Other spindle fibers do not attach to chromosomes, but instead form a scaffold that provides mechanical force to separate the daughter nuclei at the end of mitosis. From his many detailed drawings of mitosen, Walther Flemming correctly deduced, but could not prove, the sequence of chromosome movements during mitosis Figure 7.
Flemming divided mitosis into two broad parts: a progressive phase, during which the chromosomes condensed and aligned at the center of the spindle, and a regressive phase, during which the sister chromatids separated. Our modern understanding of mitosis has benefited from advances in light microscopy that have allowed investigators to follow the process of mitosis in living cells. Such live cell imaging not only confirms Flemming's observations, but it also reveals an extremely dynamic process that can only be partially appreciated in still images.
Mitosis begins with prophase, during which chromosomes recruit condensin and begin to undergo a condensation process that will continue until metaphase. In most species , cohesin is largely removed from the arms of the sister chromatids during prophase, allowing the individual sister chromatids to be resolved. Cohesin is retained, however, at the most constricted part of the chromosome, the centromere Figure 9.
During prophase, the spindle also begins to form as the two pairs of centrioles move to opposite poles and microtubules begin to polymerize from the duplicated centrosomes. Prometaphase begins with the abrupt fragmentation of the nuclear envelope into many small vesicles that will eventually be divided between the future daughter cells.
The breakdown of the nuclear membrane is an essential step for spindle assembly. Because the centrosomes are located outside the nucleus in animal cells, the microtubules of the developing spindle do not have access to the chromosomes until the nuclear membrane breaks apart.
Prometaphase is an extremely dynamic part of the cell cycle. Microtubules rapidly assemble and disassemble as they grow out of the centrosomes, seeking out attachment sites at chromosome kinetochores, which are complex platelike structures that assemble during prometaphase on one face of each sister chromatid at its centromere. As prometaphase ensues, chromosomes are pulled and tugged in opposite directions by microtubules growing out from both poles of the spindle, until the pole-directed forces are finally balanced.
Sister chromatids do not break apart during this tug-of-war because they are firmly attached to each other by the cohesin remaining at their centromeres. At the end of prometaphase, chromosomes have a bi-orientation, meaning that the kinetochores on sister chromatids are connected by microtubules to opposite poles of the spindle. Next, chromosomes assume their most compacted state during metaphase, when the centromeres of all the cell's chromosomes line up at the equator of the spindle.
Metaphase is particularly useful in cytogenetics , because chromosomes can be most easily visualized at this stage. Furthermore, cells can be experimentally arrested at metaphase with mitotic poisons such as colchicine.
Video microscopy shows that chromosomes temporarily stop moving during metaphase. A complex checkpoint mechanism determines whether the spindle is properly assembled, and for the most part, only cells with correctly assembled spindles enter anaphase. Figure 10 Figure Detail. Figure 9. The progression of cells from metaphase into anaphase is marked by the abrupt separation of sister chromatids.
A major reason for chromatid separation is the precipitous degradation of the cohesin molecules joining the sister chromatids by the protease separase Figure Two separate classes of movements occur during anaphase. During the first part of anaphase, the kinetochore microtubules shorten, and the chromosomes move toward the spindle poles.
During the second part of anaphase, the spindle poles separate as the non-kinetochore microtubules move past each other. These latter movements are currently thought to be catalyzed by motor proteins that connect microtubules with opposite polarity and then "walk" toward the end of the microtubules.
Mitosis ends with telophase, or the stage at which the chromosomes reach the poles. The nuclear membrane then reforms, and the chromosomes begin to decondense into their interphase conformations.
Telophase is followed by cytokinesis, or the division of the cytoplasm into two daughter cells. The daughter cells that result from this process have identical genetic compositions. Cheeseman, I. Molecular architecture of the kinetochore-microtubule interface. Nature Reviews Molecular Cell Biology 9 , 33—46 doi Cremer, T. Chromosome territories, nuclear architecture and gene regulation in mammalian cells. Nature Reviews Genetics 2 , — doi Hagstrom, K.
Condensin and cohesin: More than chromosome compactor and glue. Nature Reviews Genetics 4 , — doi Hirano, T. At the heart of the chromosome: SMC proteins in action. Nature Reviews Molecular Cell Biology 7 , — doi Mitchison, T. Mitosis: A history of division.
Nature Cell Biology 3 , E17—E21 doi Paweletz, N. Walther Flemming: Pioneer of mitosis research. Nature Reviews Molecular Cell Biology 2 , 72—75 doi Satzinger, H. Theodor and Marcella Boveri: Chromosomes and cytoplasm in heredity and development.
Nature Reviews Genetics 9 , — doi Chromosome Mapping: Idiograms. Human Chromosome Translocations and Cancer. Karyotyping for Chromosomal Abnormalities. Telophase: At each pole of the cell a full set of chromosomes gather together. A membrane forms around each set of chromosomes to create two new nuclei. The single cell then pinches in the middle to form two separate daughter cells each containing a full set of chromosomes within a nucleus.
This process is known as cytokinesis. Related Content:. What is a stem cell? What is a cell? What is DNA? What is a chromosome? What is a genetic disorder? What is meiosis? Mitosis versus meiosis. How helpful was this page? What's the main reason for your rating? Which of these best describes your occupation? What is the first part of your school's postcode?
How has the site influenced you or others? Thankyou, we value your feedback! The mitotic spindle extends from the poles and attaches to the kinetochores. During metaphase, the microtubules pull the sister chromatids back and forth until they align in a plane, called the equatorial plane, along the center of the cell. During anaphase, the sister chromatids are separated simultaneously at their centromeres.
The separated chromosomes are then pulled by the spindle to opposite poles of the cell. Anaphase ensures that each daughter cell receives an identical set of chromosomes. Finally, during telophase, a nuclear membrane forms around each set of chromosomes to separate the nuclear DNA from the cytoplasm. The chromosomes begin to uncoil, which makes them diffuse and less compact. Along with telophase, the cell undergoes a separate process called cytokinesis that divides the cytoplasm of the parental cell into two daughter cells.
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