Meiosis

Before meiosis — DNA replicates during S phase of interphase. Each chromosome is now two sister chromatids.

Meiosis I (reduction division) — homologous chromosomes separate.

• Prophase I — chromosomes condense; HOMOLOGOUS pairs come together (synapsis) forming bivalents. Crossing over occurs at chiasmata — segments of DNA are exchanged between non-sister chromatids of homologous chromosomes.
• Metaphase I — bivalents align at equator. The orientation of each pair is random and independent of other pairs.
• Anaphase I — homologous chromosomes (each still 2 chromatids) pulled to opposite poles. SISTER CHROMATIDS STAY TOGETHER.
• Telophase I — two haploid cells form (each with chromosomes still as 2 chromatids).

Meiosis II (equational division) — like a mitotic division on each haploid cell.

• Prophase II — new spindle forms.
• Metaphase II — chromosomes align at equator (each is 2 chromatids).
• Anaphase II — sister chromatids separate to opposite poles.
• Telophase II — four haploid cells produced; each genetically different.

Outcome. One diploid parent → four haploid daughter cells, each genetically unique.

Meiosis generates genetic variation by THREE mechanisms:

1. Crossing over (prophase I). Non-sister chromatids of homologous chromosomes exchange DNA at chiasmata. Recombines parental alleles → new allele combinations on each chromosome.

2. Independent assortment (metaphase I). Each homologous pair orients randomly with respect to other pairs. With n = 23 chromosomes in humans, this gives $2^{23} \approx 8.4 \times 10^6$ possible gamete combinations from independent assortment alone.

3. Random fertilisation. Any sperm can fertilise any egg. Each parent produces ~$2^{23}$ possible gametes, so the offspring possibilities multiply to ~$2^{46} \approx 7 \times 10^{13}$ — even before crossing over.

Mitosis vs meiosis.

Feature	Mitosis	Meiosis
Divisions	1	2
Daughter cells	2	4
Ploidy	2n → 2n	2n → n
Crossing over	No	Yes
Daughter cells genetically	Identical	Different
Function	Growth, repair, asexual reproduction	Gamete production

Non-disjunction. Failure of chromosomes (meiosis I) or chromatids (meiosis II) to separate — produces gametes with abnormal chromosome numbers → aneuploid zygotes (e.g. trisomy 21, Down syndrome).