PB Hulbert wrote
>Is there just one strand (extremely long) of DNA in a single human Chromosome?
>I ask because some books show a chromosome as a structure with 4 'arms' -
>2 short and 2 long - with bands which can be shown up by biological stains.
>These pictures seem to show symmetrical staining on the 2 similar arms eg
>on the long arms. Does that mean that the 2 arms are completely identical
>at the chemical sequence level? If so, are there 2 stands of DNA in each
>chromosome? (one in each arm).
Each chromosome exist as a pair, 23 pairs or 46 chromosomes are found in
the human genome. Chromosome pairs join at the centromere during
replication, resulting in an X-like appearance. The DNA contained in the
chromosome pairs is essentially identical, ie same genes in the same
position. However repetitive regions exist which may vary in length (these
variable regions are used in forensics for DNA fingerprinting).
Keep in mind that each strand of DNA exists as a double helix, consisting
of two complimentary strands (with A paired with T, and G paired with C)
Department of Cytogenetics and Molecular Genetics /\_/\
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Email rwallace at mad.adelaide.edu.au