Meiosis and Genetic Variation

Spec Mapping: This lesson is mapped to OCR H420 Module 2.1.6 — Cell division, diversity and cellular organisation (refer to the official OCR H420 specification document for exact wording). It develops the significance of meiosis for sexual reproduction, the events of meiosis I (reductional) and meiosis II (equational), and the molecular sources of genetic variation (crossing over, independent assortment).

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 develops the OCR H420 Module 2.1.6 content on the significance of meiosis and 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.

5.1 Prophase II

Chromosomes condense again (if they had decondensed).
Nuclear envelopes break down.
A new spindle forms at right angles to the first.

5.2 Metaphase II

Chromosomes (each still consisting of two chromatids) line up individually at the equator — not as bivalents.
Spindle fibres attach to centromeres from both poles.
Independent assortment can occur again at this stage.

5.3 Anaphase II

Centromeres divide.
Sister chromatids are pulled apart and move to opposite poles — exactly like mitotic anaphase.

5.4 Telophase II and Cytokinesis

Nuclear envelopes reform around each group of chromatids.
Chromosomes decondense.
Cytokinesis divides the cytoplasm.
Four haploid daughter cells result, each genetically unique.

6. Sources of Genetic Variation

Meiosis (together with random fertilisation) generates the variation on which evolution depends. The three sources:

6.1 Crossing Over (Prophase I)

Non-sister chromatids of homologous chromosomes exchange segments at chiasmata. This produces recombinant chromatids containing new combinations of alleles that did not exist in either parent. The positions of crossovers are random.

6.2 Independent Assortment of Homologues (Metaphase I)

Each pair of homologues orients independently at the equator. For 23 pairs of chromosomes (human), 2²³ ≈ 8.4 million combinations are possible.

6.3 Independent Assortment at Metaphase II

Because crossing over has swapped some chromatid material, sister chromatids are no longer identical, and their independent alignment in metaphase II adds further variation.

6.4 Random Fertilisation (Bonus — Not Meiosis Itself)

Any one of ~8 million possible egg combinations can fuse with any one of ~8 million possible sperm combinations, giving roughly 70 trillion possible zygotes from a single pair of parents — even before counting crossing over. No wonder siblings look different.

Source	When it happens	Variation generated
Crossing over	Prophase I	New allele combinations on each chromatid
Independent assortment of homologues	Metaphase I	New combinations of whole chromosomes
Independent assortment of chromatids	Metaphase II	Further combinations from crossed-over chromatids
Random fertilisation	At fertilisation	Pairing of two unique gametes

7. Mitosis vs Meiosis — Complete Comparison

Feature	Mitosis	Meiosis
Number of divisions	One	Two
Number of daughter cells	2	4
Chromosome number of daughter cells	2n (diploid)	n (haploid)
Genetic identity	Identical to parent	Genetically unique
Homologous chromosomes pair up?	No	Yes (prophase I)
Crossing over?	No	Yes (prophase I)
Daughter cells become	Somatic cells	Gametes
Occurs in	All body cells	Only in gonads (or equivalents)
Role	Growth, repair, asexual reproduction	Sexual reproduction, genetic variation

8. Non-Disjunction and Chromosomal Disorders

Occasionally chromosomes fail to separate correctly during meiosis — a mistake called non-disjunction. The result is a gamete with one too many or one too few chromosomes. If fertilised, this gives a zygote with a wrong chromosome number, a condition called aneuploidy.

Examples:

Trisomy 21 (Down syndrome) — three copies of chromosome 21.
Trisomy 13 (Patau syndrome) and Trisomy 18 (Edwards syndrome) — usually fatal in infancy.
XXY (Klinefelter syndrome) and XO (Turner syndrome) — sex chromosome aneuploidies.

Non-disjunction illustrates the importance of the spindle assembly checkpoint and the precision of meiosis.

9. Common Exam Mistakes

Confusing mitosis and meiosis — make the comparison table (Section 7) second nature.
Saying meiosis has one division — it has two.
Writing that DNA replicates between meiosis I and meiosis II — it does not.
Claiming homologous chromosomes separate in meiosis II — they separate in anaphase I. Sister chromatids separate in anaphase II.
Confusing bivalents (homologous pairs) with chromosomes.
Forgetting to mention both crossing over and independent assortment as sources of variation.
Saying the chromosome number is halved during cytokinesis — it is halved in anaphase I.
Writing that crossing over happens in "mitosis". It does not — it is unique to prophase I of meiosis.
Saying each gamete is "half a cell". Each gamete is a complete haploid cell.

10. Exam-Style Questions

Describe how crossing over leads to genetic variation. (4)
Explain the significance of independent assortment in meiosis. (3)
Compare the arrangement of chromosomes at the metaphase plate in mitosis and in metaphase I of meiosis. (3)
A cell with 2n = 8 undergoes meiosis. How many chromosomes will be in each daughter cell, and how many possible chromosome combinations exist due to independent assortment alone? (2)

Model answer for (1): "During prophase I of meiosis, homologous chromosomes pair up to form bivalents. Non-sister chromatids of each homologue come into contact at points called chiasmata, where they exchange segments of DNA. This produces recombinant chromatids containing new combinations of alleles that did not exist in either parent chromosome. Because the positions of chiasmata are random and vary between meioses, crossing over produces a huge variety of novel allele combinations in the resulting gametes, generating genetic variation in offspring."

Summary

Meiosis produces four haploid daughter cells from one diploid parent cell.
It consists of two divisions (meiosis I and meiosis II) without intervening DNA replication.
Meiosis I is reductional — homologues separate; crossing over in prophase I and independent assortment in metaphase I generate variation.
Meiosis II is equational — sister chromatids separate, much like mitosis.
Genetic variation arises from crossing over, independent assortment and (combined with) random fertilisation.
Meiosis is essential for sexual reproduction and evolution by supplying varied gametes.
Errors (non-disjunction) cause aneuploidies such as Down syndrome.

Lesson 9: Meiosis and Genetic Variation