is transferred to a surrogate mother.
... ... e) In the surrogate mother, the embryo (fetus) develops
to term, is delivered as a neonate, and the neonate is
genetically identical to the donor of the original donor nucleus.
     3) The authors suggest that for successful cloning, it is
probably essential for donor nuclei to contain a full complement
of genes. For nuclear transfer to work, an adult cell that has
already been programmed (differentiated) into a specific cell
type needs to be somehow reprogrammed so that it regains the
genetic totipotency of sperm cells and egg cells (germline cells)
-- the ability to guide the formation of all the different cell
types that make up an animal. An important conclusion to come
from nuclear transfer experiments is that the processes of cell
differentiation and ageing do not lead to permanent genetic
changes in non-germline cells (somatic cells).
     4) The pattern of gene expression in adult cells is very
different from that in embryonic cells. In amphibians, for
example, a number of genes expressed in embryos 5 hours after
fertilization are not expressed in differentiated adult cells.
Conversely, some genes are expressed in adult cells but not in
early embryos. When embryos are analyzed a few hours after the
transfer of adult cell nuclei, gene expression cannot be
distinguished from that in embryos grown from normal fertilized
eggs. This indicates that the exchange of cytoplasm around a
nucleus, from the cytoplasm of an adult cell to that of an egg
cell, causes a dramatic switch in gene expression in only a few
hours. A nucleus that was once part of an intestine, skin, or
muscle cell is therefore transformed into that of an embryonic
cell. Key molecules found in egg cells that may bring about
reprogramming of the genome include *nucleoplasmin and certain
embryo-specific proteins around which the DNA is wrapped (embryo-
specific *histones).
     5) The authors suggest that one of the major uses for
cloning in the future may be "therapeutic cloning"  -- the use of
cloning to generate tissue to replace tissue that has been
damaged or diseased. The essential idea here is to use as a donor
nucleus in nuclear transfer the nucleus of a somatic cell of the
individual requiring therapeutic tissue replacement. The major
advantage of the technique is that transplantation of the cloned
tissue into the original donor should occur without the tissue
rejection that now compromises the success of transplantation
procedures: the cloned tissue would be genetically identical to
the patient's tissue. The authors suggest that therapeutic
cloning is "ethically less contentious because a new person is
not produced." However, since an unfertilized human egg cell must
be used in the procedure, "as for abortion, the issue of the
deliberate destruction of a potential person is raised."
-----------
J.B. Gurdon and A. Coleman: The future of cloning.
(Nature 16 Dec 99 402:743)
QY: J.B. Gurdon [j.b.gurdon@welc.cam.ac.uk]
-----------
Text Notes:
... ... *cell-division cycle: The term "cell division cycle"
(cell cycle) refers to the ordered sequence of phases through
which a cell passes from one mitotic cell division to the next.
... ... *pronuclei: The term "pronucleus" refers to the nucleus
of either the egg cell (ovum) or the sperm cell following
fertilization. Once the ovum is fertilized, there are two
pronuclei, one originating from the ovum, the other from the
sperm cell that produced fertilization. The two nuclei do not
fuse until immediately before the first cleavage, when each
pronucleus loses its membrane to release its contents.
... ... *blastocyst: A mammalian egg in the later stages of
*cleavage but before implantation in the uterus. The blastocyst
consists of a hollow fluid-filled ball of cells and an inner cell
mass (embryonic stem cells) from which the embryo develops.
... ... *cleavage: The early and rapid division stage that
divides the fertilized egg into smaller and smaller cells
(blastomeres) while retaining the same overall size of the
embryo.
... ... *nucleoplasmin: A heat-stable acidic protein present in
the nucleus of many cell types. It forms complexes with
*histones.
... ... *histones: In *eukaryotic chromosomes, about every 200
nucleotides, the DNA double helix is coiled around a complex of 8
histone proteins, the entire assembly having the appearance of
beads on a string. The beads (nucleosomes) are in turn
supercoiled into a solenoid structure, and the entire complex of
the eukaryotic chromosome is called "chromatin". The small
histone proteins are basic (as opposed to acidic) proteins, and
they are essential in forming nucleosomes. Chemically, histones
are single polypeptide chains, molecular mass 11 to 21
kilodaltons, 25 percent lysine and arginine amino acids.
... ... *eukaryotic: Eukaryotic cells are cells having internal
membrane-bound organelles such as a nucleus.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 4Feb00
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
ON THE MEDICAL APPLICATIONS OF CLONING
Ian Wilmut, who led the research team that cloned the sheep
Dolly, presents an essay describing the general techniques of
cloning and the possible medical applications. The author makes
the following points: 1) The author says the announcement of the
sheep Dolly's birth in February 1997 attracted enormous press
interest, perhaps because Dolly drew attention to the theoretical
possibility of cloning humans. The author says this is an outcome
he hopes never comes to pass. But the ability to make clones from
cultured cells derived from easily obtained tissue should bring
numerous practical benefits in animal husbandry and medical
science, as well as answer critical biological questions. (*Note
#1) 2) The ability to produce offspring from cultured cells opens
up relatively easy ways to make genetically modified (transgenic)
animals. Such animals are important for research and can produce
medically valuable human proteins. 3) Cloning offers many other
possibilities. One is the generation of genetically modified
animal organs that are suitable for transplantation into humans.
4) Another promising area is the rapid production of large
animals carrying genetic defects that mimic human diseases such
as *cystic fibrosis. 5) The power to make animals with precisely
engineered genetic constitution could also be employed more
directly in cell-based therapies for important diseases,
including *Parkinson's disease, *diabetes, and *muscular
dystrophy. 6) Cloning could also be a means of producing herds of
cattle that lack the *prion protein gene, which makes cattle
susceptible to infection with prions, the agents that cause "*mad
cow disease". 7) The cloning technique might curtail the
transmission of genetic disease by treating an embryo with
advanced forms of gene therapy to modify the nuclei of embryonic
cells so that the subsequent fetus and child was free of a
specific genetic disease and unable to pass the disease to the
next generation. 8) The author states that none of the suggested
uses of cloning for making copies of existing people is ethically
acceptable to his way of thinking, "because they are not in the
interests of the resulting child. It should go without saying
that I strongly oppose allowing cloned human embryos to develop
so that they can be tissue donors." The author concludes: "It
nonetheless seams clear that cloning from cultured cells will
offer important medical opportunities. Predictions about new
technologies are often wrong; societal attitudes change;
unexpected developments occur. Time will tell. But biomedical
researchers probing the potential of cloning now have a full
agenda." [Editor's note: In addition to the related background
material below, further background from past issues of SW is
gathered in a report entitled "Cloning and Genetic Engineering:
Policy Aspects", the report available at
[http://scienceweek.com/swfr016.htm]
-----------
Ian Wilmut (Roslin Institute Edinburgh, UK)
Cloning for medicine.
(Scientific American December 1998)
-----------
Text Notes:
... ... *Note #1: The cloning procedure here is based on nuclear
transfer and involves the use of two cells. The recipient cell is
usually an unfertilized egg cell taken from an animal soon after
ovulation. The DNA-containing chromosomes are removed from the
recipient cell, and then the donor cell (containing the genome to
be copied) is fused with the recipient egg cell, and the new
fused cell is stimulated to begin the normal process of embryonic
development. Essentially, the key event is the apparent
reprogramming of the adult somatic donor cell genome by the egg
cell cytoplasm so that the donor genome now behaves like the
genome of a fertilized embryonic cell and normal development
results.
... ... *cystic fibrosis: An inherited disease of the exocrine
glands, primarily affecting the gastrointestinal tract and
respiratory systems. The "exocrine" glands are glands that secret
material via excretory ducts (e.g., mucous secreting glands).
... ... *Parkinson's disease: A neurological disorder first
described by James Parkinson (1817) and associated with
degeneration of a specific small region of the brain and a
resultant loss of projection to several important brain centers.
... ... *diabetes: When used without a qualifier, this refers to
diabetes mellitus, a metabolic disease in which carbohydrate
utilization is reduced and that of lipid and protein enhanced,
the disease caused by an absolute or relative deficiency of the
hormone insulin.
... ... *muscular dystrophy: This is a general term for a number
of hereditary, progressive degenerative disorders affecting
skeletal muscles, and often other organ systems as well.
... ... *prion protein: Prions are a class of poorly understood
proteins implicated in a number of exotic human neurological
diseases and in some common animal diseases such as sheep scrapie
and bovine spongiform encephalopathy in cattle ("mad cow
disease").
... ... *mad cow disease: (bovine spongiform encephalopathy)
Although the precise structure of the infectious agent that
causes prion diseases is still unknown, important features of its
molecular genetics have been revealed, and the evidence suggests
possible transmissibility of bovine spongiform encephalopathy to
humans.
-------------------
Summary & Notes by SCIENCE-WEEK  11Dec98
[For more information: http://scienceweek.com/search/search.htm]


