chromosomes

chromosomes
The nuclei of the body-cells of humans and other complex animals and plants contain a number (fixed for each species—23 in the human case) of pairs of thread-like structures. These carry the genes, the basic units of material inheritance, arranged in a linear sequence. When cells divide to form egg and sperm, chromosomes pair up and then divide in ways which randomly allocate genetic material to the resulting gametes. This process accounts for the inherited diversity among the individual progeny of sexual reproduction.