87 3.2 The nature of chromosomes complete set of 23 human chromosomes, there are ap-proximately 850 G-dark bands visible during a stage of mitosis called metaphase , which is just before the chro-mosome pairs are pulled apart. These bands have pro-vided a useful way of subdividing the various regions of chromosomes, and each band has been assigned a spe-ciﬁc number. The difference between dark- and light-staining re-gions was formerly believed to be caused by differences in the relative proportions of bases: the G-light bands were thought to be relatively GC-rich, and the G-dark bands AT-rich. However, it is now thought that the dif-ferences are too small to account for banding patterns. The crucial factor again appears to be chromatin pack-ing density: the G-dark regions are packed more densely, with tighter coils. Thus there is a higher density of DNA to take up the stain. In addition, G bands have been correlated to vari-ous other properties. For example, nucleotide-labeling
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.