I teach introductory biology at community colleges. In teaching DNA replication,
I have been bothered for some time regarding some specifics. First, I know that centromeres
are the constricted regions on each chromosome and have researched the reason for this.
What I found seems to indicated that these regions are more tightly condensed/wound than
other regions of the chromosome (heterochromatin?). I could not find any good explanation
for why the regions are more tightly wound. Is there more extensive hydrogen bonding in these
areas? Also, if it remains tightly condensed, how are these regions copied during DNA replication?
It is my understanding that tightly condensed DNA is not available to enzymes that do many of
these processes, including transcription and translation (referring to barr bodies). I may be missing
a glaring point here, but this is a niggling question that has bothered me for years. No textbook I've
encountered seems to address it and, being a generalist myself I lack expertise in this specific area.
Can you clear this up for me?
In order for DNA to be able to fit inside a tiny little nucleus it must be wound up into a
ball. But then it has to be able to be unwound without breaking it. Our DNA is wrapped
around histone proteins which allows this orderly winding. Recent research has found that
DNA that is more tightly wound is highly methylated, and is wrapped tightly around
histones. The methyl groups prevent polymerases from reading the sequence and therefore
those genes aren't expressed. Methyl groups are usually attached to a cytosine nucleotide
and are a physical block around that sequence. Genes that "turn off" in development are
methylated and are never read again. Some genes can be unwound from their histones in
order to be read, others can't. So, it makes sense to me that the DNA sequences that
make up the centromeric (is that a word?) region would not be read and therefore would
remain as heterochromatin.
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Update: June 2012