parkerbr at cc.uvcc.edu parkerbr at cc.uvcc.edu
Tue Feb 8 19:16:15 EST 1994

Follow-up to the responses on the definition of a chromosome:

The problem still remains that a new term seems to be appropriate to apply to
the "X-shaped" structure seen in metaphase. Too many are still confused by the fact
that this structure actually contains two pieces of dsDNA. We cannot even say
that it is okay to call the structure a chromosome since the two chromatids are
identical... they aren't in meiosis. A furher confusion arises due to the use
of the term chromosome to indicate genetic loci and markers in the human genome 
project.  Lewin's latest edition of Genes V defines a chromosome as "a discrete
unit of the genome carrying many genes. Each chromosome consists of a very long
molecular of duplex DNA.." My point is still that students leave biology with
the mistaken notion that the chromosome lives in resting cells as that X-shape
and that the sister chromatids are the homolog pairs.

I normally call the X-shaped structure somthing totally unrelated to DNA in my
non-majors courses---like RALPH. And stick to the definition of a chromosome as
the discrete piece of DNA. Since the non-condensed form of DNA is the form used
to make proteins, etc. this is much more useful, and less confusing. It is hard
for a student to grasp that difference when their textbooks will refer to the
X-shape as a chromosome on one page, and the rest of the book treats a
chromosome as a single DNA strand.

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