WHAT IS A CHROMOSOME

Mark D. Garfinkel garfinkl at iitmax.iit.edu
Mon Feb 7 19:49:06 EST 1994


In article <1994Feb7.135013.16931 at alw.nih.gov>
Jim Owens <jow at helix.nih.gov> writes:

>My biology degree is vintage 1967.  [...]  Has it been proved that
>eucaryotic chromatids have just one very, very long molecule of DNA?  The
>last I read, in 1978, said this was _assumed_ to be the case, but it was
>not proven.  It was considered possible that there could be a series of
>long or very long DNA molecules in a chromatid.

        Taylor did a radioisotope tracer experiment in the 1960s that,
I thought, demonstrated that each higher eukaryotic chromosome contains
a single dsDNA molecule, based on the segregation of label through
meiosis, probably oogenesis in particular. Anyone able to elaborate?

	Then there are mammalian somatic cell genetic experiments
from the 1970s, probably. The observed segregation of DNA labels
during sister chromatid exchange would only make sense if there was
one linear dsDNA per chromatid.

        As for more recent experiments, probably the best are based
upon a series of gel electrophoresis techniques developed in the 1980s
that are able to separate very large linear dsDNA molecules (100-2000
kilobases) according to size. In a number of lower eukaryotes, yeasts
& fungi, it is possible to make a one-to-one correspondence between
these large nuclear dsDNAs on the gels and genetic linkage groups (i.e.,
in Saccharomyces cerevisiae there are 16 such DNA species and 16 linkage
groups). The yeasts in question behave as perfectly good Mendelian
genetic organisms, with both haploid & diploid cell types. Physically
smaller DNA molecules correspond to genetically smaller linkage groups,
which you would expect. Nuclear DNA mass per haploid cell vs. DNA mass
per diploid cell, measured by independent means, also add up the way you
would expect if one copy of each of the 16 DNA molecules were present in
each haploid cell, and two copies of each in every diploid cell. It seems
pretty convincing to me, based on biochemical & genetic grounds that
these large dsDNA molecules are the yeast's chromosomes.

	[Oh, and if Saccharomyces cerevisiae *really* has 17 linkage
groups & 17 bands on a CHEF gel... Noted!]

-- 
Mark D. Garfinkel (e-mail: garfinkl at iitmax.acc.iit.edu)
My views are my own, which is why they're copyright 1994



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