The cell cycle consists of all the stages of growth and division for a eukaryotic cell. All eukaryotic cells go through the same basic life cycle, but different cells vary in the amount of time they spend in the various stages. The cell’s life cycle is a continuous process without a beginning or an end. As cells complete one cycle, they begin the next. Mitosis is the period when the cell divides.1 Mitosis consists of a sequence of four stages: prophase, metaphase, anaphase, and telophase. Mitosis has two purposes: i) to produce daughter cells that are identical copies of the parent cell and, ii) to maintain the proper number of chromosomes from generation to generation.
In prophase, the chromosomes become visible as they thicken to form coils upon coils. The chromosomes are not usually visible during interphase. Chromatin refers to the genetic material that is scattered throughout the nucleus. The disappearance of the nucleolus is one of the initial signs of prophase.1 Centrioles start to drift from each other and head toward opposite ends of the cell. The centrioles begin to produce a system of microtubules known as the spindle fibers and these attach themselves to a structure on each chromatid called a kinetochore. The centromere has the kinetochores as a part of it.
Metaphase is the next stage and here the chromosomes begin to line up along the equatorial plane, or the metaphase plate, of the cell. This results because the spindle fibers are connected to the kinetochore of each chromatid.1
In the next stage known as the anaphase, the sister chromatids of each chromosome break apart at the centromere and drift to opposite poles.1 The mictrotubules are responsible for pulling apart the chromatids, which begin to shorten. Each half of a pair of sister chromatids will end up on opposite poles of the cell. Non-kinetochore microtubules are the ones that elongate the cell.
Telophase is the last phase of mitosis. The nucleoli reappears and a nuclear membrane forms around each set of chromosomes.1 The nuclear membrane is ready to divide and the cytoplasm is split in a process known as cytokinesis. The cell splits along a cleavage furrow, which is produced by actin microfilaments. A cell membrane forms around each cell and they split into two distinct daughter cells. The division of the cytoplasm yields two daughter cells.
Cells spend most of their time in interphase. Interphase has three stages: G1, S and G2, During G1 of interphase, the cell produces tRNA, mRNA, ribosomes, and enzymes for everyday processes.1 During the S phase of interphase, the cell synthesizes DNA to prepare for division. During G2 of interphase, the cell produces the proteins required for the spindles. After interphase, the cell can enter mitosis. Interphase is a stage of the cell cycle during which the cell engages in normal metabolic activities and prepares for the next cell division. Most cells spend the greater part of their life in the interphase stage. After the required preparatory steps the cell proceeds into the stages of mitosis. Mitosis is the portion of the cell cycle in which the cell divides its genetic information. In the interphase stage, there are three distinct phases of cell activity: G1, S, and G2. The cell is engaged in specific activities needed to prepare for cell division during each of these parts of interphase.
During the G1 stage of interphase, the cell gathers nutrients and other resources from its environment. These activities allow the cell to perform its normal functions. Gathering nutrients allows the cell both to grow in volume and to carry out its usual metabolic roles, such as producing tRNA, mRNA, ribosomes, enzymes, and other cell components. Often, a cell stays in G1 for an extended period. This is a normal process. For cells that remain in the G1 stage for a long time, the stage is often renamed the G0 stage, because the cell is not moving forward through the cell cycle. In the G0 stage, cells may become differentiated, or specialized in their function, such as becoming nerve cells or muscle cells. The length of time cells stay in G0 varies. Some cells, such as nerve cells, that enter into the G0 stage remain there permanently while others, such as cells for bone repair, can move back into the cell cycle and continue toward mitosis. A cell that will undergo division, does so during G1 and moves to the S stage. During the S stage of interphase, DNA synthesis happens. With two copies of the genetic information, the cell can distribute copies to the daughter cells in the chromosomes. By following the cell’s chromosomes, you can follow the cell’s genetic information while mitosis creates two genetically identical cells. The structure of a chromosome consists of DNA wrapped around histone proteins to form chromatin. The individual chromatin strands are too thin and tangled to be seen with a compound microscope. As a cell gets ready to divide, the