IGCSE Biology 0610: Meiosis, Gametes and Contrasting Meiosis with Mitosis

The aquarium fish species described above has 24 chromosomes in each body cell (12 homologous pairs). In the reproductive organs of an adult fish, cells divide by meiosis to produce gametes (sperm or egg cells).

(a) State the number of chromosomes present in one gamete of this fish species, and explain why this number is different from the number in a body cell. [2]

(b) State three ways in which meiosis differs from mitosis, referring to the number of divisions involved, the number of daughter cells produced, and whether the daughter cells are genetically identical to the parent cell. [3]

(c) Explain why it is important that meiosis halves the chromosome number, in terms of what happens when two gametes fuse at fertilisation. [2]

Part (a): Chromosome number in a gamete

Meiosis is a reduction division: it produces cells with half the chromosome number of the parent cell. Since a body cell of this fish has 24 chromosomes (12 homologous pairs), each gamete produced by meiosis contains only 12 chromosomes, one chromosome from each pair. This halving is why a gamete’s chromosome number differs from a body cell’s: a body cell is diploid (chromosomes in pairs), while a gamete is haploid (single set).

Part (b): Meiosis compared with mitosis

Three key differences between meiosis and mitosis:

Number of divisions: mitosis involves a single division of the nucleus; meiosis involves two successive divisions of the nucleus.
Number of daughter cells: mitosis produces two daughter cells from one parent cell; meiosis produces four daughter cells from one parent cell.
Genetic identity: the two daughter cells from mitosis are diploid and genetically identical to the parent cell and to each other. The four daughter cells from meiosis are haploid and genetically different from the parent cell and from one another.

Part (c): Why halving the chromosome number matters

Fertilisation involves the fusion of two gametes, one from each parent, to form a zygote. If gametes were diploid (with the full body-cell chromosome number), fusing two of them would produce a zygote with double the chromosome number of the parents, and this doubling would continue with every new generation. Because meiosis halves the chromosome number, each gamete is haploid (12 chromosomes in this fish). When a haploid sperm fuses with a haploid egg at fertilisation, the resulting zygote has the diploid number restored (12 + 12 = 24), matching the chromosome number of the parents. This keeps the chromosome number constant from generation to generation.

Final answers

(a) A gamete has 12 chromosomes, half the body-cell number, because meiosis is a reduction division.
(b) Meiosis: two divisions, four daughter cells, genetically different from parent and each other. Mitosis: one division, two daughter cells, genetically identical to parent and each other.
(c) Halving the chromosome number in gametes means fertilisation restores the diploid number, keeping the chromosome number constant across generations rather than doubling it each time.