The resulting cells, known as daughter cells, each enter their own interphase and begin a new round of the cell cycle. Cell cycle is the name we give the process through which cells replicate and make two new cells.
Cell cycle has different stages called G1, S, G2, and M. G1 is the stage where the cell is preparing to divide. In many situations, division also ensures that new cells are available to replace the older cells within an organism whenever those cells die.
Prokaryotic cells, which include bacteria, undergo a type of cell division known as binary fission. This process involves replication of the cell's chromosomes, segregation of the copied DNA, and splitting of the parent cell's cytoplasm. The outcome of binary fission is two new cells that are identical to the original cell. In contrast to prokaryotic cells, eukaryotic cells may divide via either mitosis or meiosis.
Of these two processes, mitosis is more common. In fact, whereas only sexually reproducing eukaryotes can engage in meiosis, all eukaryotes — regardless of size or number of cells — can engage in mitosis. But how does this process proceed, and what sorts of cells does it produce? During mitosis, a eukaryotic cell undergoes a carefully coordinated nuclear division that results in the formation of two genetically identical daughter cells. Mitosis itself consists of five active steps, or phases: prophase, prometaphase, metaphase, anaphase, and telophase.
Before a cell can enter the active phases of mitosis, however, it must go through a period known as interphase , during which it grows and produces the various proteins necessary for division. Then, at a critical point during interphase called the S phase , the cell duplicates its chromosomes and ensures its systems are ready for cell division. If all conditions are ideal, the cell is now ready to move into the first phase of mitosis.
This page appears in the following eBook. Aa Aa Aa. Walther Flemming's drawing of chromosomes. What happens during mitosis? Figure 1: During prophase, the chromosomes in a cell's nucleus condense to the point that they can be viewed using a light microscope.
Prophase is the first phase of mitosis. During this phase, the chromosomes inside the cell's nucleus condense and form tight structures. Illustration showing the five stages of mitosis. Image credit: Genome Research Limited. A stem cell is a cell with the unique ability to develop into specialised cell types in the body. In the future they may be used to replace cells and tissues that have been damaged or lost due to disease. 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.
Cohesin is a member of the SMC, or structural maintenance of chromosomes, family of proteins. SMC proteins are DNA-binding proteins that affect chromosome architectures; indeed, cells that lack SMC proteins show a variety of defects in chromosome stability or chromosome behavior. At the end of S phase, cells are able to sense whether their DNA has been successfully copied, using a complicated set of checkpoint controls that are still not fully understood.
For the most part, only cells that have successfully copied their DNA will proceed into mitosis. The most obvious difference between interphase and mitosis involves the appearance of a cell 's chromosomes. During interphase, individual chromosomes are not visible, and the chromatin appears diffuse and unorganized. Like cohesin, condensin is an elongated complex of several proteins that binds and encircles DNA. 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
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