Describe and draw the key events and stages of meiosis that lead to haploid gametes. Recall that homologous chromosomes separate during meiosis I (a reductional division) and that sister chromatids separate during meiosis II (an equational division). Compare mitosis and meiosis. Compare the processes of oogenesis and spermatogenesis in humans, including the chromosome complements of the gametes.

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Most eukaryotes replicate sexually - a cell from one individual joins with a cell from another to create the next generation. For this to be successful, the cells that fuse must contain half the number of chromosomes as in the adult organism. Otherwise, the number of chromosomes would double with each generation! The reduction in chromosome number is achieved by the process of meiosis. In meiosis, there are usually two steps, Meiosis I and II. In Meiosis I homologous chromosomes segregate, while in Meiosis II sister chromatids segregate. Most multicellular organisms use meiosis to produce gametes, the cells that fuse to make offspring. Some single celled eukaryotes such as yeast also use meiosis.

Meiosis begins similarly to mitosis (a cell has replicated its chromosomes and grown large enough to divide), but requires two rounds of division. In the first, known as meiosis I, the homologous chromosomes separate and segregate. During meiosis II the sister chromatids separate and segregate. Note how meosis I and II are both divided into prophase, metaphase, anaphase, and telophase. After two rounds of cytokinesis, four cells will be produced, each with a single copy of each chromosome.

Meiosis is divided into two stages designated by the roman numerals I (one) and II (two). Meiosis I is called a reductional division, because it reduces the number of chromosomes inherited by each of the daughter cells. Meiosis I is further divided into Prophase I, Metaphase I, Anaphase I, and Telophase I, which are roughly similar to the corresponding stages of mitosis, except that in Prophase I and Metaphase I, homologous chromosomes pair with each other, or synapse, and are called bivalents. This is an important difference between mitosis and meiosis, because it affects the segregation of alleles, and also allows for recombination to occur through crossing-over, as described later. During Anaphase I, one member of each pair of homologous chromosomes migrates to each daughter cell (1N). Meiosis II resembles mitosis, with one sister chromatid from each chromosome separating to produce two daughter cells. Because Meiosis II, like mitosis, results in the segregation of sister chromatids, Meiosis II is called an equational division.


Query \(\PageIndex{1}\)


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Homologous Recombination

Within the synaptonemal complex during prophase 1, homologous recombination, or crossing over, occurs. These are places where DNA endonucleases break two non-sister chromatids in similar locations and then covalently reattach non-sister chromatids together to create a crossover between non-sister chromatids (4.1.1: Homologous recombination). This reorganization of chromatids will persist for the remainder of meiosis and result in recombination of alleles in the gametes.

Crossovers function to hold homologous chromosomes together during meiosis I so that they segregate successfully; they also cause the reshuffling of allele combinations to create genetic diversity, which can have an important effect on evolution.


Query \(\PageIndex{2}\)


Outcomes of meiosis

The outcome of meiosis is a cell or cells with half the number of chromosomes as the starting cell. But what combinations of chromosomes are possible?

The homologous chromosomes separate during meiosis I, but the separation of the pairs of homologs is independent of other homologs. As an example in the figure below, for a cell with two pairs of chromosomes (2n=4), there can be four possible combinations of the four chromosomes. When we add recombination and additional pairs of chromosomes, there are almost infinite combinations of chromosomes in gametes.

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Figure \(\PageIndex{2}\): Random, independent assortment during metaphase I can be demonstrated by considering a cell with a set of two chromosomes (n = 2). In this case, there are two possible arrangements at the equatorial plane in metaphase I. The total possible number of different gametes is 2n, where n equals the number of chromosomes in a set. In this example, there are four possible genetic combinations for the gametes. With n = 23 in human cells, there are over 8 million possible combinations of paternal and maternal chromosomes. (CC-BY OpenStax Figure_11_01_03.jpg)

Watch the video below to follow how 4 pair of chromosomes are passed during meiosis I and II.





Gamete maturation

In animals and plants, the cells produced at the end of meiosis need to mature before they become functional gametes.


Spermatogenesis

In most male animals, the four products of meiosis are called spermatids. They change morphology to develop tails and become functional sperm cells. In the meiotic steps of spermatogenesis, the cell divisions are equal, with the meiotic spindle aligned with the center of the cell, and the cells have equal amounts of cytoplasm, much like an average cell that has undergone mitosis. The streamlined, minimal-cytoplasm mature sperm is a product of post-meiotic differentiation, in which it gains the flagellar tail, and ejects most of its cytoplasmic material, keeping only some mitochondria to power the flagella and an acrosomal vesicle that contains the enzymes and other molecules needed to reach and fuse with (i.e. fertilize) a mature egg.

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Fertilization

The purpose of gametes is to allow reproduction generation after generation. By uniting two gametes with half the number of chromosomes, the full chromosome number is restored each generation. Remember that homologous recombination and assortment of chromosomes create a genetically diverse population of gametes.