Book : Molecular Biology
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Posted by: CHELSEA
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Normal Chromosomes

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Overview

Each species has a normal diploid number of chromosomes. Cytogenetically normal humans, for example, have 46 chromosomes (44 autosomes and two sex chromosomes). Cattle, on the other hand, have 60 chromosomes. This ratio is an important parameter for chromosome identification, and also, the ratio of lengths of the two arms allows classification of chromosomes into several basic morphologic types. Germ cells (egg and sperm) have 23 chromosomes: one copy of each autosome plus a single sex chromosome. This is referred to as the haploid number. One chromosome from each autosomal pair plus one sex chromosome is inherited from each parent. Mothers can contribute only an X chromosome to their children while fathers can contribute either an X or a Y. Cytogenetic analyses are almost always based on examination of chromosomes fixed during mitotic metaphase. During that phase of the cell cycle, DNA has been replicated and the chromatin is highly condensed. The two daughter DNAs are encased in chromosomal proteins forming sister chromatids, which are held together at their centromere. Metaphase chromosomes differ from one another in size and shape, and the absolute length of any one chromosome varies depending on the stage of mitosis in which it was fixed. However, the relative position of the centromere is constant, which means that that the ratio of the lengths of the two arms is constant for each chromosome. Centromere position and arm ratios can assist in identifying specific pairs of chromosomes, but inevitably several or many pairs of chromosomes appear identical by these criteria. The ability to identify specific chromosomes with certainty was revolutionized by discovery that certain dyes would produce reproducible patterns of bands when used to stain chromosomes. Chromosome banding has since become a standard and indispensable tool for cytogenetic analysis, and several banding techniques have been developed: • Q banding: chromosomes are stained with a fluorescent dye such as quinacrine • G banding: produced by staining with Giemsa after digesting the chromosomes with trypsin • C banding: chromosomes are treated with acid and base, then stained with Giesma stain Each of these techniques produces a pattern of dark and light (or fluorescent versus nonfluorescent) bands along the length of the chromosomes. Importantly, each chromosome displays a unique banding pattern, analogous to a "bar code", which allows it to be reliably differentiated from other chromosomes of the same size and centromeric position.



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