How do cells divide to maintain organisms and produce genetically varied gametes?
Compare mitosis and meiosis and explain how meiosis generates genetic variation
A focused answer to the WACE Year 12 Biology dot point on cell division. Compares mitosis and meiosis stage by stage and explains how crossing over, independent assortment and random fertilisation generate variation.
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What this dot point is asking
SCSA wants you to distinguish the two types of nuclear division by their purpose, their products and their key events, and to explain precisely how meiosis introduces genetic variation. Diagrams of the stages and the chromosome numbers are common stimulus.
Why cells divide
Before any division, DNA is replicated during the S phase of interphase, so each chromosome becomes two identical sister chromatids joined at a centromere. Both mitosis and meiosis start from this replicated state.
Mitosis
Mitosis produces two daughter nuclei that are genetically identical to the parent and to each other. It is used for growth, tissue repair and asexual reproduction. The diploid chromosome number (2n) is maintained.
The stages, after interphase, are:
- Prophase: chromosomes condense and become visible, the nuclear membrane breaks down, and spindle fibres form.
- Metaphase: chromosomes line up singly along the cell equator.
- Anaphase: sister chromatids are pulled apart to opposite poles.
- Telophase: nuclear membranes reform around the two sets of chromosomes.
Cytokinesis then divides the cytoplasm, giving two identical diploid cells.
Meiosis
Meiosis produces four haploid (n) gametes from one diploid (2n) cell through two consecutive divisions, meiosis I and meiosis II, with only one round of DNA replication beforehand.
Meiosis I separates homologous chromosomes (the reduction division):
- Prophase I: chromosomes condense and homologous chromosomes pair up to form bivalents. Crossing over occurs here.
- Metaphase I: bivalents line up at the equator; their orientation is random (independent assortment).
- Anaphase I: whole homologous chromosomes (still as pairs of chromatids) are pulled to opposite poles.
- Telophase I: two haploid nuclei form.
Meiosis II resembles mitosis and separates sister chromatids, producing four haploid cells in total.
How meiosis generates variation
Three mechanisms make gametes genetically unique.
- Crossing over (recombination)
- During prophase I, homologous chromosomes exchange equivalent segments at points called chiasmata. This shuffles alleles between maternal and paternal chromosomes, producing new combinations on a single chromosome.
- Independent assortment
- During metaphase I, each homologous pair lines up independently of every other pair. In humans, with 23 pairs, this alone produces over eight million (2 to the power 23) possible combinations of chromosomes in a gamete.
- Random fertilisation
- Any one of millions of genetically distinct sperm can fertilise any one of millions of distinct eggs, multiplying the variation again.
Together these processes ensure that, apart from identical twins, no two individuals produced by sexual reproduction are genetically the same.
Why the difference matters
Maintaining the diploid number through mitosis keeps body cells genetically stable so tissues function reliably. Halving the number through meiosis is essential for sexual reproduction: if gametes were diploid, the chromosome number would double every generation. The variation meiosis creates is also the raw material on which natural selection acts.