Meiosis and Genetic Variation

Meiosis: Reduction Division

Meiosis is a type of nuclear division that produces four haploid daughter cells from one diploid parent cell. Each daughter cell contains half the chromosome number of the parent and is genetically unique. Meiosis is essential for producing gametes (sex cells) in animals and spores in plants.

Why Meiosis Matters

• It halves the chromosome number, so that when two gametes fuse at fertilisation, the diploid number is restored
• It generates genetic variation among offspring through three key mechanisms: independent assortment, crossing over, and random fertilisation
• Without meiosis, chromosome number would double each generation

Overview of Meiosis

Meiosis consists of two successive divisions: meiosis I (the reduction division) and meiosis II (similar to mitosis). Each division has prophase, metaphase, anaphase, and telophase stages.

Meiosis I — The Reduction Division

Prophase I:

• Chromosomes condense and become visible
• Homologous chromosomes pair up to form bivalents (a process called synapsis)
• Each bivalent consists of four chromatids (a tetrad)
• Crossing over occurs: non-sister chromatids of homologous chromosomes exchange segments of DNA at points called chiasmata (singular: chiasma)
• The nuclear envelope breaks down and the spindle forms

Metaphase I:

• Bivalents line up along the metaphase plate (cell equator)
• The orientation of each bivalent is random — either the maternal or paternal homologue can face either pole
• This is the basis of independent assortment

Anaphase I:

• Homologous chromosomes are pulled to opposite poles by spindle fibres attached to centromeres
• Note: sister chromatids remain joined — it is whole chromosomes (each consisting of two chromatids) that separate
• This is the step that halves the chromosome number

Telophase I:

• Chromosomes may partially decondense
• The cell divides by cytokinesis to produce two haploid cells
• Each cell contains one chromosome from each homologous pair

Meiosis II — Separation of Sister Chromatids

Meiosis II resembles mitosis and occurs in both daughter cells from meiosis I:

Prophase II: Chromosomes condense; new spindle forms at right angles to the first

Metaphase II: Individual chromosomes (each of two chromatids) line up on the metaphase plate

Anaphase II: Sister chromatids are pulled apart to opposite poles as centromeres divide

Telophase II: Nuclear envelopes reform; cytokinesis produces four haploid daughter cells

Sources of Genetic Variation in Meiosis

1. Crossing Over

During prophase I, non-sister chromatids of homologous chromosomes exchange corresponding sections of DNA at chiasmata. This produces recombinant chromosomes — chromatids that carry a new combination of alleles not found in either parent chromosome.

• Crossing over can occur at many points along a chromosome
• The positions of chiasmata vary between cells, so each meiosis produces a different combination of recombinant chromatids
• Crossing over increases the number of different gamete genotypes far beyond what independent assortment alone could produce

Example: If a chromosome carries alleles A and B on one chromatid and a and b on the homologue, crossing over between the two loci can produce recombinant chromatids carrying A-b and a-B.

2. Independent Assortment

During metaphase I, the random orientation of bivalents means that the maternal and paternal homologues are distributed independently to the two poles. For an organism with n pairs of homologous chromosomes, independent assortment alone can produce 2ⁿ different combinations of chromosomes in the gametes.

• In humans (n = 23), this gives 2²³ = 8,388,608 possible combinations
• Combined with crossing over, the number of genetically distinct gametes is effectively unlimited

3. Random Fertilisation

Any male gamete can fuse with any female gamete. In humans, this means 8,388,608 × 8,388,608 = over 70 trillion possible zygote combinations from independent assortment alone (before accounting for crossing over).

Comparison of Mitosis and Meiosis

Feature	Mitosis	Meiosis
Number of divisions	One	Two
Daughter cells produced	Two	Four
Chromosome number of daughters	Diploid (2n)	Haploid (n)
Genetic identity	Identical to parent	Genetically unique
Crossing over?	No	Yes (prophase I)
Independent assortment?	No	Yes (metaphase I)
Homologous pairing?	No	Yes (bivalents in prophase I)
Where it occurs	Growth, repair, asexual reproduction	Gamete/spore production

Non-Disjunction

Occasionally, homologous chromosomes (in meiosis I) or sister chromatids (in meiosis II) fail to separate properly. This is called non-disjunction and results in gametes with an abnormal chromosome number (aneuploidy).

• If a gamete with an extra chromosome is fertilised, the resulting zygote has trisomy (2n + 1)
• If a gamete is missing a chromosome, the zygote has monosomy (2n - 1)
• Down syndrome (trisomy 21) is the most common example in humans, caused by non-disjunction of chromosome 21
• The risk of non-disjunction increases with maternal age

Meiosis and Evolution

Genetic variation produced by meiosis is the raw material for natural selection. Without the reshuffling of alleles through crossing over and independent assortment, populations would have far less variation, reducing their ability to adapt to changing environments. Sexual reproduction, underpinned by meiosis, is therefore a major driver of evolutionary change.

Exam Tips

• Be precise about WHEN each source of variation occurs — crossing over in prophase I, independent assortment in metaphase I, random fertilisation at fusion of gametes
• When asked about the difference between anaphase I and anaphase II, state clearly that homologues separate in AI but sister chromatids separate in AII
• AQA often asks you to calculate the number of possible gamete combinations — use 2ⁿ and state n is the haploid number
• In diagrams, show chiasmata as X-shaped crosses between non-sister chromatids, and use different shading for maternal vs paternal chromosomes