=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

IN FOCUS: ON THE FORMATION OF RAINDROPS
"When water evaporates from the surface of the sea or lakes, it
becomes a gas, composed of single molecules free to move
independently among other molecules of the atmosphere. Warm air
can hold more water in the vapor form than can air at lower
temperatures. When the atmosphere cools, the water molecules tend
to condense back again into tiny cloud droplets many times
smaller than raindrops -- a billion cloud droplets would not fill
the hollow of your hand. These tiny drops of water are the
substance of all the clouds that veil the sky, the mists that
rise from marshlands on a summer night, the dew that collects on
meadow grass at dawn. But they are at least a million times
smaller than raindrops. They are so light that they remain
airborne, suspended by the rapidly dancing molecules of the gases
in the atmosphere, and they do not tend to come together to form
larger drops. So for many decades meteorologists were baffled in
their attempt to explain how raindrops form. Then it was
discovered that a frozen cloud droplet is the seed that makes
raindrops. A growing ice crystal attracts water-vapor molecules,
persuading them to give up the individuality they had preserved
so carefully before the crystal appeared. As each molecule is
added to the crystal it draws in others. Like an elaborate circle
dance, an intricate, symmetrical pattern is built up in ever-
widening spheres. A unique, original masterpiece of form suddenly
materializes from what appeared to be empty space. Soon the ice
crystal becomes heavy enough to fall through the cloud, and as it
collides with cloud droplets, little splinters of ice break off,
leaving a trail of tiny fragments that act as nuclei for new
crystals. These also grow and fall and splinter. In the space of
just a minute or two, the cloud becomes a flurry of snowflakes.
As they descend into warmer air, they melt and descend as rain
upon the land -- rain to fill the lake basins and swell the
rushing rivers, to carve out canyons, and soften the harsh rock
surfaces, to tear down the mountain ranges stone by stone and
return their substance to the sea."
-----------
Louise B. Young: _The Blue Planet_
(Little, Brown & Co. 1983)


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The first issue of SCIENCE-WEEK appeared May 1, 1997, and it has
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