In contrast to cohesin, which binds two sister chromatids together, condensin is thought to bind a single chromatid at multiple spots, twisting the chromatin into a variety of coils and loops Figure 3. During mitosis, chromosomes become attached to the structure known as the mitotic spindle. In the late s, Theodor Boveri created the earliest detailed drawings of the spindle based on his observations of cell division in early Ascaris embryos Figure 4; Satzinger, Boveri's drawings, which are amazingly accurate, show chromosomes attached to a bipolar network of fibers.
Boveri observed that the spindle fibers radiate from structures at each pole that we now recognize as centrosomes, and he also noted that each centrosome contains two small, rodlike bodies, which are now known as centrioles. Boveri observed that the centrioles duplicate before the chromosomes become visible and that the two pairs of centrioles move to separate poles before the spindle assembles.
We now know that centrioles duplicate during S phase, although many details of this duplication process are still under investigation. It is now well-established that spindles are bipolar arrays of microtubules composed of tubulin Figure 5 and that the centrosomes nucleate the growth of the spindle microtubules.
During mitosis, many of the spindle fibers attach to chromosomes at their kinetochores Figure 6 , which are specialized structures in the most constricted regions of the chromosomes. The length of these kinetochore-attached microtubules then decreases during mitosis, pulling sister chromatids to opposite poles of the spindle.
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. 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. Trisomy 21 Causes Down Syndrome. X Chromosome: X Inactivation. Chromosome Theory and the Castle and Morgan Debate.
Developing the Chromosome Theory. Meiosis, Genetic Recombination, and Sexual Reproduction. Mitosis and Cell Division. Genetic Mechanisms of Sex Determination. Sex Chromosomes and Sex Determination. Sex Chromosomes in Mammals: X Inactivation. Sex Determination in Honeybees. Citation: O'Connor, C. Cells are the basic building blocks of living things.
The human body is composed of trillions of cells, all with their own specialised function. DNA or deoxyribonucleic acid is a long molecule that contains our unique genetic code. Like a recipe book it holds the instructions for making all the proteins in our bodies.
Chromosomes are bundles of tightly coiled DNA located within the nucleus of almost every cell in our body. Humans have 23 pairs of chromosomes.
Meiosis is a process where a single cell divides twice to produce four cells containing half the original amount of genetic information. These cells are our sex cells — sperm in males, eggs in females. Cells divide and reproduce in two ways, mitosis and meiosis. Mitosis results in two identical daughter cells, whereas meiosis results in four sex cells. Below we highlight the keys differences and similarities between the two types of cell division.
If you have any other comments or suggestions, please let us know at comment yourgenome. Can you spare minutes to tell us what you think of this website? Open survey. In: Facts In the Cell.
During mitosis one cell divides once to form two identical cells. The major purpose of mitosis is for growth and to replace worn out cells. If not corrected in time, mistakes made during mitosis can result in changes in the DNA that can potentially lead to genetic disorders. Mitosis is divided into five phases: 1. Interphase: The DNA in the cell is copied in preparation for cell division, this results in two identical full sets of chromosomes.
Outside of the nucleus are two centrosomes, each containing a pair of centrioles, these structures are critical for the process of cell division. During interphase, microtubules extend from these centrosomes. Prophase: The chromosomes condense into X-shaped structures that can be easily seen under a microscope.
Each chromosome is composed of two sister chromatids, containing identical genetic information. The chromosomes pair up so that both copies of chromosome 1 are together, both copies of chromosome 2 are together, and so on. At the end of prophase the membrane around the nucleus in the cell dissolves away releasing the chromosomes. The mitotic spindle, consisting of the microtubules and other proteins, extends across the cell between the centrioles as they move to opposite poles of the cell.
Metaphase: The chromosomes line up neatly end-to-end along the centre equator of the cell.
0コメント