OCR Specification focus:
‘Outline meiosis I and II; explain independent assortment and crossing over generating variation.’
Meiosis is a specialised form of cell division that produces haploid gametes from diploid cells. It ensures genetic diversity through crossing over and independent assortment.
The Role and Purpose of Meiosis
Meiosis occurs in the reproductive organs of sexually reproducing organisms, reducing the chromosome number by half to form gametes such as sperm and eggs. This maintains a constant chromosome number across generations and introduces variation essential for evolution and adaptation.
Overview of Meiosis
Meiosis consists of two sequential divisions:
Meiosis I – reduction division, separating homologous chromosomes.
Meiosis II – similar to mitosis, separating sister chromatids.
Each original cell produces four genetically distinct haploid daughter cells.
A labelled overview of Meiosis I (reduction division separating homologous chromosomes) followed by Meiosis II (separating sister chromatids). The diagram tracks chromosome behaviour across prophase, metaphase, anaphase, and telophase in both divisions, concluding with four haploid cells. This figure complements the text by mapping each stage to its core outcome. Source.
Meiosis I – The Reduction Division
Meiosis I reduces the chromosome number from diploid (2n) to haploid (n). Each stage contributes to genetic variation.
Prophase I
Chromosomes condense, becoming visible under a light microscope.
Homologous chromosomes (matching pairs, one maternal and one paternal) pair up in a process called synapsis to form bivalents.
The nuclear envelope breaks down and spindle fibres form.
Crossing over occurs during this phase.
Crossing over: The exchange of genetic material between non-sister chromatids of homologous chromosomes, forming recombinant chromatids with new allele combinations.
Chiasmata (singular: chiasma) form at the points where chromatids cross, holding homologues together until anaphase I.
This process generates genetic recombination, introducing variation between gametes.
Crossing over in Prophase I: homologous chromosomes pair and exchange equivalent DNA segments at chiasmata, creating recombinant chromatids. The allele letters (A/a, B/b) illustrate new combinations that arise after exchange. This diagram includes allele lettering to visualise recombination; the extra allelic detail exceeds the bare minimum of the OCR specification but clarifies the outcome of crossover. Source.
Metaphase I
Bivalents align randomly along the equatorial plate of the cell.
The orientation of each homologous pair is independent of others.
Independent assortment: The random distribution of maternal and paternal homologous chromosomes to gametes, producing numerous genetic combinations.
This random alignment increases genetic variation, as each gamete receives a different mix of maternal and paternal chromosomes.
Anaphase I
Spindle fibres shorten, pulling homologous chromosomes (each still consisting of two chromatids) to opposite poles.
Chiasmata separate, ensuring each new cell receives one chromosome from each homologous pair.
Telophase I and Cytokinesis
Chromosomes reach poles, and the nuclear envelope reforms around each set.
The cytoplasm divides (cytokinesis), producing two haploid daughter cells containing one chromosome from each homologous pair.
Meiosis II – The Division of Chromatids
Meiosis II resembles mitosis but involves haploid cells. There is no further replication of DNA before this division.
Prophase II
Chromosomes condense again if they had decondensed.
Spindle fibres reform, and the nuclear envelope breaks down.
Metaphase II
Chromosomes align individually along the equator.
The orientation of chromatids remains random, contributing further to variation.
Anaphase II
Centromeres divide, and sister chromatids are pulled apart to opposite poles by the spindle fibres.
Each chromatid is now considered an individual chromosome.
Telophase II and Cytokinesis
Chromosomes decondense, and nuclear envelopes reform around each set of chromosomes.
Cytokinesis occurs, resulting in four genetically unique haploid cells.
Sources of Genetic Variation in Meiosis
Genetic variation generated by meiosis is crucial for natural selection and evolutionary processes.
1. Crossing Over (Prophase I)
Exchanges DNA between homologous chromosomes.
Produces recombinant chromatids with new combinations of alleles.
The number of crossovers varies, increasing genetic possibilities.
2. Independent Assortment (Metaphase I and II)
Random orientation of homologous pairs during metaphase I and of chromatids during metaphase II.
Each gamete receives a unique combination of chromosomes.
The number of combinations possible is 2ⁿ, where n is the haploid number of chromosomes.
