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Meiosis

Biology: Cambridge International A Level, topic 16.1.1

Role of Meiosis

Meiosis is the process that produces haploid gamete cells from diploid cells. Meiosis is necessary for sexual reproduction to take place.

Sexual reproduction

Sexual reproduction

  • Gametes are the sex cells (e.g. sperm cells and egg cells) that fuse together during sexual reproduction.
  • Gametes are haploid (contain one chromosome).
  • When the gametes fuse together, a new diploid organism (an organism with two copies of every chromosome) is formed.
  • An individual inherits one copy of a chromosome from each parent.
Producing gametes

Producing gametes

  • Haploid gametes are produced from diploid cells in an organism.
  • The gametes are produced in a process called meiosis.
  • Meiosis involves two sequential cell divisions (meiosis I and meiosis II).
  • In humans, meiosis is used to produce sperm and egg cells.
  • In plants, microspore cells divide by meiosis to produce pollen grains (plant gametes).
Genetic variation

Genetic variation

  • Meiosis produces four genetically different daughter cells.
  • Each daughter cell receives a different combination of chromosomes.
  • Meiosis increases genetic diversity in this way.

Meiosis

Meiosis is the production of haploid gametes from diploid cells. The steps involved in this process are:

1) Interphase + prophase 1

1) Interphase + prophase 1

  • Interphase:
  • The chromosomes in the cells replicate to produce an identical copy of each other.
  • The replicated chromosomes are called chromatids.
  • Prophase 1:
  • The chromatids condense to form a double-armed chromosome. Each arm is made from one chromatid and is joined by a centromere.
  • Each arm is called a sister chromatid.
2) Metaphase 1

2) Metaphase 1

  • Chromosomes in diploid organisms are found in homologous pairs (chromosomes with identical genes but different alleles).
  • The chromosomes in a pair are arranged in the center of the cell.
  • The chromosomes are moved by fibers called spindle fibers.
3) Anaphase 1 + telophase 1

3) Anaphase 1 + telophase 1

  • Anaphase 1:
  • The homologous chromosomes are separated and pulled to opposite ends of the cell by the spindle fibers.
  • The separation of the chromosomes from each pair is random.
  • Telophase 1:
  • Random combinations of chromosomes are pulled to each side of the cell.
4) Cytokinesis (end of meiosis 1)

4) Cytokinesis (end of meiosis 1)

  • As the homologous chromosomes are separated, the cell divides into two cells.
  • One chromosome from a pair is separated into each cell.
  • This division concludes meiosis 1.
  • Meiosis 1 produces two genetically different cells.
5) Prophase 2 + metaphase 2

5) Prophase 2 + metaphase 2

  • Prophase 2:
  • The chromatids condense again.
  • Metaphase 2:
  • Chromosomes line up in the center of the cell.
6) Anaphase 2

6) Anaphase 2

  • Anaphase 2:
  • The sister chromatids are separated by the spindle fibers and pulled to opposite ends of the cell.
  • The separation of the sister chromatids is random.

7) Telophase 2 + cytokinesis

  • As the sister chromatids are separated, the cell divides into two cells.
  • One chromatid from a chromosome is separated into each cell.
  • This division concludes meiosis 2.
  • Meiosis 2 produces four genetically different haploid cells.

Test yourself on Meiosis

  • Which of the following statements about meiosis are true?

  • What is the role of meiosis?

  • How many chromosomes does a human gamete have if a body cell has 46?

  • Stages Involved in Meiosis

  • Which of the following statements about meiosis are true?

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