?The cell cycle, or cell-division cycle, is the series of events that take place in a cell leading to its division and duplication that produces two daughter cells. In cells without a nucleus, the cell cycle occurs via a process termed binary fission.
In cells with a nucleus, the cell cycle can be divided in three periods: interphase—during which the cell grows, accumulating nutrients needed for mitosis preparing it for cell division and duplicating its DNA—and the mitotic phase, during which the cell splits itself into two distinct cells, often called “daughter cells” and the final phase, cytokinesis, where the new cell is completely divided. The cell-division cycle is a vital process by which a single-celled fertilized egg develops into a mature organism, as well as the process by which hair, skin, blood cells, and some internal organs are renewed.
After cell division, each of the daughter cells begin the interphase of a new cycle. Although the various stages of interphase are not usually morphologically distinguishable, each phase of the cell cycle has a distinct set of specialized biochemical processes that prepare the cell for initiation of cell division. G0 phase The word “post-mitotic” is sometimes used to refer to both quiescent and senescent cells. Nonproliferative cells in multicellular eukaryotes generally enter the quiescent G0 state from G1 and may remain quiescent for long periods of time, possibly indefinitely .
The Essay on Cell Division In Animals: Mitosis, Cytokinesis, And The Cell Cycle
Cell division in animals is a two-step process involving mitosis and cytokinesis and is set up by interphase. Interphase is a growth period for the cell. In the nucleus the chromosomes are duplicated but are not yet distinguishable because they are still a form of chromatin. There is also a nucleoli, one or more, present producing ribosomes that are sent to the cytoplasm. Mitosis is the division ...
This is very common for cells that are fully differentiated. Cellular senescence occurs in response to DNA damage or degradation that would make a cell’s progeny nonviable; it is often a biochemical reaction; division of such a cell could, for example, become cancerous. Some cells enter the G0 phase semi-permanentally e. g. , some liver and kidney cells. Many cells do not enter G0 and continue to divide throughout an organism’s life, e. g. epithelial cells. Interphase Before a cell can enter cell division, it needs to take in nutrients. All of the preparations are done during interphase.
Interphase is a series of changes that takes place in a newly formed cell and its nucleus, before it becomes capable of division again. It is also called preparatory phase or intermitosis. Previously it was called resting stage because there is no apparent activity related to cell division. Typically interphase lasts for at least 90% of the total time required for the cell cycle. Interphase proceeds in three stages, G1, S, and G2, preceded by the previous cycle of mitosis and cytokinesis. The most significant event is the replication of genetic material in S phase. G1 Phase
The first phase within interphase, from the end of the previous M phase until the beginning of DNA synthesis, is called G1 . It is also called the growth phase. During this phase the biosynthetic activities of the cell, which are considerably slowed down during M phase, resume at a high rate. This phase is marked by the use of 20 amino acids to form millions of proteins and later on enzymes that are required in S phase, mainly those needed for DNA replication. Duration of G1 is highly variable, even among different cells of the same species. It is under the control of the p53 gene.
We can say that in this phase, cell increases its supply of proteins, increases the number of organelles, and grows in size. S Phase The ensuing S phase starts when DNA replication commences; when it is complete, all of the chromosomes have been replicated, i. e. , each chromosome has two chromatids. Thus, during this phase, the amount of DNA in the cell has effectively doubled, though the ploidy of the cell remains the same. During this phase, synthesis is completed as quickly as possible due to the exposed base pairs being sensitive to harmful external factors such as mutagens.
The Essay on The process of mitosis
... and final stage of mitosis is called telophase. During telophase, the elongation of the cell continues. Daughter nuclei appear at the ... the expanded chromosomes and the nucleoli also reappear. Cytokinesis, the division of the cytoplasm, occurs during telophase ... new identical daughter cell replicas of the parent cell. Mitosis is composed of four unique phases. The phases are prophase, metaphase, ...
Mitosis The relatively brief M phase consists of nuclear division . It is a relatively short period of the cell cycle. M phase is complex and highly regulated. The sequence of events is divided into phases, corresponding to the completion of one set of activities and the start of the next. These phases are sequentially known as: prophase, metaphase, anaphase, telophase cytokinesis Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two nuclei.
During the process of mitosis the pairs of chromosomes condense and attach to fibers that pull the sister chromatids to opposite sides of the cell. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic phase of the cell cycle – the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell. This accounts for approximately 10% of the cell cycle.
Mitosis occurs exclusively in eukaryotic cells, but occurs in different ways in different species. For example, animals undergo an “open” mitosis, where the nuclear envelope breaks down before the chromosomes separate, while fungi such as Aspergillus nidulans and Saccharomyces cerevisiae undergo a “closed” mitosis, where chromosomes divide within an intact cell nucleus. Prokaryotic cells, which lack a nucleus, divide by a process called binary fission. Because cytokinesis usually occurs in conjunction with mitosis, “mitosis” is often used interchangeably with “M phase”.
