Meiosis is a process of cell division that occurs exclusively in sexual cells in organisms with sexual reproduction. This process results in the formation of gametes (eggs and sperm).
Importance of meiosis
Unlike mitosis, which produces cells identical to the stem cell, meiosis produces cells with half the number of chromosomes and different from the original cell. This process is essential to:
- Keep the number of sex chromosomes constant in future generations. The formation of the zygote takes place after the fusion of two gametes.
- Generate genetic variability between organisms. This second point is possible thanks to the recombination process (see section "Genetic recombination").
Phases of meiosis
Meiosis consists of two consecutive cell divisions called meiosis I and meiosis II, which result in the formation of four haploid cells. These haploid cells can fuse with another haploid cell of the opposite sex during fertilization to form a zygote, which will develop into a new organism.
Meiosis I is a reductional division, as homologous chromosomes separate, halving the number of chromosomes in each daughter cell. This process consists of 4 phases:
- Prophase I: The cell prepares for division in much the same way as it does in mitosis. The chromosomes condense and become visible under the microscope. The homologous chromosomes pair up and a structure called bivalent or tetrad is formed. During this pairing, genetic recombination can occur through crossing over (a process detailed in the "Genetic recombination" section). This process is crucial and differentiates between the prophase of mitosis and meiosis. Also, the nuclear envelope begins to disintegrate.
- Metaphase I: Bivalents line up in the equatorial plane of the cell where microtubules of the mitotic spindle attach to the centromeres of chromosomes.
- Anaphase I: The homologous chromosomes are separated and dragged to opposite poles of the cell by the microtubules of the mitotic spindle. The separation of homologous chromosomes is crucial, as it ensures that each daughter cell receives only one chromosome from each pair. Failures in this process generate anomalies known as aneuploidies. One of the most common aneuploidies is Down syndrome, a type of trisomies.
- Telophase I: Chromosomes reach opposite poles of the cell. A new nuclear envelope forms around each set of chromosomes and the cell divides by cytokinesis, generating two haploid daughter cells.
Meiosis II is similar to a conventional mitotic division but takes place in haploid cells rather than diploid cells. The result of meiosis II is the formation of four haploid cells, each with a unique combination of chromosomes and alleles. These cells are known as gametes (eggs or sperm) and are involved in sexual reproduction.
- Prophase II: The daughter cells formed in meiosis I enter into prophase II. At this stage, the chromosomes condense, and the nuclear envelopes disappear.
- Metaphase II: The chromosomes are aligned in the equatorial plane of each daughter cell. The microtubules of the mitotic spindle attach to the centromeres of chromosomes.
- Anaphase II: The centromeres and the sister chromatids are separate, being dragged toward the opposite poles of the cell by the microtubules of the mitotic spindle. Failures in this phase may be responsible for the subsequent existence of aneuploidies.
- Telophase II: Chromosomes reach opposite poles of each daughter cell. New nuclear envelopes form around the chromosome sets and the chromosomes decondense. Daughter cells divide by cytokinesis, resulting in the formation of four haploid cells, each with half the number of chromosomes of the original cell.
Recombination, also known as crossing over, is a key process that occurs during meiosis. During the stage of meiosis I homologous chromosomes, (pairs of similar chromosomes inherited from parents), pair up and form structures called bivalent or tetrad. During this pairing, an exchange of DNA segments occurs between homologous chromosomes. These exchanges are known as crossing-overs and result in genetic recombination because alleles and genes are mixed between homologous chromosomes.
Genetic recombination is vital because it generates genetic variability in sex cells. This means that the resulting gametes will have unique combinations of alleles, increasing genetic diversity in the offspring. Recombination also helps break genetic links between genes, allowing the redistribution of alleles in the population and promoting evolution, an essential process for species adaptation.