Meiosis

Meiosis is the cell division that produces haploid gametes from diploid parent cells. It consists of two divisions:

Meiosis I - the homologous pairs are separated (reduction division). The cell goes from diploid to haploid. The key events:

• Prophase I: homologous chromosomes pair up to form bivalents. Non-sister chromatids cross over at points called chiasmata, exchanging segments of DNA - this is recombination, and it's one of the major sources of genetic variation.
• Metaphase I: bivalents align at the equator. The orientation of each bivalent is random - this is independent assortment.
• Anaphase I: homologous chromosomes are pulled to opposite poles.

Meiosis II - essentially a mitotic division of each haploid cell, separating sister chromatids.

Two mechanisms generate genetic variation in meiosis:

1\. Crossing over - during prophase I, non-sister chromatids of homologous chromosomes exchange segments at chiasmata (the basis of Autosomal linkage). This creates new combinations of alleles on chromosomes (recombinant chromosomes), producing genetic variation beyond what either parent had.

2\. Independent assortment - at metaphase I, each bivalent aligns randomly. The maternal or paternal chromosome of each pair can go to either pole independently of all other pairs. With 23 pairs in humans, this gives 2²³ = ~8 million possible combinations of chromosomes in the gametes, before even accounting for crossing over.

If meiosis fails at division II, diploid (2n) gametes are produced. If two such gametes fuse at fertilisation, a tetraploid (4n) organism results. This is a route to polyploidy, which is common in plants and can lead to rapid speciation - covered in detail in Overview.

The Stages of Meiosis in More Detail

Meiosis I (Reductive Division)

Prophase I - the most complex and important stage. Chromosomes condense. Homologous chromosomes pair up side-by-side to form bivalents (each bivalent = 2 homologous chromosomes = 4 chromatids). Non-sister chromatids from homologous chromosomes interdigitate and break at the same points, exchanging segments - this is crossing over. The points where chromatids remain joined after exchanging DNA are called chiasmata. After separation, the chromatids carry new allele combinations. The nuclear envelope breaks down; the spindle forms.

Metaphase I - bivalents (not individual chromosomes as in mitosis) align at the equator. Each bivalent's two centromeres attach to spindle fibres from opposite poles. The orientation of each bivalent is random - the maternal or paternal chromosome of each pair can face either pole - this is independent assortment.

Anaphase I - homologous chromosomes are pulled to opposite poles by the spindle. Sister chromatids remain joined at the centromere. Each pole now has one chromosome from each homologous pair - the cell is haploid (but each chromosome still consists of two chromatids).

Telophase I - nuclear envelopes may reform; cells may briefly separate. Each cell has a haploid set of chromosomes (but each chromosome = 2 chromatids).

Meiosis II (Equatorial Division)

Essentially a mitotic division of each haploid cell. Prophase II, Metaphase II (chromosomes align at equator), Anaphase II (centromeres split; chromatids separated to poles), Telophase II (four haploid cells formed). The result is four genetically different haploid cells.

Sources of Genetic Variation in Meiosis

1. Crossing over - during prophase I, non-sister chromatids exchange segments. This creates new allele combinations not present in either parent. The more chiasmata, the more recombination. This is the basis of Autosomal linkage.

2. Independent assortment - during metaphase I, each bivalent aligns randomly. The combination of which homologue goes to which pole is entirely chance. With 23 pairs in humans: 2²³ ≈ 8 million possible chromosome combinations in gametes, before crossing over.

3. Random fertilisation - any gamete from one individual can fuse with any gamete from another, adding a further layer of variation.

Meiosis vs Mitosis

Summary

• Meiosis=2 divisions→4 haploid genetically different cells
• Meiosis I: homologous chromosomes separate (reductive division) - diploid→haploid
• Prophase I: bivalents form; crossing over at chiasmata → new allele combinations
• Metaphase I: bivalents align at equator; independent assortment (random orientation)
• Anaphase I: homologs pulled apart (sister chromatids still joined)
• Meiosis II: sister chromatids separate (like mitosis)
• Sources of variation: crossing over + independent assortment + random fertilisation
• Non-disjunction→diploid gametes→polyploidy after fertilisation

AQA Exam Tips

• Bivalent definition: "a pair of homologous chromosomes (each consisting of two chromatids) held together at chiasmata." Four chromatids total.
• Chiasma vs crossing over: crossing over is the physical exchange of DNA; a chiasma is the visible point where chromatids remain joined after the exchange. They are not the same thing.
• Independent assortment vs crossing over: independent assortment produces new combinations of whole chromosomes; crossing over produces new combinations within chromosomes. Both must be mentioned if asked to explain how meiosis produces genetic variation.
• Non-disjunction: chromosomes fail to separate properly → gametes with too many or too few chromosomes → after fertilisation → aneuploidy (e.g. Down syndrome = trisomy 21) or polyploidy.
• Meiosis I is the reductive division: this is where the chromosome number halves. Meiosis II separates chromatids (like mitosis). AQA often asks at which stage the chromosome number halves.
• Why gametes must be haploid: so that when two gametes fuse at fertilisation, the diploid chromosome number is restored. If gametes were diploid, offspring would be tetraploid.