Meiosis

Where it happens

Meiosis happens in the reproductive organs, the testes and ovaries in humans, where gametes are made.

In humans the cells that undergo meiosis are the spermatogonia in the seminiferous tubules of the testes and the oogonia in the ovaries. In flowering plants, meiosis occurs in the anther, where pollen mother cells divide to form pollen grains, and in the ovule, where the embryo sac is formed.

Every one of these parent cells is diploid (2n = 46 in humans), and every product is haploid (n = 23). Meiosis is preceded by an interphase in which each chromosome is replicated into two identical sister chromatids joined at the centromere.

Inputs and outputs

• Input: one diploid parent cell with homologous chromosome pairs, one member of each pair from each parent.
• Input: DNA replication during interphase, so that each chromosome enters meiosis as two sister chromatids.
• Input: energy from respiration to build the spindle fibres and move chromosomes.
• Output: four haploid daughter cells, each with half the chromosome number of the parent cell.
• Output: daughter cells that are genetically different from one another because of crossing over and independent assortment.
• Output: in males, four functional sperm; in females, one ovum and three polar bodies that degenerate.

The steps

1. Interphase: the cell grows and replicates its DNA, so each chromosome consists of two identical sister chromatids.
2. Prophase I: chromosomes condense and homologous chromosomes pair up to form bivalents. Non-sister chromatids exchange segments at chiasmata, a process called crossing over, which produces new combinations of alleles.
3. Metaphase I: bivalents line up along the equator of the cell, with each pair oriented at random. This random orientation is called independent assortment.
4. Anaphase I: spindle fibres pull the homologous chromosomes of each pair to opposite poles. The sister chromatids stay joined, so each pole receives one chromosome from each pair.
5. Telophase I and cytokinesis: two haploid cells form, each chromosome still made of two chromatids. There is no further DNA replication before the second division.
6. Prophase II and metaphase II: in each of the two cells, a new spindle forms and the chromosomes line up singly along the equator.
7. Anaphase II and telophase II: the centromeres divide and sister chromatids are pulled to opposite poles. Nuclear membranes re-form and cytokinesis produces four haploid cells.

Why it matters and how it is controlled

Meiosis keeps the chromosome number of a species constant across generations, because the halving in gametes is balanced by the doubling at fertilisation. It also produces variation by shuffling alleles.

Variation comes from two events. Crossing over in prophase I swaps segments between non-sister chromatids, so the chromatids carry allele combinations that neither parent chromosome had.

Independent assortment in metaphase I means that which chromosome of each pair faces which pole is random, so a human can make 2 to the power of 23 different chromosome combinations in gametes before crossing over is even counted. Random fertilisation then multiplies this further.

This variation is the raw material for natural selection and is the reason siblings differ from one another. Meiosis is controlled by the same cell-cycle checkpoints that regulate mitosis; a failure of homologous chromosomes to separate in anaphase I, called non-disjunction, produces gametes with an extra or a missing chromosome, and Down syndrome (trisomy 21) is the standard example of the result.

How it is examined

You may be asked to compare meiosis with mitosis, to explain why gametes must be haploid, or to state where meiosis occurs.

Diagram questions often show a cell with two or four chromosomes and ask you to name the stage and justify the answer, for example by pointing to paired homologous chromosomes (prophase I) or single chromosomes lining up at the equator (metaphase II). Calculation-style questions give the diploid number of a hypothetical organism and ask for the number of chromosomes in a gamete, in a cell at the end of meiosis I, or in a zygote.

Essay questions frequently ask for the significance of meiosis, so prepare three points: keeping the chromosome number constant, producing genetic variation, and forming gametes for sexual reproduction.

Common misconceptions

Worked exam-style question

Question. A hypothetical animal has a diploid chromosome number of 12. (a) State the number of chromosomes in one of its gametes.

(b) A diagram shows a cell from this animal in which six pairs of homologous chromosomes are lined up at the equator. Name the stage and give one reason for your answer.

(c) Explain two ways in which meiosis produces genetic variation in the gametes of this animal.

Model answer. (a) 6, because meiosis halves the chromosome number from diploid to haploid. (b) Metaphase I, because homologous chromosomes are paired as bivalents and are lined up at the equator; in metaphase II the chromosomes would line up singly.

(c) During prophase I, crossing over occurs at the chiasmata between non-sister chromatids, exchanging segments so that chromatids carry new combinations of alleles. During metaphase I, independent assortment means each bivalent is oriented at random, so the maternal and paternal chromosomes of different pairs are distributed into the gametes in random combinations.

Source:SRC-DSKP-EN

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