However, there are many cells where mitosis and cytokinesis occur separately, forming single cells with multiple nuclei in a process called endoreplication. This occurs most notably among the fungi and slime moulds, but is found in various groups. Even in animals, cytokinesis and mitosis may occur independently, for instance during certain stages of fruit fly embryonic development. Errors in mitosis can either kill a cell through apoptosis or cause mutations that may lead to cancer. Regulation of eukaryotic cell cycle
The Term Paper on The Different Types of Cells
There are three major parts of a cell-- the nucleus, cytoplasm, and cell membrane, if these are stained appropriately, they can be easily seen under a light microscope. The nucleus (in many cell types) is the innermost and is enclosed by a thin membrane. The nucleus contains the genetic material which directs the cells function. The cytoplasm includes specialized structures called cytoplasmic ...
Regulation of the cell cycle involves processes crucial to the survival of a cell, including the detection and repair of genetic damage as well as the prevention of uncontrolled cell division. The molecular events that control the cell cycle are ordered and directional; that is, each process occurs in a sequential fashion and it is impossible to “reverse” the cycle. Role of cyclins and CDKs Two key classes of regulatory molecules, cyclins and cyclin-dependent kinases, determine a cell’s progress through the cell cycle. Leland H. Hartwell, R.
Timothy Hunt, and Paul M. Nurse won the 2001 Nobel Prize in Physiology or Medicine for their discovery of these central molecules. Many of the genes encoding cyclins and CDKs are conserved among all eukaryotes, but in general more complex organisms have more elaborate cell cycle control systems that incorporate more individual components. Many of the relevant genes were first identified by studying yeast, especially Saccharomyces cerevisiae; genetic nomenclature in yeast dubs many of these genes cdc followed by an identifying number, e. g. , cdc25 or cdc20.
Cyclins form the regulatory subunits and CDKs the catalytic subunits of an activated heterodimer; cyclins have no catalytic activity and CDKs are inactive in the absence of a partner cyclin. When activated by a bound cyclin, CDKs perform a common biochemical reaction called phosphorylation that activates or inactivates target proteins to orchestrate coordinated entry into the next phase of the cell cycle. Different cyclin-CDK combinations determine the downstream proteins targeted. CDKs are constitutively expressed in cells whereas cyclins are synthesised at specific stages of the cell cycle, in response to various molecular signals.
General mechanism of cyclin-CDK interaction Upon receiving a pro-mitotic extracellular signal, G1 cyclin-CDK complexes become active to prepare the cell for S phase, promoting the expression of transcription factors that in turn promote the expression of S cyclins and of enzymes required for DNA replication. The G1 cyclin-CDK complexes also promote the degradation of molecules that function as S phase inhibitors by targeting them for ubiquitination. Once a protein has been ubiquitinated, it is targeted for proteolytic degradation by the proteasome.
The Essay on Cell Transformation Resistant Gene
Title: Colony Transformation Lab Purpose: To study the behavior of Escherichia coli once it has been introduced to a foreign gene. Hypothesis: If the ampicillin resistant DNA is introduced into the E. coli bacteria, through the uptake of this DNA the bacteria will receive the resistant gene that will permit the bacteria to grow freely in the presence of ampicillin. Experimental Design: By ...
Active S cyclin-CDK complexes phosphorylate proteins that make up the pre-replication complexes assembled during G1 phase on DNA replication origins. The phosphorylation serves two purposes: to activate each already-assembled pre-replication complex, and to prevent new complexes from forming. This ensures that every portion of the cell’s genome will be replicated once and only once. The reason for prevention of gaps in replication is fairly clear, because daughter cells that are missing all or part of crucial genes will die.
However, for reasons related to gene copy number effects, possession of extra copies of certain genes is also deleterious to the daughter cells. Mitotic cyclin-CDK complexes, which are synthesized but inactivated during S and G2 phases, promote the initiation of mitosis by stimulating downstream proteins involved in chromosome condensation and mitotic spindle assembly. A critical complex activated during this process is a ubiquitin ligase known as the anaphase-promoting complex, which promotes degradation of structural proteins associated with the chromosomal kinetochore.
APC also targets the mitotic cyclins for degradation, ensuring that telophase and cytokinesis can proceed. Specific action of cyclin-CDK complexes Cyclin D is the first cyclin produced in the cell cycle, in response to extracellular signals . Cyclin D binds to existing CDK4, forming the active cyclin D-CDK4 complex. Cyclin D-CDK4 complex in turn phosphorylates the retinoblastoma susceptibility protein . The hyperphosphorylated Rb dissociates from the E2F/DP1/Rb complex, activating E2F.
