Meiosis

Overview of Meiosis

Meiosis begins with a single diploid cell, which contains two copies of each chromosome

After two rounds of division (meiosis I and meiosis II) the result is four non-identical haploid gametes, each with half the usual chromosome number.

Meiosis involves six stages in each division: prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis.

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Fig 1
The differences between mitosis and meiosis.

Meiosis I: Reductional Division

Meiosis I is referred to as “reductional division” because it reduces the chromosome number by half. During this phase, homologous chromosomes are separated, with each daughter cell receiving one chromosome from each pair.

• Prophase I – chromosomes replicate and condense as the nuclear envelope begins to break down. Crossing over occurs at chiasmata, where homologous chromosomes exchange genetic material, creating new allele combinations and promoting genetic diversity.
• Prometaphase I – spindle fibres attach to chromosomes at their centromeres. The chromosomes continue to condense.
• Metaphase I – homologous chromosomes align along the cell’s equator. Independent assortment occurs, where maternal and paternal chromosomes are randomly oriented to either pole, further enhancing genetic diversity.
• Anaphase I – homologous chromosomes, each still consisting of two sister chromatids, are pulled toward opposite poles of the cell.
• Telophase I – nuclear envelopes reform around each set of chromosomes at the poles.
• Cytokinesis I – cytoplasm divides, resulting in two haploid daughter cells, each containing one chromosome from each homologous pair

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Fig 2
Meiosis I, known as reductional division.

Meiosis II: Equational Division

Meiosis II is often referred to as “equational division” because the chromosome number remains unchanged. During this phase, the sister chromatids are separated, resulting in four genetically distinct haploid gametes.

• Prophase II – nuclear envelope breaks down, and a new spindle apparatus begins to form in each haploid cell.
• Prometaphase II – spindle fibres attach to the centromeres of the chromosomes, preparing them for alignment along the equator.
• Metaphase II – chromosomes align in single file along the equator.
• Anaphase II – sister chromatids are pulled to opposite poles.
• Telophase II – nuclear envelopes reform around each set of separated chromatids at opposite poles of the cell.
• Cytokinesis II – cytoplasm divides, resulting in four non-identical haploid cells.

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Fig 3
Meiosis II, known as equational division.

Clinical Relevance

Chromosomal Abnormalities

Chromosomal abnormalities can arise due to errors in chromosome number or structure during meiosis. These abnormalities are associated with a range of clinical conditions.

Errors in Chromosome Number

Errors in chromosome number are most commonly caused by nondisjunction or anaphase lag, both of which lead to aneuploidy — an abnormal number of chromosomes in the resulting gametes.

• Nondisjunction – failure of chromosomes to separate correctly during anaphase. This can happen in meiosis I or II and leads to gametes with too many or too few chromosomes.
• Anaphase lag – chromosome may lag behind during division and fail to be included in the nucleus, resulting in missing genetic material in one of the daughter cells.

Common disorders caused by errors in chromosome number include trisomies such as Down syndrome, Edwards syndrome, and Patau syndrome.

Errors in Chromosome Structure

Chromosomal structural abnormalities, such as translocations, can arise when chromosome segments are rearranged. There are two main types of translocations:

• Reciprocal translocation – involves the exchange of chromosome segments between two non-homologous chromosomes.
• Robertsonian translocation – involves the fusion of two acrocentric chromosomes at the centromere.

Translocations can be balanced (where no genetic material is lost or gained) or unbalanced (where genetic material is lost or gained), leading to potential developmental issues or miscarriage.

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Fig 4
Nondisjunction during meiosis can result in abnormal chromosome number