Metaphase II

The sister chromatids are maximally condensed and aligned at the cell’s equator.

Anaphase II

The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles (Figure 8.1). Non-kinetochore microtubules elongate the cell.

Telophase II and Cytokinesis

The chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes. If the original parent cell was diploid, as is most commonly the case, cytokinesis now separates the two into four unique haploid cells. The cells produced are genetically unique because of the random assortment of paternal and maternal homologous chromosomes and the recombination of maternal and paternal segments of chromosomes that occurs during crossover. The entire process of meiosis is outlined in Figure 8.2

Comparing Meiosis and Mitosis

Mitosis and meiosis are both forms of nuclear division in eukaryotic cells. They share some similarities but exhibit significant differences that lead to very different outcomes (Fig 8.4). Mitosis is a single nuclear division resulting in two nuclei, usually partitioned into two new cells. The nuclei resulting from a mitotic division are genetically identical to the original nucleus. They have the same number of chromosomes as each other and the parent cell: one set in the case of haploid cells and two sets in the case of diploid cells.

In contrast, meiosis consists of two nuclear divisions resulting in four nuclei that are usually partitioned into four new, genetically distinct cells. The four nuclei produced during meiosis are not genetically identical and contain half the number of chromosome sets as the original cell.

A stark difference between mitosis and meiosis occurs in meiosis I, which is a very different nuclear division from mitosis. In meiosis I, the homologous chromosome pairs physically meet and are bound to each other in the synaptonemal complex. Following this, the chromosomes develop chiasmata and undergo crossover between non-sister chromatids. In the end, the chromosomes line up along the metaphase plate as tetrads, with kinetochore fibers from opposite spindle poles attached to each kinetochore of a homolog to form a tetrad. When the chiasmata dissolve, and the tetrad is broken up, the homologs move towards opposite poles. When the homologous chromosomes (homologs) separate, the ploidy level (i.e., the number of sets of chromosomes in each future nucleus) reduces (typically from two to one). For this reason, meiosis I is referred to as a reductional division. All of these events occur only in meiosis I. There is no reduction in ploidy level during mitosis.

Meiosis II is analogous to a mitotic division. In this case, the duplicated chromosomes (only one set of them) line up on the metaphase plate with divided kinetochores attached to kinetochore fibers from opposite poles. During anaphase II, as in mitotic anaphase, the kinetochores divide, and one sister chromatid—now referred to as a chromosome—is pulled to one pole while the other sister chromatid is pulled to the other pole. If it were not for the fact that there had been a crossover, the two products of each individual meiosis II division would be identical (as in mitosis). Instead, they are different because there has always been at least one crossover per chromosome. Meiosis II is not a reduction division because, although there are fewer copies of the genome in the resulting cells, there is still one set of chromosomes, as there was at the end of meiosis I.

Life Cycles of Sexually Reproducing Organisms

Fertilization and meiosis alternate in sexual life cycles. What happens between these two events depends on the organism’s reproductive strategy. The process of meiosis reduces the chromosome number by half. Fertilization, the joining of two haploid gametes, restores the diploid condition. Some organisms have the most obvious multicellular diploid stage and only produce haploid reproductive cells. Animals, including humans, have this type of life cycle. Other organisms, such as fungi, have the most obvious multicellular haploid stage. Plants and some algae have an alternation of generations, in which they have multicellular diploid and haploid life stages that are apparent to different degrees depending on the group.

Nearly all animals employ a diploid-dominant life-cycle strategy in which the only haploid cells produced by the organism are the gametes. Early in the embryo’s development, specialized diploid cells, called germ cells, are produced within the gonads (such as the testes and ovaries). Germ cells are capable of mitosis to perpetuate the germ cell line and meiosis to produce haploid gametes. Once the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage. Fertilization occurs with the fusion of two gametes, usually from different individuals, restoring the diploid state (Figure 8.5).

Most fungi and algae employ a life-cycle type in which the “body” of the organism—the ecologically important part of the life cycle—is haploid. The haploid cells that make up the tissues of the dominant multicellular stage are formed by mitosis. During sexual reproduction, specialized haploid cells from two individuals—designated the (+) and (−) mating types—join to form a diploid zygote. The zygote immediately undergoes meiosis to form four haploid cells called spores. Although these spores are haploid like the “parents,” they contain a new genetic combination from two parents. The spores can remain dormant for various periods. Eventually, when conditions are favorable, the spores form multicellular haploid structures through many rounds of mitosis (Figure 8.6).

The third life-cycle type, employed by some algae and all plants, is a blend of the haploid-dominant and diploid-dominant extremes. Species with alternating generations have haploid and diploid multicellular organisms as part of their life cycle. The haploid, multicellular plants are called gametophytes because they produce gametes from specialized cells. Meiosis is not directly involved in the production of gametes because the organism that produces the gametes is already haploid. Fertilization between the gametes forms a diploid zygote. The zygote will undergo many rounds of mitosis and give rise to a diploid; multicellular plant called a sporophyte. Specialized cells of the sporophyte will undergo meiosis and produce haploid spores. The spores will subsequently develop into gametophytes (Figure 8.7).

Although all plants utilize some version of the alternation of generations, the relative sizes of the sporophyte and the gametophyte and their relationship vary greatly. The gametophyte organism is free-living in plants such as moss, and the sporophyte is physically dependent on the gametophyte. In other plants, such as ferns, both the gametophyte and sporophyte plants are free-living; however, the sporophyte is much larger. In seed plants, such as magnolia trees and daisies, the gametophyte is composed of only a few cells and, in the case of the female gametophyte, is completely retained within the sporophyte.

Sexual reproduction takes many forms in multicellular organisms. The fact that nearly every multicellular organism on Earth employs sexual reproduction is strong evidence for the benefits of producing offspring with unique gene combinations. However, there are other possible benefits as well.

Acknowledgements

Adapted from Clark, M.A., Douglas, M., and Choi, J. (2018). Biology 2e. OpenStax. Retrieved from https://openstax.org/books/biology-2e/pages/1-introduction