Each chromosome consists of a DNA double helix bearing a linear sequence of genes, coiled and recoiled around aggregated proteins (histones). Their number varies from species to species: humans have 22 pairs plus the two sex chromosomes (two X chromosomes in females, one X and one Y in males). During cell division, each DNA strand is duplicated, and the chromosomes condense to become visible as distinct pairs of chromatids joined at the centromere. Bacteria and viruses lack a nucleus and have a single chromosome without histones.
- Ring chromosomes, chromatid exchanges and polyploidies were also observed at some treatments.
- The sizes of the two centromeres from the homologous chromosomes are different.
- The sex chromosomes are placed to the right of the smallest autosomal chromosomes.
Late 19th century: coined in German from Greek khrōma 'color' + sōma 'body'.
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