Meiosis: Stages, Processes & Genetic Diversity PDF

Slide 3: Meiosis goes through two phases: meiosis I and meiosis II. In meiosis I, the homologous chromosomes are separated, and cells go from diploid to haploid. In meiosis II (which is more similar to mitosis), the sister chromatids are separated. Each step of meiosis (I and II) has its own prophas...

Slide 3: Meiosis goes through two phases: meiosis I and meiosis II. In meiosis I, the homologous chromosomes are separated, and cells go from diploid to haploid. In meiosis II (which is more similar to mitosis), the sister chromatids are separated. Each step of meiosis (I and II) has its own prophase, prometaphase, metaphase, anaphase, and telophase. In meiosis I, the kinetochores of sister chromatids point in the same direction; however, in meiosis II, the kinetochores of sister chromatids point in the opposite direction. Why? In meiosis I, sister chromatids stay together, move together in the same direction, whereas in meiosis II, the chromatids (and their kinetochores) separate. Slide 4: Gametic, zygotic, and sporic lifecycles. Humans have a gametic lifecycle; meiosis forms haploid gametes; however, most human cells are diploid. Fungi have a zygotic lifecycle. The fungus is typically a haploid organism, and meiosis restores the haploid form after a diploid zygote is formed as a result of fertilization. Plants have a sporic lifecycle, in which a diploid sporophyte form undergoes meiosis and forms haploid gametophyte form. Slide 5: What are the differences between mitosis and meiosis? Mitosis duplicates cells and maintains the number of chromosomes. Meiosis generates haploid cells from diploid cells, and this process in humans is used only for the making of gametes (i.e., the sperm and eggs). We will first review mitosis. Then we will do a side-by-side comparison between mitosis and meiosis, concentrating on the chromosome number and the DNA content of the cell. Slide 6: Mitosis. The human cell is a diploid cell with 46 chromosomes. There are two sets of homologous chromosomes: 2 x 23 = 46. This is a DNA content of 2N, where N equals the number of one set of homologous chromosomes. After the S phase, DNA content is 4N*. The asterisk indicates that the DNA is replicated, and there are two sister chromatids per chromosome. Mitosis restores the original 2N condition, and as a result, there are two 2N cells. Note that cells are diploid (with 46 chromosomes) throughout mitosis. In meiosis, the cell starts again with a 2N DNA content that as a result of the S phase, duplicates to 4N*. Meiosis I separates the homologous chromosomes; therefore, the 4N* diploid cell with 46 chromosomes transitions to two haploid cells with 2N* DNA content of 23 chromosomes. Note that the asterisk is there to indicate that the chromosomes are still duplicated (i.e., each chromosome has two chromatids). Meiosis II separates the sister chromatids, resulting in 4 haploid cells with 1N DNA content. Slide 7: Meiosis forms gametes in humans. From one spermatogonium, four haploid sperm cells form. From one oogonium, only one haploid egg is obtained, as the other cells become polar bodies that disintegrate. Slide 8: Genetic diversity in meiosis is produced by (1) the independent assortment and (2) recombination (crossing over). In the anaphase of meiosis I, the homologous chromosomes can separate in every possible combination: 2 to the Nth power (2N) gives the number of combinations, where N is the number of pairs of homologous chromosomes. In humans, N = 23; therefore, there are approximately 8 million possible combinations of how chromosomes might separate. Recombination (crossing over) allows the homologous chromosomes to exchange fragments; as a result, each chromosome becomes a chimera of the original. Slide 9: Recombination takes place during the elongated prophase of meiosis I: it starts in leptotene of prophase when nicks are made in zygotene, a synaptonemal complex forms and the exchange of fragments begins, and this process continues through pachytene  the exchange of fragments ends at diplotene when the points of crossing over are seen as a chiasma in diakinesis, the nuclear envelope breaks down, and the spindle forms. Slide 10: The synaptonemal complex facilitates the crossing-over; however, it is not required. It consists of chromatin, axial elements, cohesin, and various factors that enable recombination. Slide 11: A nondisjunction is an abnormal event in meiosis. It yields daughter cells with the wrong number of chromosomes (aneuploidy). If this happens in meiosis I, then four abnormal gametes form, two with an extra chromosome, two missing a chromosome. If it happens in meiosis II, two gametes are normal, one has an extra chromosome, one is missing a chromosome. Keep in mind there are four gametes only when sperm is formed. When an egg is formed, three cells become polar bodies. Therefore, let us discuss the probabilities. As a result of nondisjunction of meiosis I, there is a 50% chance of an egg with an extra chromosome and a 50% chance of missing a chromosome. Nondisjunction occurs most frequently in meiosis I of egg formation in humans and is associated with the age of the mother (e.g., Down’s Syndrome). The reason is that the human egg spends most of its lifetime in the prophase of meiosis I – in crossing over. It finishes meiosis after menstruation/fertilization. The older the mother, the longer the egg has been “locked” in crossing over, and the greater the chance of something going wrong. Slide 12: Here is the current model of meiotic recombination. It is initiated by a double-strand break or gap, followed by pairing with a homologous chromosome and strand invasion to initiate the recombinational repair process. Repair of the gap can lead to crossover (CO) or non- crossover (NCO) of the flanking regions. CO recombination is thought to occur by the Double Holliday Junction (DHJ) model, illustrated on the right. NCO recombinants are thought to occur primarily by the Synthesis Dependent Strand Annealing (SDSA) model, illustrated on the left. Most recombination events appear to be the SDSA type. Slide 13: Fundamental points. Meiosis goes through defined steps in two phases: Meiosis I and Meiosis II. Meiosis I separates homologous chromosomes making haploid daughter cells. Meiosis II, similar to mitosis, separates the sister chromatids. There are the gametic, zygotic, and sporic lifecycles, with differences in the role of meiosis in each. In humans (gametic), meiosis is required to make sperm and eggs (gametes). Can track chromosome numbers, DNA content, and diploid/haploid status, comparing mitosis and meiosis. Independent assortment and recombination create genetic diversity in meiosis. Nondisjunction results from errors in meiosis.