CoachA chromosome is one DNA molecule with its packing proteins. Between divisions it is a loose, invisible thread; at the start of mitosis it coils into a compact rod visible under the light microscope. After replication each chromosome consists of two identical chromatids, joined at the centromere: the familiar X shape. The karyotype of a species is its complete set of chromosomes, photographed at metaphase and arranged in pairs by size: a human cell has 46, in 23 pairs, the members of a pair being homologous — same genes at the same positions, one from each parent.
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Example 12.8 (Human numbers)
A human cell in metaphase holds 46 chromosomes and 92 chromatids; in anaphase, 92 chromosomes, 46 moving to each pole; each daughter, 46 chromosomes of one chromatid each, containing exactly the DNA the mother had at the start of its cycle.