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All India Coordinated Project on Taxonomy (AICOPTAX)
In the background of declining taxonomic expertise in our country, the
Ministry of Environment & Forests,
Government of India organized a two-day
National Workshop at Jaipur in February
1997. This workshop was attended by the
top taxonomic experts of the country. The
meeting identified the critical gap areas in
which taxonomic expertise in the country
was either nil or fast dwindling. One of the
recommendations of the workshop was to
develop an All India coordinated project
for capacity building in taxonomy.
Thereafter, the Ministry set up a Technical
Group to develop the All India project and
after
interministerial
consultations, the project has been
approved.
The project envisages establishment
of centres for research in identified priority
gap areas (e.g. virus, bacteria, microlepidoptera, etc.) in the field of taxonomy,
education and training (fellowships,
scholarships, chairs, career awards, etc.)
and strengthening of BSI and ZSI as the
coordinating units. The modalities of
implementing the All India project, and
prioritizing activities under the project
have been decided after detailed
consultations with experts.
Nine centres for research and two
centres for training and coordinators for
these centres have been identified in the
first phase. Subject areas identified for
establishing centres for research include
animal viruses, bacteria and archea, fungi,
lichens and bryophytes, palms, grasses and
bamboos,
helminthes
and
nematodes, insects: microlepidoptera and
mollusca. The centres for training include
one each for plant biosystematics and
animal biosystematics.
The coordinators for the centres together
with the collaborators are required to
undertake: survey, collection, identification
and preservation; maintain collections and
taxonomic
data
banks;
develop
identification manuals; and train college
teachers and students and local communities in parataxonomy.
An interactive brainstorming session
with the identified coordinators and some
collaborators was held on 3 June 1999.
A high level steering committee consisting of H. Y. Mohan Ram, C. J. Saldanha
and M. S. Jairajpuri, and representatives
from UGC, CSIR, ICAR, DBT, DST,
ICMR, ICFRE, Planning Commission,
Directors of BSI and ZSI, and some
experts has been constituted to oversee the
implementation of the project. The first
meeting of the steering committee was held
on 22 July 1999. The committee after
evaluating the proposals received from the
centres has recommended the quantum of
assistance to be released. The committee
has also recommended 14 scholarships at
the M Sc level in various identified priority
areas in the centres for research.
G. V. Sarat Babu, Ministry of Environment & Forests, Paryavaran Bhavan, CGO
Complex,
Lodhi
Road,
New
Delhi 110 003, India.
RESEARCH NEWS
Visualizing orbitals and bonds
A. G. Samuelson
Seeing is believing! There are many things
which we are skeptical about, especially
when we cannot experience them with our
five senses. Orbitals and bonds are
definitely in that category. It was not long
ago that the advancement in science which
allowed one to see and move atoms earned
for its discoverers the Nobel prize in
Physics. Now another barrier in
visualization has been scaled. One that
allows us to virtually see orbitals in atoms
where electrons are housed! Zuo et al.1 at
the Arizona State University have studied
the electron density distribution in cuprite,
Cu2O and unraveled the shape of the dz2
orbital on copper. Excess electron density
has been located in the regions away from
the O–Cu–O axis, between the tetrahedral
arrays of copper ions, making them stick
to one another!
Electron density associated with bonds
is a small fraction of the total electron
density in a molecule. In the case of
molecules with only first row elements, the
electron density associated with bonds can
be distinguished with the help of careful Xray diffraction studies2. However, in
transition metal oxides, the difficulty in
locating the bonding electrons in the
presence of core electrons is like looking
for a needle in a haystack. The researchers
solved the problem using ConvergentBeam Electron Diffraction3 (CBED) – a
new technique they had recently
developed.
CBED
gave
low-order
diffraction data from a small region in the
crystal where there was no imperfections.
Diffraction from this region allows one to
use ‘perfect-crystal theory of dynamical
diffraction’. This data was then combined
CURRENT SCIENCE, VOL. 77, NO. 9, 10 NOVEMBER 1999
with X-ray diffraction data to get structure
factors for the higher-order reflections.
