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Simple Q on chromosomes

ROGER GREEN,MEDICINE,ST.JOHN'S,NF,CAN roger at kean.ucs.mun.ca
Wed Jun 7 14:23:11 EST 1995

> Each chromosome exist as a pair, 23 pairs or 46 chromosomes are found in
> the human genome.  
	Not really. Chromosomes do NOT pair up EXCEPT during formation of 
gametes (sperm and ovum). Normally each of the 46 chromosomes exists quite 
independantly throughout all phases of the cell cycle.

> Chromosome pairs join at the centromere during
> replication, resulting in an X-like appearance.
	Again, chromosomes do NOT form pairs during normal replication.

> 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)
	The DNA sequence in a pair of homologous chromosomes (e.g. the 
two number 6 chromosomes in each cell) is significantly different in many, 
many regions. These include differences (polymorphisms) in repeat-sequence 
regions, but there are also many differnces in genes (protein-coding 
regions). If this were not the case, we would all look the same.
	The DNA in the two chromatids (strands) of a single G2 or M-phase 
chromosome is normally 100% identical in all respects.

> 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)
	This is true.

Roger C. Green,	Faculty of Medicine               Phone: (709)737-6884
Memorial University , St. John's, Newfoundland    FAX  : (709)737-7010

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