Sex chromosomes. Computer illustration of human X (left) and Y (right) chromosomes. Chromosomes are composed of deoxyribonucleic acid (DNA) and contain sections called genes that encode the body's genetic information. Each chromosome consists of two chromatids joined at their centres by the centromere, which is involved in cell division. There are 22 pairs of chromosomes and a gender determining pair. In men, this comprises one X and one Y chromosome. In women, it is two X chromosomes. The Y chromosome carries genetic information responsible for the development of male characteristics.