Meiosis and Genetic Variation

Mitosis produces genetically identical daughter cells. Meiosis is a different kind of division that halves the chromosome number and shuffles genetic material to produce four genetically unique daughter cells — the gametes. This lesson covers OCR A-Level Biology A specification point 2.1.6 (c) — the significance of meiosis — and introduces the molecular events of crossing over and independent assortment that underpin all genetic variation.

1. Why Meiosis? The Problem Meiosis Solves

Sexual reproduction involves the fusion of two gametes to form a zygote. If gametes were made by mitosis, each would contain a full set of chromosomes, and fertilisation would double the chromosome number every generation — an impossible situation.

Meiosis solves this problem by halving the chromosome number during gamete formation, from diploid (2n) in the parent cell to haploid (n) in each gamete. When two gametes fuse, the resulting zygote is diploid once again — the chromosome number is preserved across generations.

Key Definition — Meiosis: A type of nuclear division that produces four genetically non-identical haploid daughter cells from one diploid parent cell, involving two successive divisions (meiosis I and meiosis II).

A second, equally important role: meiosis generates genetic variation within a species. Each gamete it makes is genetically unique. This variation is the raw material on which natural selection acts.

2. Key Terminology

Before describing the phases, make sure you understand these terms.

Term	Meaning
Diploid (2n)	Having two complete sets of chromosomes — one from each parent. Humans are 2n = 46.
Haploid (n)	Having one set of chromosomes. Human gametes are n = 23.
Homologous chromosomes	A pair of chromosomes, one maternal and one paternal, carrying the same genes at the same loci but possibly different alleles.
Bivalent (tetrad)	A pair of homologous chromosomes held together during prophase I, consisting of four chromatids.
Chiasma (plural: chiasmata)	A point of physical contact between non-sister chromatids of homologous chromosomes, where crossing over occurs.
Crossing over	The exchange of sections of chromatid between non-sister chromatids of homologous chromosomes during prophase I.
Independent assortment	The random orientation of bivalents (meiosis I) or chromosomes (meiosis II) at the metaphase plate, producing new combinations of chromosomes in daughter cells.

3. Overview — Two Divisions, Four Daughter Cells

Meiosis consists of two successive divisions without an intervening round of DNA replication:

Meiosis I — a reductional division in which homologous chromosomes are separated. One diploid cell becomes two haploid cells (each chromosome still consisting of two chromatids).
Meiosis II — an equational division resembling mitosis, in which sister chromatids are separated. Two haploid cells become four haploid cells.

graph TD
  A[Diploid cell 2n<br/>after S phase] --> B[Meiosis I<br/>homologues separate]
  B --> C[Two haploid cells<br/>still 2 chromatids each]
  C --> D[Meiosis II<br/>chromatids separate]
  D --> E[Four haploid gametes<br/>genetically unique]

Each daughter cell contains half the chromosome number of the original and a unique combination of alleles due to crossing over and independent assortment.

4. Meiosis I in Detail

Like mitosis, meiosis I has prophase, metaphase, anaphase and telophase. The Roman numeral I distinguishes them from meiosis II.

4.1 Prophase I

Prophase I is the longest and most complex phase of meiosis. During prophase I:

Chromosomes condense and become visible.
Homologous chromosomes pair up — this is called synapsis. Each pair of homologues is a bivalent (sometimes called a tetrad, because it contains four chromatids).
Crossing over occurs. Non-sister chromatids within a bivalent exchange segments of DNA at points called chiasmata (singular: chiasma). This recombines alleles, producing new combinations that did not exist in either parent.
The nuclear envelope breaks down.
Spindle fibres form from the centrioles/poles.
Chromatin becomes maximally compact.

The significance of prophase I cannot be overstated: crossing over is one of the two main sources of genetic variation in sexually reproducing organisms. Each human prophase I has on average 40–60 crossovers, and their positions are random, so each gamete carries a unique mix of parental alleles.

graph LR
  A[Two homologous chromosomes<br/>each with two chromatids] --> B[Pair up to form bivalent]
  B --> C[Non-sister chromatids touch<br/>at chiasmata]
  C --> D[Exchange of segments<br/>crossing over]
  D --> E[Recombinant chromatids<br/>with new allele combinations]

4.2 Metaphase I

During metaphase I:

Bivalents (not individual chromosomes) line up at the equator of the cell.
The orientation of each bivalent is random and independent of all others — either maternal or paternal homologue may face either pole.
This is independent assortment, the second main source of genetic variation in meiosis.

With 23 pairs of chromosomes in humans, the number of possible orientations is 2²³ = 8,388,608 distinct combinations in each gamete — before counting crossing over.

4.3 Anaphase I

During anaphase I:

Homologous chromosomes separate and move towards opposite poles. Each pole receives one chromosome from each homologous pair.
Sister chromatids stay together (unlike mitotic anaphase).
Each pole therefore receives the haploid number of chromosomes, but each chromosome still consists of two chromatids (which may no longer be identical because of crossing over).

4.4 Telophase I and Cytokinesis

Nuclear envelopes may reform (in some species) around each group of chromosomes.
The cytoplasm divides by cytokinesis.
Two haploid cells result — but each chromosome still has two sister chromatids, so more division is needed.
In many species the chromosomes do not decondense between meiosis I and meiosis II; meiosis II begins immediately.

5. Meiosis II in Detail

Meiosis II is essentially a mitotic division of each haploid cell: sister chromatids are separated. No DNA replication occurs between meiosis I and meiosis II.

Lesson 9: Meiosis and Genetic Variation