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Mitosis Maintains the Parental Chromosome Number

We now return to the overall process of mitosis. Mitosis occurs in several stages (Figure 7-15). During prophase, the chromosomes condense into the highly compact form required for segregation. At the end of prophase, the nuclear envelope breaks down and the cell enters metaphase.

During metaphase, the mitotic spindle forms and the k i net o chores of sister Chromatids attach to the microtubules. Proper chromatid attachment is only achieved when the two kinetochores of a sister-chromatid pair are attached to microtubules emanating from opposite microtubule organizing centers. This type of attachment is called bivalent attachment (see Figure 7-15] and results in the microtubules exerting tension on the chromatid pair by pulling the sisters in opposite directions. Attachment of both chromatids to microtubules emanating from the same microtubule organizing center or attachment of only one chromatid of the pair, called monovalent attachment, does not result in tension and eventually leads to chromosome loss. The tension exerted by bivalent attachment is opposed by sister chromatid cohesion and results in all the chromosomes aligning in the middle of the cell between the two microtubule organizing centers (this position is called the metaphase plate). At this point, each sister chromatid is prepared to be segregated.

Chromosome segregation is triggered by proteolytic destruction of the cohesin molecules, resulting in the loss of sister chromatid cohesion. This loss occurs as cells enter anaphase, during which the sister chromatids separate and move to opposite sides of the cell. Once the two sisters are no longer held together, they cannot resist the outward pull of the microtubule spindle. Bivalent attachment ensures that the members of a sister-chromatid pair are pulled toward opposite poles and each daughter cell receives one copy of each duplicated chromosome.

The final step of mitosis is telophase, during which the nuclear envelope reforms around each set of segregated chromosomes. At this point, cell division can be completed by physically separating the shared cytoplasm of the two presumptive cells in a process called cytokinesis.

The Gap Phases of the Cell Cycle Allow Time to Prepare for the Next Cell Cycle Stage while also Checking that the Previous Stage Is Finished Correctly

The remaining two phases of the eukaryotic cell cycle are gap phases. Cl occurs prior to DNA synthesis and G2 between S phase and M phase. The gap phases of the cell cycle serve two purposes. They provide time for the celt to prepare for the next phase of the cell cycle and to check that the previous phase of the cell cycle has been completed appropriately. For example, prior to entry into S phase, most cells must reach a certain size and level of protein synthesis to ensure that there will be adequate proteins and nutrients to complete the next round of DNA synthesis. If there is a problem with a previous step in the cell cycle, cell cycle checkpoints arresl the cell cycle to provide time for the cell to complete that step. For example, cells with damaged DNA arrest the cell cycle in Gl before DNA synthesis or in G2

microti! Nes

replicating chromosomes membrane microti! Nes organizing center rings telophase metaphase anaphase cytokinesis

attachment broken cohesion rings attachment daughter cell daughter cell interphase prophase telophase cytokinesis daughter cell daughter cell metaphase anaphase attachment broken cohesion rings attachment

FIGURE 7-15 Mitosis in detail.

Prior to mitosis, the chromosomes are in a deconcJensed state called interphase. During prophase chromosomes are condensed and de-iangled in preparation for segregation and the nuclear membrane surrounding the chromosomes breaks down ;ri most eukaryotes. During metaphase, each sister-chromatid pair attaches to opposite poles of the mitotic spindle. Anaphase is initiated by the loss of sister chromatid cohesion resulting tn the separation of sister chromatids. Telophase is distinguished by the loss of chromosome condensation and the reformation of the nuclear membrane around the two populations of segregated chromosomes. Cytokinesis is the final event of the cell cycle during which the cellular membrane surrounding the two nudeĀ« constricts and eventually completely separates into two daughter cells. At! DMA molecules are double-stranded.

before: mitosis to prevent either event from occurring with damaged chromosomes. This delay allows time for the damage to be repaired before the cell cycle continues.

Meiosis Reduces the Parental Chromosome Number

A second type of eukaryotic cell division is specialized to produce cells that have half the number of chromosomes than the parental cell. Like the mitotic cell cycle, the meiotic cell cycle includes a Gl. S, and an elongated G2 phase (Figure 7-16). During the meiotic S phase, each chromosome is replicated and the daughter chromatids remain associated as in the mitotic S phase. Cells that enter meiosis must be diploid and thus contain two copies of each chromosome, one derived from each parent. After DNA replication, these related sister-chromatid pairs, called homologs, pair with one another and recombine. Recombination between the homologs creates a physical linkage between the two homologs that is required to connect the two related sister-chro-matid pairs during chromosome segregation. We will discuss the details of meiotic recombination in Chapter 10.

The most significant difference between the mitotic and meiotic cell cycles occurs during chromosome segregation. Unlike mitosis, during which there is a single round of chromosome segregation, chromosomes participating in meiosis go through two rounds of segregation known as meiosis I and D. bike mitosis, each of these segregation events includes a prophase, m eta phase, and anaphase stage. During the metaphase of meiosis I, &Isq called metaphase 1, the homologs attach to opposite poles of the microti! bule-based spindle. This attachment is mediated by the kinetochore. Because both kinetochores of each sister-chromatid pair are attached to the same pole of the microtubule spindle, this interaction is referred to as monovalent attachment (in contrast to the bivalent attachment seen in mitosis, in which the kinetochores of each sister-chromatid pair bind to opposite poles of the spindle). As in mitosis, the paired homologs initially resist the tension of the spindle pulling them apart. In the case of meiosis I, this is mediated through the physical connections between the homologs, or crossovers, that are induced by recombination. This resistance also requires sister-chromatid cohesion along the arms of the sister chromatids. When cohesion along the arms is eliminated during anaphase I, the homologs are released from one another and segregate to opposite poles of the cell. Importantly, die cohesion between the sisters is maintained near the centromere, resulting in the sister chromatids remaining paired.

The second round of segregation during meiosis, meiosis II, is very similar to mitosis. The major difference is that a round of DNA replication does not precede this segregation event, instead, a spindle is formed in association with each of the two newly separated sister chromatid pairs. As in mitosis, during metaphase II, these spindles attach in a bivalent manner to the kinetochores of each sister-chromatid pair. The cohesion that remains at the centromeres after meiosis I is critical to oppose the pull of the spindle. As in mitosis, anaphase II is initiated by the elimination of centromere cohesion. At this point there are four sets of chromosomes in the cell, each of which contains only one copy of each chromosome. A nucleus forms around each set of chromosomes, and then the cytoplasm is divided to form four haploid cells. These cells are now ready to mate to form new diploid cells.

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