Simple Q on chromosomes

David Curtis dcurtis at hgmp.mrc.ac.uk
Thu May 25 06:37:10 EST 1995

In article <3pvtc3$hns at decaxp.harvard.edu> robison at mito.harvard.edu (Keith 
Robison) writes:

>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. 

Slightly fuller reply (hoping he doesn't get flamed for making mistake but 
obvious now that people who really know about chromosomes aren't going to 
bother answering):

1) Yes, one continuous length of DNA in a chromosome. Actually, it's 
double-stranded so talking about a strand is slightly confusing.

2) The chromosomes replicate prior to cell division to form the characteristic 
X shapes. I think the correct terminology is that they form sister chromatids 
joined at the centromere. So the bit above the centromere replicates to form 
the two short arms of the chromosome, and the bit below forms the two long 
arms of the chromosome. Unless you have the chromosome upside down. Pairs of 
chromosomes are different - except for sex chromosomes in males, there are two 
of each "type" of chromosome in a cell - two chromosome 1's, two chromosome 
2's, etc. These are called homologous pairs. They join up and swap bits (that 
dreaded word about which we should have a whole newsgroup, Recombination) 
during meiosis, which is the process leading to gamete formation.

There's a pretty crude diagram available via my home page (chase teaching, 
lecture notes, etc) but there's also a pointer round there to a more detailed 
document called the Molecular Genetics Primer which might be more help.

Disclaimer: I'm only a psychiatrist.

Dave Curtis (dcurtis at hgmp.mrc.ac.uk)

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