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2.3: Meiosis

Most eukaryotes replicate sexually - a cell from one individual joins with a cell from another to create the next generation. In order 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.

2.3.1   Meiosis I and II

Meiosis begins similarly to mitosis (a cell has replicated its chromosomes and grown large enough to divide), but requires two rounds of division (Figure 2.8). 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.

Cells that will undergo the process of meiosis are called meiocytes and are diploid (2N). Meiosis is divided into two stages designated by the roman numerals I and II.  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 (Figs. 2.12).  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.

Figure 2.12:  Meiosis in Arabidopsis (n=5).  Panels A-C show different stages of prophase I, each with an increasing degree of chromosome condensation.  Subsequent phases are shown: metaphase I (D), telophase I (E), metaphase II (F), anaphase II (G), and telophase II (H). (PLoS Genetics-Chelysheva, L. et al (2008) PLoS Genetics-PD)

In meiosis I replicated, homologous chromosomes pair up, or synapse, during prophase I, lining up in the middle of the cell during metaphase I, and separating during anaphase I. For this to happen the homologous chromosomes need to be brought together while they condense during prophase I. These attachments are formed in two ways. Proteins bind to both homologous chromosomes along their entire length and form the synaptonemal complex (synapse means junction). These proteins hold the chromosomes in a transient structure called a bivalent. The proteins are released when the cell enters anaphase I.

Within the synaptonemal complex a second event, crossingover, occurs. These are places where DNA repair enzymes break the DNA two non-sister chromatids in similar locations and then covalently reattach non-sister chromatids together to create a crossover between non-sister chromatids. 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 they segregate successfully and they also cause the reshuffling of gene/allele combinations to create genetic diversity, which can have an important effect on evolution (see Chapter 7).

2.3.2    Stages of Prophase I

In meiosis, Prophase I is divided up into five visual stages, that are steps along a continuum of events. Leptotene, zygotene, pachytene, diplotene and diakinesis. From interphase, a cell enters leptotene as the nuclear material begins to condense into long visible threads (chromosomes). During Zygotene homologous chromosomes begin to pair up (synapse) and form an elaborate structure called the synaptonemal complex along their length. At pachytene homologous chromosomes are fully synapsed (two chromosomes and four chromatids) to form bivalentsCrossing over takes place in pachytene. After this, the pairing begins to loosen and individual chromatids become apparent in diplotene. This is when the consequences of each crossing over event can be seen as a chiasma (plural: chiasmata). Diakinesis follows as the chromosomes continue to condense and individualize. This is followed by metaphase I were the paired chromosomes orient on the metaphase plate in preparation for segregation (reductional).

2.3.3  Meiosis II and Gamete Maturation

At the completion of meiosis I there are two cells, each with one, replicated copy of each chromosome (1N). Because the number of chromosomes per cell has decreased (2->1), meiosis I is called a reductional cell division. In the second part of meiosis the chromosomes will once again be brought to the middle of the cell, but this time it is the sister chromatids that will segregate during anaphase.

After cytokinesis there will be four cells, each containing only one unreplicated chromosome of each type. Meiosis II resembles mitosis in that the number of chromosomes per cell is unchanged - both are equational cell divisions – but in meiosis II all cells won’t have the same genetic composition. There will be allelic differences among the gametes.

In animals and plants the cells produced by meiosis need to mature before they become functional gametes. In male animals the four products of meiosis are called spermatids. They grow tails and become functional sperm cells. In female animals the gametes are eggs. In order that each egg contains the maximum amount of nutrients only one of the four products of meiosis becomes an egg. The other three cells end up as tiny disposable cells called polar bodies. In plants the products of meiosis reproduce a few times using mitosis as they develop into functional male or female gametes.

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