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.
|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.|
|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 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.
|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.|
|A second cell division follows: The copies of the chromosomes separate.|
|So the parent (original) cell gives rise to four sex cells. The nucleus of each sex cell contains half of the original genetic material.|
|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.|
|If a sperm cell fertilizes an egg cell, the nuclei of the two cells combine.|
|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.|
Menche N (Ed). Biologie Anatomie Physiologie. Munich: Urban und Fischer; 2016.
Pschyrembel. Klinisches Wörterbuch. Berlin: De Gruyter; 2017.
Schmidt R, Lang F, Heckmann M. Physiologie des Menschen: mit Pathophysiologie. Berlin: Springer; 2017.
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