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Mitosis, Meiosis, and Karyotype Analysis

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Figures (6)

Figure 10.5 The cell cycle in multicellular organisms consists of interphase and the mitotic phase. During interphase, the cell grows and the nuclear DNA is duplicated. Interphase is followed by the mitotic phase. During the mitotic phase, the duplicated chromosomes are segregated and distributed into daughter nuclei. Following mitosis, the cytoplasm is usually divided as well by cytokinesis, resulting in two genetically identical daughter cells.
Figure 10.6 G2 of Interphase – The last stage of interphase is the second gap period, G2. During this stage, cells grow, replenish energy and synthesize needed macromolecules, such as proteins and lipids. Mitosis - When G2 is complete, the cell will enter mitosis. Although there are 5 phases in mitosis, with the exception of the metaphase to anaphase transition, these phases are not discrete and happen as a continuous process. Prophase is the first stage in mitosis. The nuclear envelope begins to break down and chromosomes condense and are now visible. Spindle fibers start to appear and centrosomes begin to move towards opposite poles. Prometaphase - Chromosomes continue to condense and are more visible. Kinetochores appear at the centromere and kinetochore microtubules attach. Centrosomes continue to move towards opposite poles. Metaphase – The mitotic spindle is fully developed and centrosomes are at opposite poles. Chromosomes are aligned at the “equatorial plate”, and each sister chromatid rests on one side of the plate, with spindle fibers attached to them. Anaphase - Sister chromatids are pulled apart by spindle fibers and are separated from each other. Each chromatid is now a chromosome. Telophase and Cytokinesis - Chromosomes arrive at opposite poles and start to decondense and become less visible. The nuclear envelope reassembles and begins to surround each new set of chromosomes. The mitotic spindle assembly breaks down and the division of the cytoplasm begins via cytokinesis. This physical separation into two cells are remarkably different processes in plant and animal cells. Credit: Rao, A., Hawkins, A.and Fletcher, S. Department of Biology, Texas A&M University.
Figure 10.7 During prometaphase, mitotic spindle microtubules from opposite poles attach to each sister chromatid at the kinetochore. In anaphase, the connection between the sister chromatids breaks down, and the microtubules pull the chromosomes toward opposite poles.
Figure 10.8 Mitotic Cell at Metaphase. The microtubule spindle has completed the alignment of chromosomes at the metaphase plate in preparation for the separation of sister chromatids during anaphase. Credit: Fletcher, S., Ryan, K. and Rao, A., Department of Biology, Texas A&M University.
Figure 10.9 Animal cell cleavage - Contraction of actin filaments by myosin pull the “equator” of the cell causing an invagination of the cell (a fissure called a cleavage furrow). The cleavage furrow gets continuously deeper until it eventually divides the cell into two new, independent daughter cells. Plant cell wall formation - A mixture of enzymes, proteins, and glucose molecules are transported via vesicles to the center of the cell. These vesicles continuously build upon each other until a completely new cell wall has emerged and two new cells are formed, independent of one another. Credit: Rao, A., Ryan, K., Hawkins, A.and Fletcher, S. Department of Biology, Texas A&M University.
Figure 10.10 Slowly scan whitefish blastula cells with the high-power objective as illustrated in image (a) to identify their mitotic stage. (b) A microscopic image of the scanned cells is shown. (credit “micrograph”: modification of work by Linda Flora; scale-bar data from Matt Russell)

Key Points

  • Mitosis is a process of equal cell division, where each of the new cells receives the same number of chromosomes as the original cell.
  • Meiosis Syngamy is the way for organisms to become more genetically diverse, but since it increases the amount of chromosomes, it needs to be balanced by meiosis.
  • Perhaps the most well­known example of a chromosome problem is Down syndrome, in which cells have an extra copy of chromosome 21 (see page 11).
  • Prophase is the longest, nucleus disintegrates (except some protists like fungi) and the DNA is super-spiralized into chromosomes ("archived").
  • Metaphase (the “change phase”): All the chromosomes are aligned in a plane called the metaphase plate, or the equatorial plane, roughly midway between the two poles of the cell.
  • Anaphase lag occurs when the movement of one chromatid is impeded during anaphase.

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