WHAT IS A CHROMOSOME

Mark D. Garfinkel garfinkl at iitmax.iit.edu
Fri Feb 4 09:36:02 EST 1994


In article <bl14.23.0 at cornell.edu>
bl14 at cornell.edu (Barbara E. Liedl) writes:
>
>Why not use chromatid?  That is the correct term. [...]
	Yes. Or "sister chromatids," which is the way I remember textbooks
treating the post-replication condensed chromosome.

>I am intrigued with the idea of not teaching the stage names until they
>understand what is happening during the two processes.  [inter/pro/meta/ana]
	Another idea is to introduce molecular concepts of the cell cycle,
which subdivide interphase into G1, S, G2. First, it breaks the mystique
about "resting cells," second, it provides a referent to when DNA
replication occurs.

>I am curious if any one has a great way to explain the difference between n,
>x and C and then relate this to the cell cycle.  These terms are used as
>follows: tomato has 2n=2x=24 chromosomes and a somatic cell has a
>DNA content of 2C. [...]
	I learned n as the number of chromosomes in a haploid set, and C
as the DNA *mass* (measured in kilodaltons or picograms, or in base-pairs)
of that haploid set. For a diploid cell, therefore, during the G1-stage
there are 2n chromosomes and 2C DNA content. During S phase, C-value
continuously varies, from 2C to 4C, as replication forks operate. This
continuous variation in DNA mass/cell is readily seen using fluorescence-
activated cell sorting of synchronized cell populations. During G2-phase
a diploid cell has 4n chromosomes, but they form 2n sets of sister
chromatids that are presumably still joined at their centromeres. It is
during M phase, prophase in particular, when the chromatin condenses,
that the LM-visible morphology becomes evident and the cytological dance
of the chromosomes begin.

	The above is my take on the subject & how to present it.
-- 
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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