Cell Division: Mitosis and Meiosis

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Mitosis is a single nuclear division (2n → 2n) that yields two genetically identical daughter cells; used for growth, repair, and asexual reproduction in somatic cells.
Meiosis is two successive divisions after one DNA replication (2n → n), producing four genetically non-identical haploid gametes for sexual reproduction.
Stages of mitosis follow the mnemonic PMAT: Prophase, Metaphase, Anaphase, Telophase, then cytokinesis.
Meiosis I is reductional (homologous chromosomes separate); Meiosis II is equational (sister chromatids separate) and resembles mitosis.
Crossing over in Prophase I and independent assortment at Metaphase I generate genetic variation — this is the unique payoff of meiosis.
High-yield NECO pointers: know the 2n→2n vs 2n→n outcome, the site of each division (gonads vs body cells), and a 5–6 row comparison table.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

The Cell Cycle Before Division

Before mitosis or meiosis begins, the cell passes through interphase, which has three sub-phases: G1 (cell growth and organelle production), S (DNA replication — each chromosome becomes two sister chromatids joined at the centromere), and G2 (preparation for division, including synthesis of spindle proteins). A non-dividing cell exits into G0.

Stages of Mitosis (PMAT)

Prophase: Chromatin condenses into visible chromosomes; the nuclear envelope breaks down; centrioles move to opposite poles and organise spindle fibres.
Metaphase: Chromosomes line up single-file along the equator (metaphase plate), attached by spindle fibres to opposite centromeres.
Anaphase: Sister chromatids separate at the centromere and are pulled to opposite poles; each pole now has a complete set of chromosomes.
Telophase: Nuclear membranes reform around the two sets; chromosomes decondense; followed by cytokinesis (cleavage furrow in animal cells, cell plate in plant cells).

Trap: A chromosome with two chromatids is still one chromosome, not two — only after anaphase separation do they count as two chromosomes.

Meiosis: Two Divisions, One Replication

Meiosis I (reductional) separates homologous chromosomes; meiosis II (equational) separates sister chromatids. In Prophase I, homologous pairs form bivalents (tetrads) and crossing over exchanges segments between non-sister chromatids at chiasmata. Metaphase I lines up bivalents randomly at the equator — this is independent assortment, the second source of variation. Anaphase I pulls homologues apart; telophase I yields two haploid cells. Meiosis II then proceeds like mitosis, splitting chromatids to give four haploid gametes.

Mitosis vs Meiosis at a Glance

Feature	Mitosis	Meiosis
Number of divisions	1	2
Daughter cells produced	2	4
Chromosome number change	2n → 2n	2n → n
Genetic identity of daughters	Identical	Non-identical
Crossing over	Absent	Present (Prophase I)
Biological role	Growth, repair, asexual reproduction	Gamete formation, genetic variation
Occurs in	Somatic (body) cells	Germ cells of gonads

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Mechanism of Crossing Over

During Prophase I, the synaptonemal complex holds homologous chromosomes in tight synapsis. Recombinase enzymes nick non-sister chromatids at corresponding points, and the broken segments swap and rejoin. Each resulting chromatid now carries a mixture of maternal and paternal alleles, so all four gametes from one parent cell carry different allele combinations.

Why Chromosome Number Stays Constant

Humans have 46 chromosomes (2n = 46). Without halving, fertilisation would double the number each generation. Meiosis halves the number in gametes (n = 23), so fertilisation restores 2n = 46 in the zygote. This is why meiosis is described as a reductional division.

Edge Cases and Common Mistakes

Spermatogenesis vs oogenesis: Both undergo meiosis I and II. Oogenesis produces one ovum and three polar bodies because cytoplasm divides unequally; spermatogenesis produces four equal spermatids.
Plants: Mitosis forms a cell plate from vesicle fusion at the equator; animal cells use a cleavage furrow. Meiosis in flowering plants occurs in anthers and ovaries.
Independent assortment formula: For an organism with k chromosome pairs, the number of possible gamete chromosome combinations (ignoring crossing over) is 2ᵏ. Humans: 2²³ ≈ 8.4 million combinations.

Exam trap: If asked “where does meiosis occur?”, do not answer “in the uterus” — the correct answer is the gonads (ovaries/testes), specifically in primary germ cells.

Worked Example

A cell has 2n = 8 chromosomes. After meiosis, each gamete contains 4 chromosomes (n = 4). If 3 of the 4 pairs undergo crossing over, how many recombinant chromatids are present after Prophase I? Answer: Each crossover event involves 2 of the 4 chromatids in a bivalent, so 3 crossovers = 6 recombinant chromatids out of 16 total chromatids in the original tetrad stage.

NECO Exam Strategy

Cell Division carries roughly 4% of the NECO SSCE Biology paper, typically 1–2 objectives plus a 2–4 mark theory item. Past questions ask candidates to: (a) draw and label prophase/metaphase/anaphase, (b) complete a comparison table (5–6 rows), (c) state three significances of meiosis, and (d) explain why chromosome number remains constant across generations. Spend ~2 minutes per short-answer, label diagrams clearly, and always include the chromosome number outcome in any comparison answer.

Practice Prompts

State four differences between mitosis and meiosis, and in each case state whether the daughter cells are haploid or diploid.
Describe three ways in which meiosis introduces variation, naming the stage at which each occurs.

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Cell Division: Mitosis and Meiosis