Mitosis

Somatic cells divide by mitosis, which produces two nearly identical daughter cells, each containing the same number of chromosomes as the original cell (Figure 3.3). Prior to the onset of mitosis, the chromosomes within the nuclear membrane are not visible by light microscopy, because they are not condensed. Cells not actively undergoing mitosis are in the interphase state.

The cell cycle consists of a coordinated set of processes by which a cell replicates all its components and is divided into two nearly identical daughter cells. The coordination of cell growth and periodic chromosome replication and division during the cell cycle has important implications for understanding the evolution of cells, development, and for human medicine (Edgar and Lehner 1996, Novak et al. 1997, Dobie et al. 1999, Zachariae 1999, Zhang 1999).

The cell cycle consists of four phases: Gi ^ S ^ G2 ^ M. DNA synthesis and chromosome duplication take place during the portion of the cell cycle called the S phase (for synthesis), but does not occur during the G1 and G2 phases of the cell cycle. The M phase represents mitosis, in which the duplicated chromosomes and the cytoplasm are divided into two daughter cells. G1 is the gap between mitosis and DNA synthesis (S),

Cell Interphase

Figure 3.3. Mitosis of a diploid cell involves duplication of each homologous chromosome and its distribution to the daughter cells. DNA replication occurs during interphase. During prophase the two daughter chromatids are attached to each other at the centromere. During metaphase, the chromosomes line up on the metaphase plate, and during anaphase the daughter chromatids separate and begin moving to opposite poles. During telophase the nuclear membranes reform, and two identical daughter cells with a complete complement of chromosomes have been produced.

Figure 3.3. Mitosis of a diploid cell involves duplication of each homologous chromosome and its distribution to the daughter cells. DNA replication occurs during interphase. During prophase the two daughter chromatids are attached to each other at the centromere. During metaphase, the chromosomes line up on the metaphase plate, and during anaphase the daughter chromatids separate and begin moving to opposite poles. During telophase the nuclear membranes reform, and two identical daughter cells with a complete complement of chromosomes have been produced.

whereas G2 is the gap between S and mitosis (M). The length of a cell cycle varies by cell type, but typically lasts approximately 18 to 24 hours, with the process of mitosis requiring 0.5 to 2 hours (Figure 1.14).

Mitosis is divided into four main stages: prophase, metaphase, anaphase, and telophase (Figure 3.3). In prophase, the nuclear envelope is still intact and each chromosome condenses to form two visible, thin threads (chromatids) within the nucleus. Because chromosome duplication occurred in the S phase, each chromosome consists of two chro-matids connected at the centromere. The centromere is the attachment point for the spindle fibers that will draw each of the newly divided chromosomes into their respective nuclei later in mitosis. In late prophase, the nuclear membrane disappears and a mitotic spindle begins to form.

During prometaphase, the spindle develops. The spindle is a complex structure consisting of centrosomes (two centrioles oriented at right angles to one another) and microtubules (hollow protein cylinders consisting of tubulin). The two bundles of fibers extend between the opposite poles of the cell and attach to the centromere of each chromosome (Wolf 1995, Gonzalez et al. 1998). Then, the chromosomes move toward the center of the cell in a plane equidistant from the spindle poles. By the end of metaphase, the duplicated chromosomes are lined up on the metaphase plate and are at their most condensed stage, making it easy to examine them for differences in morphology.

During the next stage, anaphase, the centromeres divide; the two sister chromatids now have their own centromeres, and so have become independent chromosomes. These newly separated chromosomes move toward the opposite poles. At the end of anaphase, a complete set of chromosomes lies near each opposite pole.

During telophase, the chromosomes have reached the spindle poles and the cleavage furrow within the cytoplasm has become visible. The nuclear membrane reforms around each group of chromosomes, the chromosomes decondense, cleavage progresses, and the spindle disappears. The mitochondria often align parallel to the spindle, which may guarantee that they are distributed to both daughter cells. The cytoplasm is divided by a gradually deepening furrow, and a new cell membrane forms. If all has gone well, the result should be the formation of two nearly identical cells with perfectly duplicated genetic information in the nucleus and in the mitochondria within the cytoplasm.

Check points occur during the cell cycle to ensure that the genetic information is duplicated perfectly. During the check points, the genetic material is monitored for integrity and status of replication before the cells commit either to replicate the DNA during S phase, or to segregate it during mitosis (Elledge 1996). If the cell cycle were not well regulated, the cell would be subject to genetic instability or death.

The cell cycle is regulated by protein complexes consisting of cyclins and cyclin-dependent protein kinases (King et al. 1996, Stillman 1996, Piwnica-Worms 1999). The checkpoints involve signal-transduction pathways whose effectors interact with the cyclin/cyclin-dependent protein kinases to block the cell cycle. Blocking the cell cycle allows time for repair of damage at G1 (before DNA replication) or just before mitosis at the G2 DNA-damage checkpoint.

Chromosome replication takes place during the S phase, and the duplicated chromosomes remain physically connected (as sister chromatids) until anaphase of mitosis. The cohesion of the sister chromatids is what permits chromosome segregation to take place long after duplication, and this cohesion is due to a multisubunit complex called cohesin. Cleavage of one of cohesin's subunits appears to trigger separation of the sister chromatids at the onset of anaphase (Nasmyth et al. 2000). The ability of eukary-otic cells to delay segregation of the replicated chromosomes until long after they have been duplicated distinguishes the eukaryotic cell cycle from that of bacteria, in which chromosome segregation starts immediately after DNA replication is initiated. The separation of chromosome duplication and segregation has played a central role in the evolution of eukaryotic organisms (Nasmyth et al. 2000). Mitotic chromosome condensation, without which large genomes cannot be partitioned between daughter cells at cell division, would not be possible if chromosome segregation coincided with DNA replication. The gap (G2) between S and M phases in the cell cycle thus makes possible the evolution of large genomes that can be transmitted safely to daughter cells.

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Responses

  • Janina
    Why is dna replication not possible in prophase?
    5 years ago
  • Brigitte Gerber
    Why are drosophila mitotic visible?
    5 years ago
  • meneaduc
    What happen during prophase mitosis?
    5 years ago
  • kyllikki
    WHAT STAGE OF MITOSIS INVOLVES THE FORMATION OF CHROMOSOMES?
    5 years ago
  • annett
    What happens in G1 phase of a diploid mitosis?
    4 years ago
  • elanor
    What occurs during the g2 phase of interphase?
    4 years ago

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