Chromosomes and DNA

Chromosomes carry genetic information in a molecule called DNA.

A type of cell division called mitosis ensures that when a cell divides each new cell produced has the same genetic information.

DNA exists in a cell's nucleus within structures called chromosomes. Each section of a chromosome that contains the code for the production of a particular protein is called a gene.

Each chromosome is made from a single molecule of DNA, but when a cell is ready to divide, the DNA copies itself, then coils and condenses to form the chromosomes that we see in micrographs.

Each human body cell contains 46 chromosomes. These can be arranged into 23 pairs.

Each chromosome in a pair carries the same types of genes. The 23rd pair are the sex chromosomes:

• In females, the two chromosomes are identical in shape. There are two X chromosomes. Females are referred to as XX.
• In males, one of the chromosomes is a different in shape. There is an X and also a Y chromosome. Males are referred to as XY