chromosome

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