How are sex cells made (meiosis)?

The formation of sex cells (meiosis) is a central part of human reproduction: The process of meiosis results in the formation of egg cells and sperm cells. 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 and an egg cell combine, the resulting cell has a full set of genetic information again. That is how each parent passes on half of their own genetic material to their children, making meiosis a special type of cell division in which the genetic material of a parent cell is first duplicated, and then divided twice. This is the only way the body can produce egg cells and sperm cells with half a set of chromosomes:

	The nucleus of the original cell contains the full set of genetic information: 46 chromosomes.
	Before the original cell divides the first time, all of the genetic material is replicated: One copy is made of each chromosome.
	The DNA strands become more tightly packed into a condensed form and each chromosome and its copy remain closely attached.

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

	Unlike in “normal” cell division (mitosis), in meiosis the chromosome pairs first line up along the center 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.
	The cells then divide again and the copies of the chromosomes separate.
	So the parent cell ultimately gives rise to four sex cells. The nucleus of each sex cell contains half of the original genetic material in 23 chromosomes.
	In men, these cells develop into sperm cells.
	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.
	When a sperm cell and an egg cell meet, the two cells combine and their cell nuclei fuse.
	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. This cell can now start developing into a baby.

Alberts B. Lehrbuch der Molekularen Zellbiologie. Weinheim: Wiley-VCH; 2021.

Brandes R, Lang F, Schmidt R. Physiologie des Menschen: mit Pathophysiologie. Berlin: Springer; 2019.

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How are sex cells made (meiosis)?

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Updated on November 5, 2026

Next planned update: 2029

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Institute for Quality and Efficiency in Health Care (IQWiG, Germany)

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