WHAT IS A CHROMOSOME
Barbara E. Liedl
bl14 at cornell.edu
Fri Feb 4 01:42:58 EST 1994
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
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?
Dept. of Plant Breeding
"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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