The process of growth requires body cells to divide and multiply constantly. Cells also divide to replace those that have become worn out. When a cell divides, its genetic material is copied. This type of cell division is called mitosis. A slightly different process of cell division, called meiosis, results in the production of egg and sperm cells. In this process, the resulting cells have only one of each pair of chromosomes and the maternal and paternal genes have been reassorted to create a new mix of genetic information.
Before a cell can divide to make new body cells or egg and sperm cells, the DNA in the cell must be copied. Each of the two strands in the original DNA acts as a template against which two new strands are built.
The original DNA double helix splits open at several points along its length. This process produces areas where there are two separate single strands.
New free bases (units of DNA) are attached to both of the single strands of DNA. The order in which the bases join to the single DNA strands is determined by the DNA bases that are already present on the single strand.
While the bases attach to the strand, each of the two newly formed double strands start to twist. The process continues along the whole length of the DNA, eventually producing two identical double DNA strands.
When body cells divide, their genetic material has to be duplicated so that each new cell has a complete set of genes. This process of division is called mitosis and results in cells identical to the original cell. In the diagram here, only four chromosomes are shown for simplicity.
The DNA in each chromosome is copied to form two identical copies of each chromosome joined in the centre by the centromere.
The membrane around the nucleus breaks down and threads form across the cell. The chromosomes line up on the threads.
The duplicated chromosomes are pulled apart by the threads. The single chromosomes move to opposite sides of the cell.
A nuclear membrane forms around each set of chromosomes. The cell begins to divide into two new cells.
Two new cells form. Each cell has a central nucleus containing an identical set of chromosomes.
Sperm cells in males and egg cells in females are produced by a form of cell division called meiosis. In this process, the amount of genetic material in the new cells is halved during two stages of cell division, so that a complete set of genes is obtained when an egg and sperm fuse. During meiosis, the two chromosomes of a particular pair exchange genetic material from one to the other. Each of the resulting egg or sperm cells then has a slightly different mixture of genes from the chromosomes of the original pair.
DNA in the chromosomes is duplicated to form X-shaped double chromosomes. Each of these is joined in the centre by a structure called a centromere.
The membrane around the nucleus disappears. Matching chromosomes pair and usually exchange genetic material.
Each of the duplicated chromosomes now has a mixture of genetic material. Threads form in the cell to pull the pairs of chromosomes apart.
The cell divides to produce two new cells. Each new cell has a full set of 23 duplicated chromosomes from the original cell.
The duplicated chromosomes line up. More threads attach to each chromosome. Each duplicated chromosome is pulled apart to form two single chromosomes.
The two cells divide to produce four cells from the original single cell, each with half the amount of genetic material. Each cell has a slightly different combination of the genes that were on the chromosomes that paired at the start of the process.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.