genome-size

Christian Moews Christian.Moews at darmstadt.netsurf.de
Thu Mar 14 17:53:48 EST 1996


Allthough I did not follow this discussion from the beginning, I would
like to ad my personal view, acknowledging the possible risk of
redundancies:

It is tempting to  correlate genome-size with the complexity of an
organism, because at the first sight, genome seems to equal
information.
This is true, but only if one looks on this problem with the eyes of
an computational scientist (this is not ment as an insult). The
crucial point is whether or not this information is primarily used by
the organism of question (transcribed by RNA-polymerase to get
proteins or structural RNA etc.) or if this DNA-information can serve
for other tasks.

Just to mention some, here are just two points:
1) excess DNA can serve as squelching agent for integration of
retroviral DNA. The smaller the genome, the higher is the probability
of a retroviral genome to interrupt an exon of the host-cell.
2)excess DNA can be considered as a library of genetic information.
This information is permanently reorganized, also under the influence
of retroviral transposition (retrotransposons are considered to be
inoparative, rudimentary retroviruses). This can help an organism to
adapt more quickly to environmental changes.

With these two considerations one can easily understand, why
single-celled protozoans could have rather high genome size, which is
solely limitted by the physiological effort it takes to duplicate DNA
during mitosis. E. coli might be a good example for the latter fact
(aroung 4x10^6 bp).

So, the size of a genome might reflect simply a ratio between the
evolutionary beneftis mentioned above and the availability of
resources. Higher organisms (like the amniotes) spend a tremendous
amount of energy on ecologicaly derived, inter- and intraspecific
interactions like looking for mating partners etc., so that resources
will hardly limit genome size. The low variation found in these
classes might reflect a upper limit which does still allow efficient
and secure replication, even in rapidly deviding tissues (There is a
high evolutionary presure because failure in cell-cycle control might
kill the entire organism after a short period of time).

To come to a conclusion, one should look on the amount of proteins a
organism needs to survive when judging about complexity. A difficult
task when you look on the results of knock-out studies in various
species which reveal high redundancy also at this point.

Chris

Christian.Moews at darmstadt.netsurf.de




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