Genetic diversity from meiosis

Meiosis is the specialised type of cell division used by sexually reproducing organisms to produce gametes (sperm and egg cells in animals, pollen and ovules in plants). 

The main purpose of meiosis and sexual reproduction is to introduce genetic variation, which enables species to adapt to changing environments and drives evolution.

Meiosis differs from mitosis in several crucial ways:

1. Like mitosis, meiosis begins with DNA replication to duplicate the genetic material. However, instead of one division, meiosis involves two nuclear divisions. This results in the formation of four genetically unique haploid daughter cells rather than the two identical diploid cells produced by mitosis.

2. During meiosis, the chromosome number is halved from the normal diploid number (2n) to the haploid number (n). This halving is essential to maintain a constant chromosome number from generation to generation. When two haploid gametes fuse during fertilisation, the diploid chromosome number is restored. Thus, in sexually reproducing organisms, meiosis is always followed by fertilisation at some point in the life cycle.

3. Meiosis introduces significant genetic variation among the daughter cells, making it highly unlikely for two identical gametes to form. 

Stages of meiosis

First Meiotic Division: The homologous chromosomes are separated, resulting in two cells, each containing a unique combination of maternal and paternal chromosomes. This division reduces the chromosome number from diploid (2n) to haploid (n).

Second Meiotic Division: This is similar to mitosis, where sister chromatids (duplicated chromosomes) are separated. This results in four haploid daughter cells, each genetically different from the others due to independent segregation and crossing over.

Genetic diversity from meiosis

There are three main sources of genetic variation in sexual reproduction:

Independent Segregation: During the first meiotic division, homologous chromosomes (one set from each parent) line up randomly at the cell’s equator. Because each homologous pair aligns independently of others, the distribution of maternal and paternal chromosomes into the resulting gametes is random. This independent assortment leads to a variety of possible chromosome combinations in the final gametes. In fact, the possible combinations of chromosomes after meiosis can be calculated using the expression 2ⁿ, where n is the number of homologous chromosome pairs.

Crossing Over: While homologous chromosomes are aligned during the first division, they can exchange segments of DNA in a process called crossing over. This occurs at points called chiasmata, where chromatids break and rejoin, swapping genetic material between maternal and paternal chromosomes. This exchange of genetic material results in new combinations of alleles on each chromosome, further contributing to genetic diversity in the resulting gametes.

Random Fertilisation: Genetic variation is further increased by random fertilisation. During fertilisation, any one of the numerous male gametes can fuse with any one of the numerous female gametes. Since each gamete has a unique genetic makeup, the combination formed during fertilization is unique, making each zygote genetically distinct. The possible number of different combinations in the resulting zygote can be calculated using the formula (2ⁿ)², where n is the number of homologous chromosome pairs.

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