Equipped with this data, they were able to
determine the charge density map of the
crystal in real space very accurately. A
theoretical electron density map was
generated assuming a spherical charge
density around the Cu+ ion and the O2–
ion. A difference map between the
theoretical and experimental electron
densities provided some amazing pictures.
Before we delve into the pictures they
have obtained, let us take a moment to
understand the structure of Cu2O. The
cuprite structure stands out and is an
unique lattice. Among the oxides, Ag2O
and Pb2O are those that adopt a similar
structure. The metal ions form a face
centered cubic lattice. The oxide ions are
found at positions 0.25, 0.25, 0.25, and
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RESEARCH NEWS
0.75, 0.75, 0.75 of the unit cell. This
results in a tetrahedral coordination of
copper ions around each oxygen and a
linear coordination geometry for each
copper (see Figure 1). What is strange is
that each copper ion finds itself in the
neighbourhood of 12 copper ions at a
distance of 3.02 Å. Since copper is present
in the + 1 oxidation state – it has a filled
shell of electrons (3s2, 3p6, and 3d10) –
these close contacts should be purely
repulsive, very much like the interaction of
two helium atoms in close proximity. Only
worse since electrostatic factors are also
unfavourable. However, in several
molecular complexes much shorter Cu(I)–
Cu(I) distances have been observed
engendering controversial explanations for
the last twenty years!4 The results of Zuo
et al. appear to have shed some light on the
matter. Let us see how.
As mentioned earlier, if the two ions
have spherical electron density around
them, and only electrostatic interactions
are present, the difference map should have
revealed no regions of electron density
depletion or accumulation. Instead, Zuo et
al.1 found a region of electron density
depletion at each copper along the O–Cu–
O axis (see Figure 2) exactly in the shape
of the dz2 orbital found in chemistry text
books! Generation of a hole in this axis is
favourable
and
encourages
better
electrostatic interaction between the
positively charged copper and negatively
charged oxide ions, leading to stabilization
of the lattice.
The question of where the electron
density from copper has been transferred
to and why, needs to be addressed. To
answer these questions we return to the
controversy regarding copper–copper
bonding in cluster complexes. Merz and
Hoffman5 had suggested on the basis of
EHT calculations and symmetry arguments
that there are two ways by which
repulsive interaction between copper(I)
centers are mitigated. One is the escape of
electron density into ligand orbitals having
the right symmetry. A second possibility
is mixing of the copper 3d orbitals with
empty 4s or 4p orbitals which would
release some electron density from the
filled shell and allow for ‘soft’ bonding
between the metal centers. Cuprite adopts
the second option. Due to symmetry
around the copper ion, mixing of the 4s and
the 3dz2 orbitals occurs. A linear
combination of the 3dz2 and 4s orbitals
1132
Figure 1. Perspective view of the unit
cell of cuprite: blue balls are copper ions
at the corners and centers of the faces of
a cube; red balls are oxide ions. Two
faces of the unit cell are marked.
Figure 2. Only one dz2 orbital is shown
for clarity. Tetrahedrally coordinated
oxygen is shown at the center of a cube.
Each copper(I) ion is coordinated to two
oxide ions.
results in reduced electron density along
the z axis. Excess electron density would
be in the other combination pushing the
electron density into a region of space
between the copper ions. What is amazing
is that Zuo et al.1 have located these
regions of excess electron density in the
tetrahedral voids between the copper ions
revealing significant bonding interactions
between the copper centers! In fact, they
have calculated the electron density shared
between copper ions to be as high as 0.22
electrons! It is surprising that they do not
find any distortion of the electron cloud
around oxide ions. Presumably the more
symmetrical tetrahedral arrangement of
copper ions around oxygen has masked the
distortions of the oxide ion electron
density.
