Is the Cell Cycle is One Continuous Process
The most basic function of the cell cycle is to duplicate accurately the vast amount of DNA in the chromosomes and then segregate the copies precisely into two genetically identical daughter cells. These processes define the two major phases of the cell cycle. DNA duplication occurs during S phase (S for synthesis), which requires 10–12 hours and occupies about half of the cell-cycle time in a typical mammalian cell. After S phase, chromosome segregation and cell division occur in M phase (M for mitosis), which requires much less time (less than an hour in a mammalian cell). M phase involves a series of dramatic events that begin with nuclear division, or mitosis. As discussed in detail in Chapter 18, mitosis begins with chromosome condensation: the duplicated DNA strands, packaged into elongated chromosomes, condense into the much more compact chromosomes required for their segregation. The nuclear envelope then breaks down, and the replicated chromosomes, each consisting of a pair of sister chromatids, become attached to the microtubules of the mitotic spindle. As mitosis proceeds, the cell pauses briefly in a state called metaphase, when the chromosomes are aligned at the equator of the mitotic spindle, poised for segregation. The sudden separation of sister chromatids marks the beginning of anaphase, during which the chromosomes move to opposite poles of the spindle, where they decondense and reform intact nuclei. The cell is then pinched in two by cytoplasmic division, or cytokinesis, and cell division is complete (Figure 17-2).
Figure 17-2
The events of eucaryotic cell division as seen under a microscope. The easily visible processes of nuclear division (mitosis) and cell division (cytokinesis), collectively called M phase, typically occupy only a small fraction of the cell cycle. The other, (more...)
Most cells require much more time to grow and double their mass of proteins and organelles than they require to replicate their DNA and divide. Partly to allow more time for growth, extra gap phases are inserted in most cell cycles—a G 1 phase between M phase and S phase and a G 2 phase between S phase and mitosis. Thus, the eucaryotic cell cycle is traditionally divided into four sequential phases: G1, S, G2, and M (Figure 17-3). G1, S, and G2 together are called interphase. In a typical human cell proliferating in culture, interphase might occupy 23 hours of a 24 hour cycle, with 1 hour for M phase.
Figure 17-3
The phases of the cell cycle. The cell grows continuously in interphase, which consists of three phases: DNA replication is confined to S phase; G1 is the gap between M phase and S phase, while G2 is the gap between S phase and M phase. In M phase, the (more...)
The two gap phases serve as more than simple time delays to allow cell growth. They also provide time for the cell to monitor the internal and external environment to ensure that conditions are suitable and preparations are complete before the cell commits itself to the major upheavals of S phase and mitosis. The G1 phase is especially important in this respect. Its length can vary greatly depending on external conditions and extracellular signals from other cells. If extracellular conditions are unfavorable, for example, cells delay progress through G1 and may even enter a specialized resting state known as G 0 (G zero), in which they can remain for days, weeks, or even years before resuming proliferation. Indeed, many cells remain permanently in G0 until they or the organism dies. If extracellular conditions are favorable and signals to grow and divide are present, cells in early G1 or G0 progress through a commitment point near the end of G1 known as Start (in yeasts) or the restriction point (in mammalian cells). After passing this point, cells are committed to DNA replication, even if the extracellular signals that stimulate cell growth and division are removed.
The Cell-Cycle Control System Is Similar in All Eucaryotes
Some features of the cell cycle, including the time required to complete certain events, vary greatly from one cell type to another, even in the same organism. The basic organization of the cycle and its control system, however, are essentially the same in all eucaryotic cells. The proteins of the control system first appeared over a billion years ago. Remarkably, they have been so well conserved over the course of evolution that many of them function perfectly when transferred from a human cell to a yeast cell. We can therefore study the cell cycle and its regulation in a variety of organisms and use the findings from all of them to assemble a unified picture of how eucaryotic cells divide. In the following section, we briefly review the three eucaryotic systems in which cell-cycle control is commonly studied—yeasts, frog embryos, and cultured mammalian cells.
The Cell-Cycle Control System Can Be Dissected Genetically in Yeasts
Yeasts are tiny, single-celled fungi whose mechanisms of cell-cycle control are remarkably similar to our own. Two species are generally used in studies of the cell cycle. The fission yeast Schizosaccharomyces pombe is named after the African beer it is used to produce. It is a rod-shaped cell that grows by elongation at its ends. Division occurs by the formation of a septum, or cell plate, in the center of the rod (Figure 17-4A). The budding yeast Saccharomyces cerevisiae is used by brewers, as well as by bakers. It is an oval cell that divides by forming a bud, which first appears during G1 and grows steadily until it separates from the mother cell after mitosis (Figure 17-4B).
Figure 17-4
A comparison of the cell cycles of fission yeasts and budding yeasts. (A) The fission yeast has a typical eucaryotic cell cycle with G1, S, G2, and M phases. In contrast with what happens in higher eucaryotic cells, however, the nuclear envelope of the (more...)
