Simple Q on chromosomes

Keith Robison robison at mito.harvard.edu
Wed May 24 13:19:47 EST 1995

PB HULBERT (P.B.Hulbert at bradford.ac.uk) 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).
: More biochemical books show  the DNA of the Chromosome as a single 'line'
: pinched in at the 'waist', with marked sites at which endonucleases act.

1) Yes.  There is one continuous DNA molecule per chromosome, but...
2) Those pictures are of replicated chromosomes.  Each of
   the little X-s you see are really two paired chromosomes, as you
   are seeing the results of replication. 

Keith Robison
Harvard University
Department of Cellular and Developmental Biology
Department of Genetics / HHMI

robison at mito.harvard.edu 

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