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The Cell Cycle In Living Organisms
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| The Cell Cycle In Living Organisms
The cell cycle is the recurring sequence of events that includes the duplication of a cell's contents and its subsequent division. This SparkNote will focus on following the major events of the cell cycle as well as the processes that regulate its action. In this and the following SparkNotes on cell reproduction, we will see how the cell cycle is an essential process for all living organisms. In single-cell organisms, each round of the cell cycle leads to the production of an entirely new organism. Other organisms require multiple rounds of cell division to create a new individual. In humans and other higher-order animals, cell death and growth are constant processes and the cell cycle is necessary for maintaining appropriate cellular conditions. 
Figure %: The Cell Cycle As we discussed in theIntroduction to Cell Reproduction, the goal of cellular reproduction is to create new cells. The cell cycle is the means by which this goal is accomplished. While its duration and certain specific components vary from species to species, the cell cycle has a number of universal trends.
The copied contents of the cell must migrate to opposite ends of the cell. The cell must physically split into two separate cells. We will discuss the general organization of the cell cycle by reviewing its two major phases: M Phase (for mitosis) and interphase. Interphase is generally split into three distinct phases including one for DNA replication. We will finish with a discussion of the elements that control a cell's passage through these various stages. The cell cycle is very highly regulated to prevent constant cell division and only allows cell that have met certain requirements to engage in cell division. How long do the different stages of the cell cycle take? Replication is one of the hallmark features of living matter. The set of processes known as the cell cycle which are undertaken as one cell becomes two has been a dominant research theme in the molecular era with applications that extend far and wide including to the study of diseases such as cancer which is sometimes characterized as a disease of the cell cycle gone awry. Cell cycles are interesting both for the ways they are similar from one cell type to the next and for the ways they are different. To bring the subject in relief, we consider the cell cycles in a variety of different organisms including a model prokaryote, for mammalian cells in tissue culture and during embryonic development in the fruit fly. Specifically, we ask what are the individual steps that are undertaken for one cell to divide into two and how long do these steps take? 
Figure 1: The 150 min cell cycle of Caulobacter is shown, highlighting some of the key morphological and metabolic events that take place during cell division. M phase is not indicated because in Caulobacter there is no true mitotic apparatus that gets assembled as in eukaryotes. Much of chromosome segregation in Caulobacter (and other bacteria) occurs concomitantly with DNA replication. The final steps of chromosome segregation and especially decatenation of the two circular chromosomes occurs during G2 phase.
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≈80 min, BNID 104923) where the DNA gets replicated and G2 (second Growth phase, ≈25 min, BNID 104924) where chromosome segregation unfolds leading to cell division (final phase lasting ≈15 min). Caulobacter crescentus provides an interesting example of the way in which certain organisms get promoted to “model organism’’ status because they have some particular feature that renders them particularly opportune for the question of interest. 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. coli. 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. For the characteristic cell cycle time of 20 hours in a HeLa cell, almost half is devoted to G1 (BNID 108483) and close to another half is S phase (BNID 108485) whereas G2 and M are much faster at about 2-3 hours and 1 hour, respectively (BNID 109225, 109226). 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. (Adapted from “The Cell Cycle – Principles of Control” by David Morgan.) How does the length of the cell cycle compare to the time it takes a cell to synthesize its new genome? A decoupling between the genome length and the doubling time exists in eukaryotes due to the usage of multiple DNA replication start sites. For mammalian cells it has been observed that for many tissues with widely 538
varying overall cell cycle times, the duration of the S phase where DNA replication occurs is remarkably constant. For mouse tissues such as those found in the colon or tongue, the S phase varied in a small range from 6.9 to 7.5 hours (BNID 111491). Even when comparing several epithelial tissues across human, rat, mouse and hamster, S phase was between 6 and 8 hours (BNID 107375). These measurements were carried out in the 1960s by performing a kind of pulse-chase experiment with the radioactively labeled nucleotide thymidine. During the short pulse, the radioactive compound was incorporated only into the genome of cells in S phase. By measuring the duration of appearance and then disappearance of labeled cells in M phase one can infer how long S phase lasted The fact that the duration of S phase is relatively constant in such cells is used to this day to estimate the duration of the cell cycle from a knowledge of only the fraction of cells at a given snapshot in time that are in S phase. For example, if a third of the cells are seen in S phase which lasts about 7 hours, the cell cycle time is inferred to be about 7 hours/(1/3) ≈20 hours. Today these kinds of measurements are mostly performed using BrdU as the marker for S phase. We are not aware of a satisfactory explanation for the origin of this relatively constant replication time and how it is related to the rate of DNA polymerase and the density of replication initiation sites along the genome. The diversity of cell cycles is shown in Figure 2 and depicts several model organisms and the durations and positioning of the different stages of their cell cycles. An extreme example occurs in the mesmerizing process of embryonic development of the fruit fly Drosophila melanogaster. In this case, the situation is different from conventional cell divisions since rather than synthesizing new cytoplasmic materials, mass is essentially conserved except for the replication of the genetic material. This happens in a very synchronous manner for about 10 generations and a replication cycle of the thousands of cells in the embryo, say between cycle 10 and 11, happens in about 8 minutes as shown in Figure 2 (BNID 103004,103005, 110370). This is faster than the replication times for any bacteria even though the genome is ≈120 million bp long (BNID 100199). A striking example of the ability of cells to adapt their temporal dynamics. жүктеу/скачать 18,66 Mb. Достарыңызбен бөлісу:
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