Despite their outward differences, the two yeast species share a number of features that are extremely useful for genetic studies. They reproduce almost as rapidly as bacteria and have a genome size less than 1% that of a mammal. They are amenable to rapid molecular genetic manipulation, whereby genes can be deleted, replaced, or altered. Most importantly, they have the unusual ability to proliferate in a haploid state, in which only a single copy of each gene is present in the cell. When cells are haploid, it is easy to isolate and study mutations that inactivate a gene, as one avoids the complication of having a second copy of the gene in the cell.
Many important discoveries about cell-cycle control have come from systematic searches for mutations in yeasts that inactivate genes encoding essential components of the cell-cycle control system. The genes affected by these mutations are known as cell-division-cycle genes, or cdc genes. Many of these mutations cause cells to arrest at a specific point in the cell cycle, suggesting that the normal gene product is required to get the cell past this point.
A mutant that cannot complete the cell cycle cannot be propagated. Thus, cdc mutants can be selected and maintained only if their phenotype is conditional—that is, if the gene product fails to function only in certain specific conditions. Most conditional cell-cycle mutations are temperature-sensitive mutations, in which the mutant protein fails to function at high temperatures but functions well enough to allow cell division at low temperatures. A temperature-sensitive cdc mutant can be propagated at a low temperature (the permissive condition) and then raised to a higher temperature (the restrictive condition) to switch off the function of the mutant gene. At the higher temperature, the cells continue through the cell cycle until they reach the point where the function of the mutant gene is required for further progress, and at this point they halt (Figure 17-5). In budding yeasts, a uniform cell-cycle arrest of this type can be detected by just looking at the cells: the presence or absence of a bud, and bud size, indicate the point in the cycle at which the mutant is arrested (Figure 17-6).
Figure 17-5
The behavior of a temperature-sensitive cdc mutant. (A) At the permissive (low) temperature, the cells divide normally and are found in all phases of the cycle (the phase of the cell is indicated by its color). (B) On warming to the restrictive (high) (more...)
Figure 17-6
The morphology of budding yeast cells arrested by a cdc mutation. (A) In a normal population of proliferating yeast cells, buds vary in size according to the cell-cycle stage. (B) In a cdc15 mutant grown at the restrictive temperature, cells complete (more...)
The Cell-Cycle Control System Can Be Analyzed Biochemically in Animal Embryos
While yeasts are ideal for studying the genetics of the cell cycle, the biochemistry of the cycle is most easily analyzed in the giant fertilized eggs of many animals, which carry large stockpiles of the proteins needed for cell division. The egg of the frog Xenopus, for example, is over 1 mm in diameter and carries 100,000 times more cytoplasm than an average cell in the human body (Figure 17-7). Fertilization of the Xenopus egg triggers an astonishingly rapid sequence of cell divisions, called cleavage divisions, in which the single giant cell divides, without growing, to generate an embryo containing thousands of smaller cells (Figure 17-8). In this process, almost the only macromolecules synthesized are DNA—required to produce the thousands of new nuclei—and a small amount of protein. After a first division that takes about 90 minutes, the next 11 divisions occur, more or less synchronously, at 30-minute intervals, producing about 4096 (212) cells within 7 hours. Each cycle is divided into S and M phases of about 15 minutes each, without detectable G1 or G2 phases.
Figure 17-7
A mature Xenopus egg, ready for fertilization. The pale spot near the top shows the site of the nucleus, which has displaced the brown pigment in the surface layer of the egg cytoplasm. Although this cannot be seen in the picture, the nuclear envelope (more...)
Figure 17-8
Oocyte growth and egg cleavage in Xenopus. The oocyte grows without dividing for many months in the ovary of the mother frog and finally matures into an egg. Upon fertilization, the egg cleaves very rapidly—initially at a rate of one division (more...)
The cells in early embryos of Xenopus, as well as those of the clam Spisula and the fruit fly Drosophila, are thus capable of exceedingly rapid division in the absence of either growth or many of the control mechanisms that operate in more complex cell cycles. These early embryonic cell cycles therefore reveal the workings of the cell-cycle control system stripped down and simplified to the minimum needed to achieve the most fundamental requirements—the duplication of the genome and its segregation into two daughter cells. Another advantage of these early embryos for cell-cycle analysis is their large size. It is relatively easy to inject test substances into an egg to determine their effect on cell-cycle progression. It is also possible to prepare almost pure cytoplasm from Xenopus eggs and reconstitute many events of the cell cycle in a test tube (Figure 17-9). In such cell extracts, one can observe and manipulate cell-cycle events under highly simplified and controllable conditions.
