A chromosome consists of a very long molecule of DNA with associated proteins found in the nucleus of the cell. There are 46 chromosomes in a human diploid cell bearing the normal complement of chromosomes, made up of 22 pairs of autosomes (numbered 1 to 22) and two sex chromosomes (either XX or XY). Many genes are found on a given chromosome. For autosomal genes (i.e. those residing on chromosomes 1 to 22), a given cell has two copies of each gene, one on each pair of auto-somes. Chromosomes constitute a discrete unit of the genome and vary in size. Chromosomes become visible on microscopy as distinct nuclear bodies during cell division (mitosis) when they are highly condensed, notably during metaphase (the point in mitosis when the condensed chromosomes align with each other) when particular banding patterns can be seen after staining with specific dyes (Fig. 1.2). During metaphase the centromere is visible as a constriction in the chromosome, seen as the point where the two sister chromatids are held together. The centromere is pivotal to the process of cell division and control. 'Chromatids' refers to each of the two copies of a replicated chromosome during the process of cell division (mitosis or mei-osis), specifically at the time they are joined at the centromere; on separation (anaphase) the two are said to be 'daughter chromosomes'. The nature of chromosomes and the structural genetic variation that can be seen at a microscopic level are explored in Chapter 3.
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