Cell Division, Chromosomes, and DNA

New cells are produced by cell division. The type of division determines whether the new cells are genetically identical to the parent or genetically varied. Understanding this is the foundation for everything in genetics and inheritance.

The DNA → Chromosome → Gene → Allele Hierarchy

Human body cells contain 46 chromosomes arranged in 23 pairs of homologous chromosomes. Each pair carries the same genes, but may carry different alleles.

• Diploid (2n) — the full set of paired chromosomes; found in body cells (46 in humans)
• Haploid (n) — half the chromosome number; found in gametes (23 in humans)

Mitosis

Mitosis produces two genetically identical daughter cells from one parent cell. It is used for:

• growth of the organism
• repair of damaged tissue
• asexual reproduction in organisms that reproduce without sex

What happens in mitosis

Before mitosis begins, DNA is replicated — each chromosome makes an exact copy of itself. The cell then divides:

1. Prophase — chromosomes condense and become visible; nuclear envelope breaks down
2. Metaphase — chromosomes line up along the centre of the cell
3. Anaphase — copies of each chromosome are pulled to opposite ends of the cell
4. Telophase / Cytokinesis — nuclear envelopes reform around each set; cell divides

Outcome: 2 diploid cells, each genetically identical to the original.

Asexual Reproduction and Mitosis

In asexual reproduction, all offspring come from a single parent by mitosis. Because mitosis produces genetically identical cells, all offspring are clones — they are genetically identical to the parent. This is an advantage when conditions are stable and the parent is well-adapted, but a disadvantage if conditions change, as there is no variation to select from.

Meiosis

Meiosis produces four haploid cells from one diploid parent cell. It occurs in the gonads (testes and ovaries) to produce gametes (sperm and eggs).

Why meiosis is necessary

During fertilisation, a sperm (n = 23) fuses with an egg (n = 23) to produce a zygote (2n = 46). If gametes were produced by mitosis, they would have 46 chromosomes, and fertilisation would double the chromosome number each generation. Meiosis halves the chromosome number, ensuring the correct number is restored after fertilisation.

What happens in meiosis

Meiosis consists of two divisions (Meiosis I and Meiosis II):

Meiosis I — separation of homologous pairs

• Homologous chromosomes pair up
• Crossing over occurs — sections of DNA exchange between chromosomes, creating new allele combinations
• Homologous pairs separate to opposite poles

Meiosis II — separation of sister chromatids

• Similar to mitosis; the two cells from Meiosis I divide again
• Sister chromatids separate

Outcome: 4 haploid cells, each genetically different from the others.

Meiosis I Stage Detail

Past papers have asked for events in named stages of Meiosis I:

The most important distinction is that Meiosis I separates homologous chromosomes, while Meiosis II separates sister chromatids.

Sources of Genetic Variation in Meiosis

Two mechanisms create variation:

1. Crossing over — during Meiosis I, homologous chromosomes exchange segments. This shuffles alleles, creating chromosomes with new combinations not present in either parent.
2. Independent assortment — the orientation of each homologous pair at the cell centre is random. This means each gamete receives a random mix of maternal and paternal chromosomes.

These two mechanisms together produce enormous diversity in the gametes, even from the same two parents.

Mitosis vs Meiosis — Comparison

Link to Inheritance

The gametes produced by meiosis each carry one allele for every gene. When two gametes fuse at fertilisation, the zygote receives one allele from each parent — this is the basis of the inheritance patterns covered in the Heredity and Genetics page.

FLASHCARD SET • 11 CARDSCell Division, Chromosomes, and DNA