Zuo’s experiment is definitely a great
technological achievement. However, it is
the choice of system to study that was
significant. The symmetry around copper
is such that only one of the d orbitals
mixed with a higher lying 4s orbital. This
allowed a clear picture of the d orbital to
emerge. Secondly, it solved a long standing
puzzle about the stability of cuprite lattice
where Cu+ centers were in close proximity
to other Cu+ centers. It confirmed the weak
bonding between copper ions through d + s
mixing. Interestingly, the other possibility
mentioned by Merz and Hoffman5, was
recently verified by Bera and coworkers6
who synthesized a series of complexes
where the ligands controlled the Cu–Cu
distances. Through ab initio calculations,
on model systems, they confirmed the role
of bridging ligands in affecting Cu–Cu
distances and explained the anomalous
variations in trinuclear copper clusters.
Can Zuo’s experiment now be carried
out on more complex molecular systems?
One area where physicists and chemists
want help is with the electronic structure
of superconductors. In this case there are
CuO2 planes. Theory predicts that the
holes are located on the oxygen. At temperatures below Tc , will they be able to see
the holes? Time will tell. Seeing orbitals
and bonds definitely makes one salute
those who dared to postulate them without
being able to see!
1. Zuo, J. M., Kim, M., O’Keeffe, M. and
Spence, J. C. H., Nature, 1999, 401, 49.
2. Coppens, P., X-ray Charge Densities and
Chemical Bonding, Oxford, New York,
1997.
3. Zuo, J. M., Mater. Trans. J. I. M., 1998,
39, 938–946.
4. Poblet, J. M. and Benard, M., Chem.
Commun., 1998, 1179–1180.
5. Merz, K. M. and Hoffman, R., Inorg.
Chem., 1988, 27, 2120–2127.
6. Bera, J. K., Nethaji, M. and Samuelson,
A. G., Inorg. Chem., 1999, 38, 218–228.
ACKNOWLEDGEMENTS. I thank Prof.
K. L. Sebastian for helpful discussions and
Prof. R. Hoffmann for helpful comments.
A. G. Samuelson is in the Department of
Inorganic and Physical Chemistry, Indian
Institute of Science, Bangalore 560 012,
India.
CURRENT SCIENCE, VOL. 77, NO. 9, 10 NOVEMBER 1999
NEWS
CURRENT SCIENCE, VOL. 77, NO. 9, 10 NOVEMBER 1999
1133
RESEARCH NEWS
Regulation of chromosome condensation and sister
chromatid separation
U. C. Lavania, Seshu Lavania and Y. Vimala
To ensure faithful transmission of
genetic information through mitosis
and meiosis, the DNA is
replicated during interphase to
generate a pair of sisters that
remain linked together by a
molecular ‘glue’ to ensure proper
alignment on the mitotic spindle.
The chromosomes condense and
line up on the mitotic spindle, then
the two sisters separate and
migrate to the opposite poles,
followed by cell cleavage yielding
identical daughter cells. The data
generated in the recent years have
provided meaningful insights in
understanding chromosome
condensation1,2, sister chromatid
separation3,4, and elucidation of
large-scale chromosome
organization5. The salient features
relating to organized chromosome segregation
deciphered recently are highlighted
here.
Chromosome condensation
The DNA of eukaryotic
chromosomes must be elaborately
folded to fit within the confines of
the nucleus. As such, the
replicated DNA strands are
systematically compacted during
interphase through fundamental
process of chromosomal
condensation – for each
chromosome this means packing
of about 4 cm of DNA into a rod
10 µm long, and 1 µm in
diameter6. The degree of folding
changes locally through chromatin
remodelling to allow specific
transcription of individual genes
and globally to allow chromosome
segregation during cell cycle. As
cells enter mitosis, chromosome
condensation during
prometaphase resolves the bulk of
each chromatid’s chromatin from
that of its sister7. Early attempts to
elucidate the process of
condensation suggested that there
are major changes in the phosphorylation of histones as cells
enter mitosis8, and one protein
kinase that performs this
phosphorylation (the complex of
Cdc2 and cyclin B) is the principal
biochemical activity that induces
mitosis9. Despite this correlation,
however, the importance of
histone phosphorylation in mitotic chromosome
condensation remained unclear.
Using Xenopus egg extract as the
experimental system that lacks
transcription activity, it has now
been shown that a five-subunit
protein complex dubbed as
‘condensin’ is essential for mitotic
chromosome condensation1.