Figure 17-9
Studying the cell cycle in a cell-free system. A large batch of activated frog eggs is broken open by gentle centrifugation, which also separates the cytoplasm from other cell components. The undiluted cytoplasm is collected, and sperm nuclei are added (more...)
The Cell-Cycle Control System of Mammals Can Be Studied in Culture
It is not easy to observe individual cells in an intact mammal. Most studies on mammalian cell-cycle control therefore use cells that have been isolated from normal tissues or tumors and grown in plastic culture dishes in the presence of essential nutrients and other factors (Figure 17-10). There is a complication, however. When cells from normal mammalian tissues are cultured in standard conditions, they often stop dividing after a limited number of division cycles. Human fibroblasts, for example, permanently cease dividing after 25–40 divisions, a process called replicative cell senescence, which we discuss later.
Figure 17-10
Mammalian cells proliferating in culture. The cells in this scanning electron micrograph are rat fibroblasts. (Courtesy of Guenter Albrecht-Buehler.)
Mammalian cells occasionally undergo mutations that allow them to proliferate readily and indefinitely in culture as "immortalized" cell lines. Although they are not normal, such cell lines are used widely for cell-cycle studies—and for cell biology generally—because they provide an unlimited source of genetically homogeneous cells. In addition, these cells are sufficiently large to allow detailed cytological observations of cell-cycle events, and they are amenable to biochemical analysis of the proteins involved in cell-cycle control.
Studies of cultured mammalian cells have been especially useful for examining the molecular mechanisms governing the control of cell proliferation in multicellular organisms. Such studies are important not only for understanding the normal controls of cell numbers in tissues but also for understanding the loss of these controls in cancer (discussed in Chapter 23).
Cell-Cycle Progression Can Be Studied in Various Ways
How can one tell at what stage an animal cell is in the cell cycle? One way is to simply look at living cells with a microscope. A glance at a population of mammalian cells proliferating in culture reveals that a fraction of the cells have rounded up and are in mitosis. Others can be observed in the process of cytokinesis. The S-phase cells, however, cannot be detected by simple observation. They can be recognized, however, by supplying them with visualizable molecules that are incorporated into newly synthesized DNA, such as 3H-thymidine or the artificial thymidine analog bromo-deoxyuridine (BrdU). Cell nuclei that have incorporated 3H-thymidine are visualized by autoradiography (Figure 17-11A), whereas those that have incorporated BrdU are visualized by staining with anti-BrdU antibodies (Figure 17-11B).
Figure 17-11
Labeling S-phase cells. (A) The tissue has been exposed for a short period to 3H-thymidine and the labeled cells have been visualized by autoradiography. Silver grains (black dots) in the photographic emulsion over a nucleus indicate that the cell incorporated (more...)
Typically, in a population of cells that are all proliferating rapidly but asynchronously, about 30–40% will be in S phase at any instant and become labeled by a brief pulse of 3H-thymidine or BrdU. From the proportion of cells in such a population that are labeled (the labeling index), one can estimate the duration of S phase as a fraction of the whole cell cycle duration. Similarly, from the proportion of these cells in mitosis (the mitotic index), one can estimate the duration of M phase. In addition, by giving a pulse of 3H-thymidine or BrdU and allowing the cells to continue around the cycle for measured lengths of time, one can determine how long it takes for an S-phase cell to progress through G2 into M phase, through M phase into G1, and finally through G1 back into S phase.
Another way to assess the stage that a cell has reached in the cell cycle is by measuring its DNA content, which doubles during S phase. This approach is greatly facilitated by the use of DNA-binding fluorescent dyes and a flow cytometer, which allows large numbers of cells to be analyzed rapidly and automatically (Figure 17-12). One can also use flow cytometry to determine the lengths of G1, S, and G2 + M phases, by following over time a population of cells that have been preselected to be in one particular phase of the cell cycle: DNA content measurements on such a synchronized population of cells reveal how the cells progress through the cycle.
Figure 17-12
Analysis of DNA content with a flow cytometer. This graph shows typical results obtained for a proliferating cell population when the DNA content of its individual cells is determined in a flow cytometer. (A flow cytometer, also called a fluorescence-activated (more...)
Summary
Cell reproduction begins with duplication of the cell's contents, followed by distribution of those contents into two daughter cells. Chromosome duplication occurs during S phase of the cell cycle, whereas most other cell components are duplicated continuously throughout the cycle. During M phase, the replicated chromosomes are segregated into individual nuclei (mitosis), and the cell then splits in two (cytokinesis). S phase and M phase are usually separated by gap phases called G1 and G2, when cell-cycle progression can be regulated by various intracellular and extracellular signals. Cell-cycle organization and control have been highly conserved during evolution, and studies in a wide range of systems—including yeasts, frog embryos, and mammalian cells in culture—have led to a unified view of eucaryotic cell-cycle control.
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Source: https://www.ncbi.nlm.nih.gov/books/NBK26869/
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