EQUATION
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Number of possible chromosome combinations (C) = 2ⁿ
C = Number of genetically distinct combinations possible
n = Haploid chromosome number of the organism (no units)
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For example, in humans (n = 23), there are approximately 8.4 million possible combinations before considering crossing over.
3. Random Fertilisation
Although not part of meiosis, random fertilisation further increases variation, as any gamete can fuse with any gamete of the opposite sex, compounding the diversity produced during meiosis.
The Importance of Genetic Variation
Increases adaptability: Populations with greater genetic diversity can adapt more effectively to environmental changes.
Drives evolution: Variation provides raw material for natural selection to act upon.
Prevents uniformity: Reduces the likelihood that all individuals will share detrimental traits.
Ensures species survival: Populations with diverse genotypes are more resilient to diseases and environmental pressures.
Comparison of Meiosis and Mitosis
While both are forms of nuclear division, meiosis differs fundamentally in purpose and outcome:
Chromosome number: Meiosis halves it (diploid → haploid); mitosis maintains it.
Number of divisions: Meiosis has two; mitosis has one.
Genetic outcome: Meiosis produces genetically varied gametes; mitosis produces identical somatic cells.
Location: Meiosis occurs in germ cells; mitosis occurs in body cells.
This distinction is vital to understanding how sexual reproduction promotes genetic diversity compared with asexual reproduction.
FAQ
Crossing over occurs during Prophase I of meiosis when homologous chromosomes exchange DNA segments, producing new allele combinations without changing existing genes.
In contrast, mutation changes the DNA sequence itself, potentially altering the function of a gene.
Crossing over shuffles existing genetic material.
Mutation creates entirely new genetic variants.
Together, they contribute to variation, but crossing over is a controlled, regular event in gamete formation, whereas mutations are spontaneous and rare.
Correct alignment ensures that homologous chromosomes pair accurately in synapsis, allowing proper crossing over at equivalent loci.
If alignment is incorrect:
Unequal crossing over may occur, leading to insertions or deletions.
Non-disjunction can happen later, producing gametes with abnormal chromosome numbers.
Accurate pairing therefore safeguards balanced genetic exchange and successful chromosomal segregation.
No, crossing over typically happens between non-sister chromatids of homologous chromosomes because they carry different alleles.
If it occurred between sister chromatids, no genetic variation would be produced since sister chromatids are genetically identical copies.
Thus, only exchanges between non-sister chromatids contribute to new combinations of alleles in gametes.
The frequency of crossing over depends on several factors:
Chromosome length: longer chromosomes usually undergo more crossovers.
Distance between genes: genes further apart on a chromosome are more likely to be separated by crossing over.
Environmental and cellular conditions: temperature, radiation, or chemical exposure can sometimes alter crossover frequency.
However, crossover number is tightly regulated to maintain genetic stability while ensuring variation.
Even without crossing over, independent assortment during Metaphase I randomly distributes maternal and paternal chromosomes into gametes.
For a species with n chromosome pairs, there are 2ⁿ possible combinations of chromosomes.
In humans (n = 23), this equals over 8 million combinations.
Thus, each gamete receives a unique genetic mix, ensuring diversity even before fertilisation occurs.
Practice Questions
Question 1 (2 marks)
During Prophase I of meiosis, crossing over occurs. Explain how crossing over contributes to genetic variation.
Mark Scheme:
(1 mark) Correctly states that crossing over involves the exchange of genetic material between non-sister chromatids of homologous chromosomes.
(1 mark) Explains that this results in new combinations of alleles (recombinant chromatids), increasing genetic diversity in gametes.
Question 2 (5 marks)
Describe and explain how meiosis results in the formation of four genetically distinct haploid cells from one diploid parent cell.
Mark Scheme:
(1 mark) States that meiosis involves two divisions: Meiosis I and Meiosis II.
(1 mark) Describes that Meiosis I separates homologous chromosomes, reducing the chromosome number from diploid to haploid.
(1 mark) Explains that crossing over during Prophase I produces recombinant chromatids with new allele combinations.
(1 mark) Explains that independent assortment during Metaphase I randomly distributes maternal and paternal chromosomes, generating varied combinations.
(1 mark) States that Meiosis II separates sister chromatids, producing four haploid cells that are genetically different due to crossing over and independent assortment.
