Meiosis
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Genes are the hereditary units passed down from parents to offspring. They are encoded in chromosomes. Humans have 23 pairs of chromosomes. One chromosome of each pair is inherited from the mother, and one from the father.
Despite their different origins, both chromosomes of a pair are the same length and have the same set of genes, centromere position and staining pattern. Thus, they are called homologous chromosomes.
Maternal and paternal chromosomes have slightly different genetic code within the genes. These differences distinguish versions of the same gene (alleles) and determine which traits an individual will exhibit.
The only exception to chromosome homology is the pair of sex chromosomes, which determine an individual’s sex. In humans, these are X and Y chromosomes.
The human Y chromosome is much smaller than the X chromosome.
Females have two copies of X chromosomes, and thus have 23 pairs of homologous chromosomes in total. Males have both X and Y chromosomes, thus have 22 pairs of homologous chromosomes and 1 pair of non-homologous chromosomes.
Non-sex chromosomes are called autosomes.
Cells that have two sets of chromosomes are called diploid. Diploid human cells have 46 chromosomes each. Such cells make up nearly all cells of the human body and are called somatic cells.
The remaining cells are gametes – cells that are involved in sexual reproduction.
Gametes have only one set of chromosomes and thus are haploid. Each gamete carries 23 chromosomes.
Different versions of gametic cells are produced in male and female organisms: in humans, the male gametes are sperm cells, while the female gametes are egg cells.
Gametes must be haploid, because after fertilisation – fusion of two parental cells to produce an offspring – the first resulting cell will be diploid, and will give rise to all other diploid somatic cells via mitosis.
The opposite process, division of a diploid precursor cell into haploid gametes, is called meiosis.
Meiosis is a type of cell division distinct from mitosis.
Both processes are preceded by chromosome replication; however, meiosis involves two consecutive cytokinesis steps, producing four, genetically non-identical daughter cells. This contrasts with the two identical cells produced by mitotic cell division.
Each meiotic division proceeds via stages that are very similar to mitosis. However, there are a few crucial differences.
In meiosis I, homologous chromosomes are separated. This means that pairs of chromosomes, rather than individual chromosomes, line up on the metaphase plate. The cell can orient maternal and paternal chromosomes towards either pole: this is called random assortment. After meiosis I, each daughter cell will have received a random mix of maternal and paternal chromosomes, which increases the genetic variation of gametes produced. Each cell has only one set of duplicated chromosomes, i.e. is haploid.
There is another process that is unique to meiosis. During prophase I, as each chromosome aligns with its homolog, crossing over takes place.
DNA of two non-sister chromatids, i.e., chromatids belonging to one paternal and one maternal chromosome, is broken and rejoined such that these chromatids have now swapped their genetic material.
The points where crossing over has occurred are called chiasmata. They keep homologous chromatids together until anaphase I, when they are finally separated.
Crossing over effectively scrambles fragments of genetic material between maternal and paternal chromosomes, increasing the number of gene and allele combinations that may be inherited.
The phases of meiosis 1 show differences to the phases in mitotic cell division:
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Meiosis II then proceeds similarly to meiosis, in which all duplicated chromosomes are separated into sister chromatids.
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A brief comparison of meiosis and mitosis is given in the table below.
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In sexual reproduction, the nucleus of a haploid sperm cell and the nucleus of a haploid egg cell, fuse, becoming a diploid cell called a zygote. The zygote then divides by mitosis, generating all somatic cells of the offspring.
Each gamete in each parent is slightly different, and the random combination of these gametes results in an offspring with unique traits. The offspring will then produce their own gametes, each with a unique set of genetic material. Together, these processes maintain an astonishing degree of genetic variation and make each individual truly unique.