Condensin converts interphase
chromatin into mitotic-like
chromosomes by reconfiguring
DNA by introducing an ordered
global positive writhe in the
presence of topoisomerase I and
adenosine triphosphate2. Such
CURRENT SCIENCE, VOL. 77, NO. 9, 10 NOVEMBER 1999
knotting requires ATP hydrolysis
and cell cycle-specific
phosphorylation of condensin.
Comparison of the condensin
complex from interphase and
mitotic extracts reveals that three
of its subunits become
phosphorylated in the mitotic
extract and that only the mitotic
form of the complex has the ability
to supercoil DNA. Cdc2 is likely
to be the kinase that phosphorylates and activates condensin
that may trigger mitotic
chromosome condensation; and
depletion of the former would
enable decondensation1,6. Based
on specific study on mitotic
chromosomes assembled in vitro,
it is estimated that there is one unit
of condensin for
5–10 kb of DNA2. Kimura et al.2
further suggest that positive
solenoidal supercoiling is a
mitosis-specific strategy for
chromatin organization.
Sister chromatid separation
One of the most dramatic events
of the eukaryotic cell cycle is the
separation of sister chromatids at
the metaphase-to-anaphase
transition, that otherwise remain
paired along the entire length till
their attachment to the mitotic
spindle. It has long been
suspected that destruction of sister
chromatid cohesion, rather than a
major change in traction exerted
1133
RESEARCH NEWS
by
the spindle, is responsible for
sudden separation of sister
chromatids at the metaphase-toanaphase transition. Cohesion
between sisters resists the pulling
forces exerted by microtubules
attached to sister kinetochores10
and thereby ensures that sister
chromatids attach to microtubules
emanating from opposite spindle
poles11.
There are important clues as to
the molecular nature of the
cohesive structures that hold
sisters together and the mechanism
by which it is suddenly broken at
the onset of anaphase12. Cohesion
between sister chromatids is
established during DNA
replication, and depends on a
multisubunit protein complex
called ‘cohesin’3,13. In the budding
yeast, Saccharomyces
cerevisiae, cohesin is comprised
of at least four subunits: Scc1,
Scc3, Smc1 and Smc3 (refs 13,
14). A similar cohesin complex
has been implicated in sister
chromatid cohesion in Xenopus
extracts7. The SMC (structural
maintenance of chromosomes)
proteins were originally identified
in yeast as key elements of
chromosome segregation and have
since been recognized in a wide
range of organisms15. Attachment
of sister kinetochores to the
mitotic spindle during mitosis
generates forces that would
immediately split sister chromatids
were it not opposed by cohesion.
Cohesion is essential for the
alignment of chromosomes in
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metaphase but must be abolished
for sister separation to start during
anaphase. Uhlman et al.3 have
demonstrated that in the budding
yeast sister chromatid separation
at anaphase onset is promoted by
cleavage of the cohesin subunit
Scc1. The loss of sister chromatid
cohesion in fact depends on a
separating protein (separin) called
Esp1. Using a mutant Scc1 that is
resistant to Esp1-dependent
cleavage and which blocks both
sister chromatid separation and
the dissociation of Scc1 from
chromosomes, it has been shown
that Esp1 causes dissociation of
Scc1 from chromosomes when
sister chromatids separate3. Esp1
may therefore have a direct role in
removing Scc1 from
chromosomes by stimulating its
cleavage by proteolysis. The
evolutionary conservation of separins indicates that the proteolytic
cleavage of cohesion proteins
might be a general mechanism for
triggering anaphase.
Is cleavage of cohesin a cause
or consequence of sister
separation? With the development
of methods to locate specific
sequences in fixed nuclei by
in situ hybridization and
localization of fluorescent binding
proteins, it became possible to
identify proteins – called Pds1 in
budding yeast that had to be
destroyed by the ‘anaphasepromoting complex’ to enable the
sisters to separate16,17. So, two
different kinds of proteolysis are
needed to initiate sister separation.
The first is activation of the
anaphase-promoting complex, the
enzyme which leads to the
wholesome destruction of Pds1.
