Chapter Review:11.1 Sexual Reproduction Requires Meiosis (figure 11.2)Meiosis reduces the number of chromosomes.Eggs and sperm are haploid (1n) cells, which contain one set of all chromosomes, and products of meiotic division.Sexual life cycles have both haploid and diploid stages.During fertilization, or syngamy, the fusion of two haploid gametes results in a diploid (2n) zygote, which contains two sets of chromosomes. Meiosis and fertilization constitute a reproductive cycle in sexual organisms as they alternate between diploid and haploid chromosome numbers. Somatic cells divide by mitosis and form the body of an organism.Germ-line cells are set aside early in animal development.Cells that eventually will form haploid gametes by meiosis are called germ-line cells. These are set aside early in development in animals.11.2 Features of MeiosisHomologous chromosomes pair during meiosis.The pairing of homologous chromosomes, called synapsis, occurs during early prophase I. Paired homologues are often joined by the synaptonemal complex (figure 11.3). During synapsis, crossing over occurs between homologous chromosomes, exchanging chromosomal material. Because the homologues are paired, they move as a unit to the metaphase plate during metaphase I. During anaphase I, homologues of each pair are pulled to opposite poles, producing two cells that each have one complete set of chromosomes.Meiosis features two divisions with one round of DNA replication.Meiosis II is like mitosis but without replication of DNA. Sister chromatids are pulled to opposite poles to yield four haploid cells.11.3 The Process of Meiosis (figures 11.6 & 11.7)Prophase I sets the stage for the reductive division.Meiotic cells have an interphase period similar to mitosis with G1, S, and G2phases. This is followed by prophase I in which homologous chromosomes align along their entire length. The sister chromatids are held together by cohesin proteins. Homologues exchange chromosomal material by crossing over, which assists in holding the homologues together during meiosis I. The nuclear envelope disperses and the spindle apparatus forms.During metaphase I, paired homologues align.Spindle fibers attach to the kinetochores of the homologues; the kinetochores of sister chromatids behave as a single unit. Homologues of each pair become attached by kinetochore microtubules to opposite poles, and homologous pairs move to the metaphase plate as a unit. The orientation of each homologous pair on the equator is random; either the maternal or paternal homologue may be oriented toward a given pole.Anaphase I results from the differential loss of sister chromatid cohesion along the arms.During anaphase I the homologues of each pair are pulled to opposite poles as kinetochore microtubules shorten. Loss of sister chromatid cohesion on the arms but not at the centromeres allows homologues to separate, but sister chromatids to stay together. This is due to the loss of cohesin proteins on the arms but not at the centromere. At the end of anaphase I each pole has a complete set of haploid chromosomes, consisting of one member of each homologous pair. Because of the random orientation of homologous pairs at metaphase I, meiosis I results in the independent assortment of maternal and paternal chromosomes in gametes.Telophase I completes meiosis I.During telophase I the nuclear envelope re-forms around each daughter nucleus. This phase does not occur in all species. Cytokinesis may or may not occur after telophase I.Achiasmate segregation of homologues is possible.Although homologues are usually held together by chiasmata, some systems are able to segregate chromosomes without this.Meiosis II is like a mitotic division without DNA replication.A brief interphase with no DNA replication occurs after meiosis I. During meiosis II, cohesin proteins at the centromeres that hold sister chromatids together are destroyed, allowing each to migrate to opposite poles of the cell. The result of meiosis I and II is four cells, each containing haploid sets of chromosomes that are not identical. Once completed, the haploid cells may produce gametes or divide mitotically to produce even more gametes or haploid adults.Errors in meiosis produce aneuploid gametes.Errors occur during meiosis because of nondisjunction, the failure of chromosomes to move to opposite poles. It may result in aneuploid gametes: one gamete with no chromosome, and another gamete with two copies of a chromosome.11.4 Summing Up: Meiosis Versus MitosisFour distinct features of meiosis I are not found in mitosis: Maternal and paternal homologues pair, and exchange genetic information by crossing over; the kinetochores of sister chromatids function as a unit during meiosis I, allowingsister chromatids to cosegregate during anaphase I; kinetochores of sister chromatids are connected to a single pole in meiosis I and to opposite poles in mitosis; and DNA replication is suppressed between meiosis I and meiosis II.Homologous pairing is specific to meiosis.How homologues find each other during meiosis is not known. The proteins of the synaptonemal complex do not seem to be conserved in different species, but there are meiosis-specific cohesin proteins. These are involved in the differential destruction of cohesins on the arms versus the centromere during meiosis I. The recombination process that occurs between paired homologues is better known. This process uses proteins involved in DNA repair and startswith a double-stranded break in DNA.Sister chromatid cohesion is maintained through meiosis I but released in meiosis II.Shugoshin protein protects centromeric cohesin in anaphase I, so that sister chromatids remain connected. Cohesins on the arms are not protected and are thus degraded during anaphase I, allowing homologues to move to opposite poles.Sister kinetochores are attached to the same pole during meiosis I.Kinetochores of sister chromatids must be attached to the same spindle fibers (monopolar attachment) to segregate together.Replication is suppressed between meiotic divisions.Suppression of replication may be related to the maintenance of some cyclin proteins that are degraded at the end of mitosis.Meiosis produces cells that are not identical.Because of the independent assortment of homologues and the process of crossing over, gametes show great variation.Vocabulary:gamete - A haploid reproductive cell.somatic cell - Any of the cells of a multicellular organism except those that