Meiosis

Key definition

• Meiosis is a reduction division in which the chromosome number is halved from diploid to haploid resulting in genetically different cells.
  • Remember that diploid is the total count of chromosomes and haploid is only half the diploid number. It is also the number of chromosomes in a single complete set, or also the number of chromosomes in the gametes.

Meiosis in sexual reproduction

• If human sperm cells and egg cells both had 46 chromosomes, the zygote formed at fertilization would have 92 chromosomes.
  • If this continued generation after generation, the number of chromosomes in the nuclei would double every generation. But, the number remains at 46 chromosomes.
• The number remains at 46 because there is a different sort of nuclear division in the life-cycle in the production of gametes. This type of nuclear division is a reduction division called meiosis.
  • Meiosis is involved in the production of gametes.
• Meiosis halves the number of chromosomes so egg cells and sperm cells have only 23 chromosomes each.
  • The zygote has 46 chromosomes. 23 from the mother (in the egg) and 23 from the father (in the sperm).
• In sexual reproduction, the number of chromosomes stays constant from generation to generation.
• Meiosis occurs in sex organs in humans, the ovaries and testes.
• In flowering plants, meiosis occurs in the anthers and the ovules.
• Since the daughter cells are genetically different, this contributes to the genetic variation that appears in the next generation following sexual reproduction.
  • Genetic variation is important in natural selection.

Chromosomes and meiosis

• This diagram shows what happens to one pair of chromosomes during meiosis.
  • There are two divisions of the cell, resulting in four cells. Each of these four cells has one chromosome, so the number has been halved to give a haploid gamete.
  • The pair of chromosomes shown in the first cell are different from each other. One is paternal in origin as it was inherited from the father, and one is maternal as it was inherited from the mother.
• When the 23 pairs of chromosomes separate in the first division of meiosis, the daughter cells gain different combinations of paternal and maternal chromosomes.
  • This is one way in which the haploid cells produced in meiosis are genetically different from one another.

Differences between mitosis and meiosis

• This diagram visually compares mitosis and meiosis.