WHAT IS A CHROMOSOME

[Barbara E. Liedl]

Why not use chromatid?  That is the correct term.  Below is the definition 
in "Glossary of Genetics", 5th edition, R. Rieger, A. Michaelis, M.M. Green

Chromatid - one of the two visibly distinct longitudinal subunits of all 
reduplicated chromosomes (a half-chromosome) which become visible between 
early prophase and metaphase of mitosis and between diplotene and the second 
metaphase of meiosis.  After these stages the chromatids are known as "
daughter chromosomes".  The chromatids are those longitudinal units of the 
chromosome which become separated during anaphase of mitosis and anaphase II 
of meiosis.  Sister chromatids are the two chromatids derived from one and 
the same chromosome during its replication in interphase as opposed to 
nonsister chromatids derived from homologous chromosomes.  Normally, 
chromatids of homologous chromosomes carry identical genetic information (
identical linkage groups), apart from allelic differences between nonsister 
chromatids and are similar morphologically.  Each chromatid contains a 
continuous DNA double helix.

If you teach genetics, I strongly urge you to get a copy of this glossary 
and/or another one (Dictionary of Genetics, R.C. King and W.D. Stansfield).  
I use both ALL the time for research and class preparation.  The Glossary 
helped me survive a killer cytogenetics course I had my first year in grad 
school and will be a lifetime companion.

I am intrigued with the idea of not teaching the stage names until they 
understand what is happening during the two processes.  Unfortunately, I 
just finished going the standard mitosis and meiosis lectures and lab for 
the Plant Genetics course I teach.  During the lab (3 hours) we had them 
looking at prepared slides and at the end taught them how to do mitotic 
squashes on garlic root tips.  Overall report from the students was 
they now appreciate the prepared slides.  In addition, they were really 
proud of themselves when they found any of the stages they knew in their own 
slide.   

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.  Explaining x (ploidy) isn't too bad.  But n and C 
content thru the cell cycle always is a problem for the students.  Lately 
we tell them to draw what the chromosomes are doing during the cell cycle 
and count the centromeres to get the n # and count the chromatids  
(replicated chromosomes) to get a C #.  Any ideas?

Barbara Liedl
Cornell University
Dept. of Plant Breeding

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"And he gave it for his opinion, that whoever could make two ears of corn 
or two blades of grass to grow upon a spot of ground where only one grew 
before, would deserve better of mankind, and do more essential service to 
his country than the whole race of politicans put together." 

Jonathan Swift in "Gulliver's Travels"
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