Cell Reproduction: Mitosis and Meiosis

Mitosis and meiosis are two ways that cells reproduce.

By mitosis a cell splits to create two identical copies of the original cell.

In meiosis cells split to form new cells with half the usual number of chromosomes, to produce gametes for sexual reproduction.

DNA is a very large molecule shaped like a twisted ladder. The shape is a double helix.Long strands of DNA make up chromosomes. These are found in the nucleus of a cell.

Mitosis is a type of cell division. Mitosis occurs wherever more cells are needed. It produces two new cells that are identical to each other, and to the parent cell. The process of growth and division is called the cell cycle.

The cycle starts as the number of organelles - the different parts of the cell - increases. This is to ensure that each of the two new cells receives copies of all the organelles.

Before a cell divides, its chromosomes are copied exactly. The DNA molecule is made of two strands. As each of the two strands separate, new strands are made alongside each of them, thereby making two new copies.

Meiosis is a different kind of cell division. It is used to produce male and femalegametes. A human body cell contains 46 chromosomes arranged in 23 pairs. The gametes are sperm or eggs, and only contain half as many chromosomes(23). This is why meiosis is sometimes called reduction division.

At fertilisation, the nuclei of the sperm and an egg join to form the zygote. The zygote contains 23 pairs of chromosomes - 23 single chromosomes from thesperm, and 23 single chromosomes from the egg, thereby creating the correct number of 46 chromosomes for all body cells. It also means the zygote contains a complete set of chromosomes from each parent.