Thread-like structures in the nuclei of cells. Chromosomes carry inherited information in the form of genes, which govern all cell activity and function. Each chromosome contains up to several thousand genes arranged in single file along a long double filament of DNA. The sequence of chemical units, or bases, in the DNA provides the coded instructions for cellular activities. All an individual’s body cells (with the exception of egg or sperm cells) carry precisely the same chromosomal material copied by a process of cell division from the original material in the fertilized egg. Each human cell normally contains 46 chromosomes made up of 23 pairs. Half of each pair is of maternal and half of paternal origin. 22 pairs are autosomal chromosomes, which are the same in both sexes; the remaining pair is made up of 2 sex chromosomes. In females, the sex chromosomes are a pair of X chromosomes. In males, 1 is an X chromosome and the other is a Y chromosome. One sex chromosome (an X) originates from the mother’s egg and the other (an X in girls; a Y in boys) from the father’s sperm.


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