MeiosisAQA A-Level Biology: Revision notes

Meiosis is a form of nuclear division that produces daughter cells that are genetically different from each other. In animals it produces gametes (sperm and eggs); in plants it produces the cells that give rise to gametes or spores, depending on the life cycle.

Body cells are diploid (2n): they contain homologous chromosomes in pairs, one of each pair inherited from each parent. Gametes must be haploid (n), with one chromosome from each pair, so that fertilisation restores the diploid number rather than doubling it each generation.

Before meiosis, DNA replicates in interphase, so every chromosome consists of two sister chromatids. Meiosis then involves two nuclear divisions and gives, usually, four haploid cells from one diploid parent cell.

• Meiosis I (reduction division): homologous chromosomes pair up and are then separated into two cells. Each cell has half the chromosome number (haploid), but each chromosome still has two chromatids.
• Meiosis II: in each of the two cells, the sister chromatids of each chromosome separate. This gives four cells, each haploid, each chromosome now a single chromatid.

You only need the process in enough detail to show how two divisions give four haploid cells; the names of the phases within each division are not required.

You may be given the chromosome content of a parent cell and asked to complete diagrams showing the cells after each division. Use this method, worked for a parent cell with 2n = 4 (one long pair and one short pair).

1. Parent cell (after replication): 4 chromosomes, each of two chromatids.
2. After meiosis I: 2 cells, each with 2 chromosomes (one long, one short), each still of two chromatids.
3. After meiosis II: 4 cells, each with 2 chromosomes (one long, one short), each a single chromatid.

Always draw one chromosome of each homologous pair in every cell. A cell with both chromosomes of a pair, or with two long and no short chromosome, is wrong. If 2n = 24, the cells after meiosis I have 12 chromosomes, and so do the four cells after meiosis II.

The two processes have different outcomes because they have different jobs: mitosis makes genetically identical cells for growth, repair and asexual reproduction, whereas meiosis makes genetically different haploid cells for sexual reproduction.

• Divisions: mitosis has one; meiosis has two.
• Cells produced: mitosis gives two; meiosis gives usually four.
• Chromosome number: stays diploid in mitosis; halved to haploid in meiosis.
• Genetic make-up: identical to the parent and to each other in mitosis; different from each other in meiosis.
• Homologous chromosomes: stay in the same cell in mitosis; separate in meiosis I.

Meiosis occurs wherever the chromosome number halves, from diploid to haploid. In a life cycle diagram or description, find the step where cells go from 2n to n.

• In animals, meiosis forms the gametes, so the body cells are diploid and the gametes are haploid.
• In some plants, algae and fungi, meiosis forms spores, and the haploid stage may then make gametes by mitosis.
• Fertilisation (gametes fusing) restores the diploid number and is never the stage where meiosis happens.

If a step keeps the number the same (n to n or 2n to 2n), it is mitosis.

Worked example: a haploid organism with 9 chromosomes forms a zygote with 18 chromosomes. The zygote later forms spores with 9 chromosomes. Meiosis must occur when the zygote forms spores, because 18 is halved to 9 there.