The human body

How are sex cells made (meiosis)?

The formation of sex cells is a central part of human reproduction: In fertilization, an egg cell and a sperm cell combine. These sex cells are also called reproductive cells or gametes. Sperm cells are produced in men's testicles and egg cells are produced in women's ovaries. Sex cells are different from other cells in one special way: Put simply, they only have one half of the total amount of human genetic information. When a sperm cell fertilizes an egg cell, the resulting cell has a full set of genetic information again.

Sex cells are formed through a particular kind of cell division called meiosis. Unlike in normal cell division (mitosis), the genetic material of the original (parent) cell is divided up twice.

Illustration: The nucleus of the original cell contains the full set of genetic information As in most other cells in the body, the nucleus of the original cell contains the person's full set of genetic information. This information is stored in the form of long threads (strands) of DNA – in the chromosomes.
Illustration: Before the original cell divides, the genetic material is copied Before the original cell divides the first time, all of the genetic material is replicated: One copy is made of each chromosome.
Illustration: The DNA strands condense and become visible The DNA strands become more tightly packed into a condensed form and can be seen under a microscope. Each chromosome and its copy remain attached in one place.

As in many other living things, chromosomes in humans always come in pairs.

Illustration: The pairs of chromosomes line up along the center of the cell Unlike in “normal” cell division (mitosis), in meiosis the chromosome pairs first line up along the center of the cell.
Illustration: The chromosome pairs are separated and move to opposite sides of the cell The pairs separate there. The individual chromosomes then move to opposite sides of the cell, together with their attached copy. The cell membrane starts pinching inward to form two separate cells.
Illustration: During the second cell division, the copies of the chromosomes separate A second cell division follows: The copies of the chromosomes separate.
Illustration: Four sex cells develop from the parent cell So the parent (original) cell gives rise to four sex cells. The nucleus of each sex cell contains half of the original genetic material.
Illustration: In men, the four sex cells develop into sperm cells In men, these cells develop into sperm cells.
Illustration: In women, only one egg cell is formed during meiosis In women, only one of the four sex cells becomes an egg cell that can be fertilized. During meiosis, it receives most of the cell body of the parent cell. The three smaller cells – referred to as polar bodies – break down and disappear.
Illustration: One egg cell and one sperm cell combine during fertilization If a sperm cell fertilizes an egg cell, the nuclei of the two cells combine.
Grafik: Eine neue Zelle mit vollständigem Erbgut ist entstanden A new cell with a complete set of genetic information is produced – one half is from the mother, and the other half is from the father. The fertilized egg cell can now start developing into a baby.