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division, the two daughter cells and the parent all are identical in chromosomal content, and, with a few exceptions, all the cells in the body should have chromosomes identical with those of the fertilized egg. The process sometimes errs: some cells have too many or too few chromosomes, and some differentiated tissues (such as liver) might have some cells with a different chromosome number (Therman and Susman 1993). But for the most part, cells throughout the body are identical in chromosomal composition.2 The most important exception occurs in the development of the reproductive cells. During formation of sperms and eggs, the process of reduction division (meiosis)a chromosomal duplication followed by two cell divisions—halves the number of chromosomes from 46 to 23. Thus, sperms and eggs have only one member of each chromosome pair. The double number, 46, is restored by fertilization. A cell (or organism) with two sets of chromosomes is diploid. A cell, such as an egg, with one set is haploid.

Chromosomes vary greatly in size, but the two members of a homologous pair (one maternal and one paternal) are identical in microscopic appearance, except for the sex-chromosome pair, X and Y, in which the male-determining Y is much smaller than the X. A set of 23 chromosomes with the genetic information they contain is termed the genome.

A chromosome is a very thin thread of DNA, surrounded by other materials, mainly protein. If straightened out, an average chromosome would be an inch or more long. But it is arranged as coils within coils and so can be packed into a cell only a thousandth of an inch in diameter. The DNA thread is not visible in an ordinary microscope, and a stained chromosome is more rod-like than thread-like during the mitotic stages when it is most visible.

The DNA thread is actually double—two strands coiled around each other like a twisted rope ladder with stiff wooden steps (Figure 2. 1). The basic chemical unit of DNA is the nucleotide, consisting of a base (a half-step in the ladder) and a sugar-phosphate complex (the adjacent section of the rope). There are four kinds of bases, designated A, G, T, and C; A stands for adenine, G for guanine, T for thymine, and C for cytosine. The nucleotides of one DNA strand pair up in a specific fashion with those of the other to form the ladder; because of their specific size and complementary shape, T always pairs with A, and G with C. A DNA strand has a chemical directionality that is defined by the antisymmetry of the chemical connections between the successive sugars and phosphates in the two strands. In double-stranded DNA, the two strands run in opposite directions.

Because of the pairing rule just described, if we know the sequence of nucleotides on one strand, we automatically know the sequence on the other strand. A short segment of double-stranded DNA is shown below; the arrows indicate opposite directionality of the two strands.

2 More important for our purpose, tissues with different numbers of chromosomes (except for some malignancies) have the same DNA content as diploid cells.

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