chromatin coils and becomes visible as a chromosome. As chromosomes become more visible at the beginning of mitosis, you can see two threadlike parts lying side by side. Each parallel thread is called a chromatid.1 A chromatid is one of two parallel parts of a chromosome. Each chromatid contains one DNA molecule. After DNA synthesis, the chromosome contains two DNA molecules, one in each chromatid. Sister chromatids are the 2 chromatids of a chromosome that were produced by replication and that contain the identical DNA. The centromere is the sequence of bases at the site where the sister chromatids are attached. During interphase, when chromosome replication occurs, the two strands of the DNA molecule unzip and two identical double-stranded DNA molecules are formed, which remain attached at the centromere. Each chromatid contains one of these DNA molecules. The term dyad is sometimes used to refer to the two identical chromatids, to show that there are two double-stranded DNA molecules, one in each chromatid. The genetic data is contained within DNA. The final stage of interphase is G2 and this is where final preparations are made for mitosis. The cell manufactures the cellular components necessary to divide successfully, like the proteins it will use to move the chromosomes. The nuclear membrane is intact at this point in the cell cycle. The chromatin has been replicated, but it is uncoiled and so the individual chromosomes are not visible. The nucleolus is the site of ribosome production and it is still visible during the G2 stage.1
Cells utilise a centralized control system to check whether appropriate conditions have been achieved before passing three key checkpoints in the cell cycle. Cell growth is checked at the G1 checkpoint. The G1 checkpoint makes the main decision of whether the cell should divide, delay division, or enter a resting stage and it is located near the end of G1 and just before entry into S phase. If conditions are favorable for cell division, the cell begins to copy their DNA to begin the S phase. The G1 checkpoint is where complex eukaryotes normally arrest the cell cycle if environmental conditions make cell division unfavorable or if the cell passes into the extended resting period called G0. DNA replication is assessed at the G2 checkpoint. The second checkpoint, the G2 checkpoint, triggers the start of M phase. If this checkpoint is passed, the cell initiates the many molecular processes that signal the beginning of mitosis. Mitosis is assessed at the M checkpoint. The third checkpoint, the M checkpoint, occurs at metaphase and triggers the exit from mitosis and cytokinesis and the beginning of G1.
Cancer can be caused by chemicals like those in cigarette smoke, by environmental factors such as UV rays that damage DNA, or in some instances by viruses that circumvent the cell’s normal growth and division controls. Whatever the immediate cause, however, all cancers are characterized by unrestrained cell growth and division. The cell cycle never stops in a cancerous line of cells. Cancer results from damaged genes failing to control cell division. One particular gene seems to be a key regulator of the cell cycle known as p53. This gene plays a key role in the G1 checkpoint of cell division. If the p53 protein detects damaged DNA, it halts cell division and stimulates the activity of special enzymes to repair the damage. The gene p53 allows cell division to continue once the DNA has been repaired. In instances where the DNA cannot be repaired, p53 would then direct the cell to activate apoptosis (cell suicide). The p53 protein typically monitors DNA and destroys cells with irreparable damage to their DNA. Abnormal p53 protein does not halt cell division nor does it repair DNA. Cancer results as damaged cells proliferate in the body. By stopping division in damaged cells, p53 work to prevent the formation of tumors. Scientists have found, through various tests, that p53 is itself damaged beyond use in most human cancers they have examined. This supports the theory that it is precisely because p53 is non-functional that these cancer cells are allowed undergo cell division without being halted at the G1 checkpoint. As more and more damage occurs to these cells, they become cancerous.
1) Maton A, Hopkins JJ, LaHart S, Quon Warner D, Wright M, Jill D (1997). Cells: Building Blocks of Life. New Jersey: Prentice Hall. pp. 70–4
2) Wilbur, Beth, editor. The World of the Cell, Becker, W.M., et al., 7th ed. San Francisco, CA; 2009.
3) Kent, M. (2000). Advanced Biology. Pages 246 -247. Oxford University Press.
4) M.B.V. Roberts, J. M. (1985) Biology for CXC. Pages 266 – 267. Cheltenham: Thomas Nelson and Sons Limited.
5) Slack, J.M.W. (2013) Essential Developmental Biology. Wiley-Blackwell, Oxford.
6) Hayflick L; Moorhead PS (December 1961). “The serial cultivation of human diploid cell strains”. Exp. Cell Res. 25: 585–621.