This, in turn, frees Esp1 to
introduce two surgical snips in
Scc1, thereby destroying the
cohesin complex17. A similar
functional situation has also been
observed for vertebrate sister
chromatid separation as well,
where a protein called ‘securin’
which is analogous to Pds1 in
budding yeast and Cut 2 in fission
yeast has been identified4. An
analysis of related data17 indicates
that changes in the cohesin
subunits are responsible for the
differences between chromosome
segregation in mitosis and meiosis,
and a single change in
chromosomal protein may be
enough to cause the altered
pattern of chromosome
segregation that is responsible for
sexual reproduction.
Further, besides identification of
proteins involved in sister
chromatid cohesion, efforts have
also been made to identify DNA
elements involved in the process.
Using a budding yeast minichromosome centromere assay,
Megee and Koshland18 were able
to identify a centromeric element
CDEIII that was necessary (but
not sufficient) for cohesion,
suggesting that the centromere
cassette contains DNA elements
that mediate sister chromatid
cohesion, although, there may be
another DNA element outside the
cassette that mediates cohesion. Their data,
CURRENT SCIENCE, VOL. 77, NO. 9, 10 NOVEMBER 1999
RESEARCH NEWS
however, do emphasize that at
least in budding yeast, cohesion
and kinetochore activities are
coordinated through a common
sequence element. Their
observations further suggested that
cohesion factors may bind to
chromosomes nonspecifically like
histones or specifically to multiple
sites. In either case, the DNA
elements are functionally
redundant.
1. Kimura, K., Hirano, R., Kobayashi, T.
and Hirano, T., Science, 1998, 282,
487–490.
2. Kimura, K., Rybenkov, V. V., Crisona,
N. J., Hirano, T. and Cozzarelli, N. R.,
Cell, 1999, 98, 239–248.
3. Uhlmann, F., Lottspeich, F. and
Nasmyth, K., Nature, 1999, 400, 37–
42.
4. Zou, H., McGarry, T. J., Bernal, T. and
Kirschner, M. W., Science, 1999, 285,
418–422.
5. Lavania, U. C., Curr. Sci., 1999, 77,
216–218.
6. Murray, A. W., Science, 1998, 282,
425–427.
7. Losada, A., Hirano, M. and Hirano, T.,
Genes Dev., 1998, 12, 1003–1012.
8. Hendzel, M. J., Wei, Y., Mancini, M.
A., Hooser, A. V., Ranalli, T., Brinkley,
B. R., Bazett-Jones, D. P. and Allis,
C. D., Chromosoma, 1997, 106, 348–
360.
9. Morgan, D. O., Nature, 1995, 374,
131–134.
10. Nicklas, R. B., Annu. Rev. Biophys.
Chem., 1998, 17, 431–449.
11. Rieder, C. L. and Salmon, E. D., Trends
Cell Biol., 1998, 8, 310–318.
12. Nasmyth, K., Trends Biochem. Sci.,
1999, 24, 98–104.
13. Toth, A., Ciosk, R., Uhlman, P.,
Galova, M., Schleifer, A. and Nasmyth,
K., Genes Dev., 1999, 13, 320–333.
14. Klein, F., Mahr, P., Galova, M.,
Buonomo, S. B. C., Michaelis, C., Nairz,
K. and Nasmyth, K., Cell, 1999, 98,
91–103.
CURRENT SCIENCE, VOL. 77, NO. 9, 10 NOVEMBER 1999
15. Hirano, T., Genes Dev., 1999, 13, 11–
19.
16. Yamamoto, A., Guacci, V. and
Koshland, V., J. Cell Biol., 1996, 133,
99–110.
17. Murray, A., Nature, 1999, 400, 19–20.
18. Megee, P. C. and Koshland, D., Science,
1999, 285, 254–257.
U. C. Lavania is in the Central
Institute of Medicinal and
Aromatic Plants, Lucknow
226 015, India; Seshu Lavania
is in the Botany Department,
Lucknow University, Lucknow
226 007, India;
and Y. Vimala is in the Botany
Department, Ch. Charan Singh
University,
Meerut 250 005, India.
1135