Compare mitosis and meiosis and explain how meiosis generates genetic variation.

The cell cycle, the stages and roles of mitosis and meiosis, and how meiosis produces genetically varied haploid gametes, for TCE Biology Unit 3.

Generated by Claude Opus 4.87 min answerUpdated 2026-06-02

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The cell cycle

A dividing cell passes through a repeating sequence called the cell cycle. Most of the cycle is interphase, made of three phases: G1 (cell growth), S (DNA replication, when each chromosome is copied into two identical sister chromatids), and G2 (preparation for division). Division itself is the mitotic (M) phase, followed by cytokinesis, when the cytoplasm splits. Checkpoints control progression through the cycle, and loss of this control can lead to uncontrolled division and cancer.

Mitosis

Mitosis produces two daughter nuclei that are genetically identical to the parent cell and to each other. It is used for growth, tissue repair, and asexual reproduction. The stages are:

Prophase: chromosomes condense and become visible as two sister chromatids; the nuclear membrane breaks down; spindle fibres form.
Metaphase: chromosomes line up single file along the cell equator.
Anaphase: sister chromatids are pulled apart to opposite poles.
Telophase: chromosomes reach the poles, nuclear membranes reform, and chromosomes decondense.

Cytokinesis then divides the cytoplasm, giving two diploid cells. Because there is no pairing of homologous chromosomes and no crossing over, the products are clones of the parent.

Meiosis

Meiosis produces four haploid gametes from one diploid cell, through two successive divisions, meiosis I and meiosis II, with only one round of DNA replication beforehand.

Meiosis I separates homologous chromosomes (the matching pairs, one from each parent):

Prophase I: chromosomes condense, homologous chromosomes pair up to form bivalents, and crossing over occurs.
Metaphase I: homologous pairs line up at the equator; their orientation is random (independent assortment).
Anaphase I: whole homologous chromosomes are pulled to opposite poles, halving the chromosome number.
Telophase I and cytokinesis: two haploid cells form, each chromosome still as two chromatids.

Meiosis II resembles mitosis and separates the sister chromatids of each chromosome, producing four haploid cells in total.

How meiosis generates variation

Meiosis is a major source of genetic variation, in three ways:

Crossing over: during prophase I, homologous chromosomes exchange segments at points called chiasmata, producing recombinant chromosomes with new allele combinations.
Independent assortment: in metaphase I, each homologous pair lines up independently, so maternal and paternal chromosomes are mixed randomly between the gametes.
Random fertilisation: although this happens after meiosis, it multiplies the variation by combining any two gametes.

Together these processes mean that, apart from identical twins, no two offspring are genetically the same.

Comparing mitosis and meiosis

Mitosis gives two identical diploid cells in one division, with no crossing over and no pairing of homologues. Meiosis gives four varied haploid cells in two divisions, with crossing over and independent assortment. Mitosis supports growth and repair; meiosis supports sexual reproduction. Errors in meiosis, such as non-disjunction (chromosomes failing to separate), can lead to gametes with the wrong chromosome number and conditions such as Down syndrome.

Exam-style practice questions

Practice questions written in the style of TASC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

TCE 20237 marksA diploid organism has a chromosome number of

2n = 8

. Compare mitosis and meiosis for this organism by stating, for each process, the number of daughter cells produced, the chromosome number of each daughter cell, and whether the daughters are genetically identical to the parent. Then explain two ways meiosis generates genetic variation.

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A 7 mark answer compares the two processes numerically and gives two sources of variation.

Mitosis: Produces $2$ daughter cells, each with $8$ chromosomes (diploid, $2n$ ), genetically identical to the parent cell.
Meiosis: Produces $4$ daughter cells, each with $4$ chromosomes (haploid, $n$ ), genetically different from the parent and from each other.
Source of variation 1 - independent assortment: In metaphase I the homologous pairs line up randomly, so each gamete gets a random mix of maternal and paternal chromosomes.
Source of variation 2 - crossing over: In prophase I homologous chromosomes exchange segments at chiasmata, producing new combinations of alleles on a chromosome.

Markers reward correct numbers for both processes ( $2$ vs $4$ cells; $8$ vs $4$ chromosomes; identical vs varied) and two valid named sources of variation.

TCE 20215 marksExplain why it is essential that gametes are produced by meiosis rather than mitosis, and explain what would happen to the chromosome number over generations if gametes were produced by mitosis.

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A 5 mark answer links meiosis to halving the chromosome number and reasons through the alternative.

Why meiosis for gametes. Meiosis halves the chromosome number, producing haploid ( $n$ ) gametes. When two haploid gametes fuse at fertilisation, the diploid ( $2n$ ) number is restored. This keeps the chromosome number constant from one generation to the next.

If gametes were made by mitosis. Mitosis keeps the diploid number, so gametes would be $2n$ . At fertilisation two $2n$ gametes would fuse to give $4n$ , then $8n$ , and so on, doubling the chromosome number every generation. This would disrupt gene balance and be lethal.

Markers reward the halving/restoring role of meiosis and the clear consequence of doubling chromosome number each generation.