VMBO Year 3 · Organs and cells · Section 1.6
Reduction division: meiosis
What is reduction division?
Sexual reproduction involves sex cells, or gametes: sperm cells and egg cells. These develop through a special type of cell division called meiosis, or reduction division. The chromosome number is halved. Most human body cells contain 46 chromosomes, whereas a human sex cell has 23. During fertilisation, a sperm nucleus and an egg nucleus combine, restoring the total to 46 chromosomes.
How does meiosis work?
DNA is copied once, followed by two divisions. In the first division, the two members of each chromosome pair separate into different cells. In the second, sister chromatids separate. Four cells with half the original chromosome number result. In sperm production these can develop into four sperm cells. In egg production, division of the cytoplasm is unequal: usually one large egg cell and smaller polar bodies are formed.
Differences between sperm and egg cells
A sperm cell is small and has a tail that enables it to swim. An egg cell is large and cannot swim by itself. It is moved along the oviduct by cilia and muscular contractions. These differences suit the cells' roles in reproduction.
How is chromosomal sex determined?
In the usual XX/XY pattern, each egg carries an X chromosome, and each sperm carries either an X or a Y chromosome. An X-bearing sperm produces an XX combination, usually associated with female development. A Y-bearing sperm produces XY, usually associated with male development. The sperm therefore determines this chromosome combination at fertilisation. Variations in chromosomes and sexual development also occur.
Glossary
- Egg cells
- Large female reproductive cells that cannot swim by themselves.
- Sex cells
- Reproductive cells with one chromosome from each pair, also called gametes.
- Sex chromosomes
- Chromosomes involved in sexual development; the usual human combinations are XX and XY.
- Meiosis
- Two successive divisions following one round of DNA replication, halving the chromosome number for sexual reproduction.
- Reduction division
- Another name for meiosis, in which chromosome pairs separate and cells receive one chromosome from each pair.
- Sperm cells
- Small male reproductive cells that swim using a tail.
- X chromosome
- A sex chromosome present in both usual XX and XY combinations.
- XX
- The usual sex-chromosome combination associated with female development.
- XY
- The usual sex-chromosome combination associated with male development.
- Y chromosome
- A sex chromosome usually involved in triggering male development when present with an X chromosome.