Mitosis and meiosis: WACE Year 12 Biology
“Compare mitosis and meiosis and explain how meiosis generates genetic variation”
Mitosis produces two genetically identical diploid cells for growth and repair, while meiosis produces four genetically varied haploid gametes through two divisions, with variation arising from crossing over, independent assortment and random fertilisation.
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.
In mitosis, sister chromatids separate once and the chromosome number stays diploid. In meiosis, homologous chromosomes separate in meiosis I (halving the number) and sister chromatids separate in meiosis II.
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.
Compare the products of each division
Start with one diploid cell containing, say, 2n = 4 chromosomes. Mitosis gives two cells, each 2n = 4 and genetically identical. Meiosis gives four cells, each n = 2 and genetically varied. The halving from 2n to n in meiosis is exactly what allows fertilisation to restore the diploid number in the next generation without doubling it each time.
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.
- Saying meiosis has one division
- Meiosis has two divisions (I and II) but only one round of DNA replication.
- Putting crossing over in meiosis II
- Crossing over occurs in prophase I, when homologous chromosomes are paired.
- Confusing what separates when
- Homologous chromosomes separate in anaphase I; sister chromatids separate in anaphase II (and in mitotic anaphase).
- Calling mitosis a source of variation
- Mitosis normally produces identical cells; meiosis is the source of variation.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2021-style7 marksCompare mitosis and meiosis with respect to their purpose, the number of divisions, the number and type of daughter cells produced, and the genetic outcome.Show worked answer →
A 7 mark compare answer should address each point of comparison for both processes.
- Purpose
- Mitosis is for growth, tissue repair and asexual reproduction; meiosis produces gametes for sexual reproduction.
- Number of divisions
- Mitosis is a single division; meiosis has two consecutive divisions (meiosis I and II) after one round of DNA replication.
- Number and type of daughter cells
- Mitosis produces two diploid () cells; meiosis produces four haploid () cells.
- Genetic outcome
- Mitosis produces cells genetically identical to the parent and each other; meiosis produces genetically varied cells (through crossing over and independent assortment).
- Chromosome separation
- In mitosis sister chromatids separate; in meiosis I homologous chromosomes separate (reduction division) and in meiosis II sister chromatids separate.
Markers reward a clear comparison on each criterion, not just a description of one process.
WACE 2023-style5 marksExplain why meiosis must halve the chromosome number when producing gametes, and what would happen to the chromosome number across generations if gametes were produced by mitosis instead.Show worked answer →
A 5 mark explain answer needs the reduction rationale plus the consequence.
Why halve. Meiosis is a reduction division: it takes a diploid () cell and produces haploid () gametes. At fertilisation two haploid gametes fuse, restoring the diploid number () in the zygote. Halving in meiosis is what keeps the chromosome number constant from one generation to the next.
If gametes were diploid. If gametes were made by mitosis they would be diploid (). Fusing two diploid gametes would give a zygote, and the chromosome number would double every generation, disrupting gene balance and normal development.
Markers reward the reduction-then-restoration logic and the doubling consequence of diploid gametes.