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. Prenatal Screen Detects Fetal Abnormalities. Synteny: Inferring Ancestral Genomes.
Telomeres of Human Chromosomes. Chromosomal Abnormalities: Aneuploidies. Chromosome Abnormalities and Cancer Cytogenetics. Copy Number Variation and Human Disease. Genetic Recombination. Human Chromosome Number. Laub et al. Arguably the best-characterized prokaryotic cell cycle is that of the model organism Caulobacter crescentus. One of the appealing features of this bacterium is that it has an asymmetric cell division that enables researchers to bind one of the two progeny to a microscope cover slip while the other daughter drifts away enabling further study without obstructions.
In this case, the cell-cycle progression goes hand in hand with the differentiation process giving readily visualized identifiable stages making them preferable to cell-cycle biologists over, say, the model bacterium E.
The behavior of mammalian cells in tissue culture has served as the basis for much of what we know about the cell cycle in higher eukaryotes. The eukaryotic cell cycle can be broadly separated into two stages, interphase, that part of the cell cycle when the materials of the cell are being duplicated and mitosis, the set of physical processes that attend chromosome segregation and subsequent cell division. The rates of processes in the cell cycle, are mostly built up from many of the molecular events such as polymerization of DNA and cytoskeletal filaments whose rates we have already considered.
The stage most variable in duration is G1. In less favorable growth conditions when the cell cycle duration increases this is the stage that is mostly affected, probably due to the time it takes until some regulatory size checkpoint is reached. Though different types of evidence point to the existence of such a checkpoint, it is currently very poorly understood. Historically, stages in the cell cycle have usually been inferred using fixed cells but recently, genetically-encoded biosensors that change localization at different stages of the cell cycle have made it possible to get live-cell temporal information on cell cycle progression and arrest.
Figure 2: Cell cycle times for different cell types. Each pie chart shows the fraction of the cell cycle devoted to each of the primary stages of the cell cycle. The area of each chart is proportional to the overall cell cycle duration. Cell cycle durations reflect minimal doubling times under ideal conditions. The proteins of the kinetochore attract and bind mitotic spindle microtubules.
During prometaphase , the nuclear envelope is fully broken down and chromosomes are attached to microtubules from both poles of the mitotic spindle, which begin to move them toward the middle of the cell.
Figure 3. Once a mitotic fiber attaches to a chromosome, the chromosome will be oriented until the kinetochores of sister chromatids face the opposite poles. Eventually, all the sister chromatids will be attached via their kinetochores to microtubules from opposing poles. During metaphase , all the chromosomes are aligned in a plane called the metaphase plate , or the equatorial plane, midway between the two poles of the cell.
At this time, the chromosomes are maximally condensed. During anaphase , the sister chromatids separate at the centromere. Each chromatid, now called a chromosome, is pulled rapidly toward the centrosome to which its microtubule is attached. The cell becomes visibly elongated oval shaped as the polar microtubules slide against each other at the metaphase plate where they overlap. During telophase , the chromosomes reach the opposite poles and begin to decondense unravel , relaxing into a chromatin configuration.
Nuclear envelopes form around the chromosomes, and nucleosomes appear within the nuclear area. The activity below will walk you through mitosis—providing you with the chance to review the different steps of the process and how they work together. Click here for a text-only version of the activity. Cytokinesis is the second main stage of the mitotic phase during which cell division is completed via the physical separation of the cytoplasmic components into two daughter cells.
Division is not complete until the cell components have been apportioned and completely separated into the two daughter cells. Although the stages of mitosis are similar for most eukaryotes, the process of cytokinesis is quite different for eukaryotes that have cell walls, such as plant cells.
In cells such as animal cells that lack cell walls, cytokinesis follows the onset of anaphase. A contractile ring composed of actin filaments forms just inside the plasma membrane at the former metaphase plate.
The actin filaments pull the equator of the cell inward, forming a fissure. The furrow deepens as the actin ring contracts, and eventually the membrane is cleaved in two Figure 4.
Figure 4. During cytokinesis in animal cells, a ring of actin filaments forms at the metaphase plate. The ring contracts, forming a cleavage furrow, which divides the cell in two. In plant cells, Golgi vesicles coalesce at the former metaphase plate, forming a phragmoplast.
0コメント