The mixture of a single (but double stranded) long string of genetic material (DNA) wound around supporting proteins. There are 46 chromosomes (23 pairs) in every normal human cell (other than the germ cells). Each cell therefore contains all the genetic information needed to make a human being. But itís only in the first few days of the embryo that all of a cellís DNA is accessible: once cells differentiate to have special purposes only the DNA they need remains unmasked. Chromosomes are located in the cellís nucleus and come in pairs, so that each cell has two alleles of each gene. See also chromosomal cross-over.
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