Chromosomes are structures within cells that carry DNA, RNA, and proteins. Each chromosome is DNA tightly wound around proteins that support its structure. Chromosomes are not visible in the cell unless the cell is dividing and much of the knowledge concerning chromosomes is learned by observing cells during division. 

In humans, chromosomes are classified in two ways: autosomes and sex chromosomes. Autosomes carry all genetic, hereditary information, except for certain genetic traits related to a person's sex, which are carried in the sex chromosomes. Each human cell has 23 pairs of chromosomes (twenty two autosomes and one sex chromosome).