Activation of E2F results in transcription of various genes like cyclin E, cyclin A, DNA polymerase, thymidine kinase, etc. Cyclin E thus produced binds to CDK2, forming the cyclin E-CDK2 complex, which pushes the cell from G1 to S phase that show cell cycle progression defects. Genome-wide studies using high throughput technologies have identified the transcription factors that bind to the promoters of yeast genes, and correlating these findings with temporal expression patterns have allowed the identification of transcription factors that drive phase-specific gene expression.
The Term Paper on The Cell Cycle
... of the cell cycle. Cyclin-dependent kinases (Cdks) a G1 Cdk (Cdk4) an S-phase Cdk (Cdk2) an M-phase Cdk (Cdk1) Their levels in the cell remain fairly ... active repression of the genes needed for mitosis. Cancer cells cannot enter G0 and are destined to repeat the cell cycle indefinitely. Mitosis is ...
The expression profiles of these transcription factors are driven by the transcription factors that peak in the prior phase, and computational models have shown that a CDK-autonomous network of these transcription factors is sufficient to produce steady-state oscillations in gene expression).
Experimental evidence also suggests that gene expression can oscillate with the period seen in dividing wild-type cells independently of the CDK machinery. Orlando et al. used microarrays to measure the expression of a set of 1,271 genes that they identified as periodic in both wild type cells and cells lacking all S-phase and mitotic cyclins .
Of the 1,271 genes assayed, 882 continued to be expressed in the cyclin-deficient cells at the same time as in the wild type cells, despite the fact that the cyclin-deficient cells arrest at the border between G1 and S phase. However, 833 of the genes assayed changed behavior between the wild type and mutant cells, indicating that these genes are likely directly or indirectly regulated by the CDK-cyclin machinery. Some genes that continued to be expressed on time in the mutant cells were also expressed at different levels in the mutant and wild type cells.
These findings suggest that while the transcriptional network may oscillate independently of the CDK-cyclin oscillator, they are coupled in a manner that requires both to ensure the proper timing of cell cycle events. While oscillatory transcription plays a key role in the progression of the yeast cell cycle, the CDK-cyclin machinery operates independently in the early embryonic cell cycle. Before the midblastula transition, zygotic transcription does not occur and all needed proteins, such as the B-type cyclins, are translated from maternally loaded mRNA. DNA replication and DNA replication origin activity
Analyses of synchronized cultures of Saccharomyces cerevisiae under conditions that prevent DNA replication initiation without delaying cell cycle progression showed that origin licensing decreases the expression of genes with origins near their 3′ ends, revealing that downstream origins can regulate the expression of upstream genes. This confirms previous predictions from mathematical modeling of a global causal coordination between DNA replication origin activity and mRNA expression, and shows that mathematical modeling of DNA microarray data can be used to correctly predict previously unknown biological modes of regulation.
The Essay on Ultraviolet and Mutated Cell Cycle
2.How does sunlight contribute to the development of melanoma? 3.What does it mean to be predisposed to getting cancer? If you inherit a mutated cell cycle gene, does that automatically mean that you will get cancer some day? If you inherit a mutated cell cycle gene and participate in risky behaviors such as sunbathing, does that mean that you will automatically get cancer some day? How does ...
Checkpoints Cell cycle checkpoints are used by the cell to monitor and regulate the progress of the cell cycle. Checkpoints prevent cell cycle progression at specific points, allowing verification of necessary phase processes and repair of DNA damage. The cell cannot proceed to the next phase until checkpoint requirements have been met. Several checkpoints are designed to ensure that damaged or incomplete DNA is not passed on to daughter cells. Two main checkpoints exist: the G1/S checkpoint and the G2/M checkpoint.
G1/S transition is a rate-limiting step in the cell cycle and is also known as restriction point. An alternative model of the cell cycle response to DNA damage has also been proposed, known as the postreplication checkpoint. p53 plays an important role in triggering the control mechanisms at both G1/S and G2/M checkpoints. Role in tumor formation A disregulation of the cell cycle components may lead to tumor formation. As mentioned above, when some genes like the cell cycle inhibitors, RB, p53 etc. mutate, they may cause the cell to multiply uncontrollably, forming a tumor.
Although the duration of cell cycle in tumor cells is equal to or longer than that of normal cell cycle, the proportion of cells that are in active cell division in tumors is much higher than that in normal tissue. Thus there is a net increase in cell number as the number of cells that die by apoptosis or senescence remains the same. The cells which are actively undergoing cell cycle are targeted in cancer therapy as the DNA is relatively exposed during cell division and hence susceptible to damage by drugs or radiation.
This fact is made use of in cancer treatment; by a process known as debulking, a significant mass of the tumor is removed which pushes a significant number of the remaining tumor cells from G0 to G1 phase . Radiation or chemotherapy following the debulking procedure kills these cells which have newly entered the cell cycle. The fastest cycling mammalian cells in culture, crypt cells in the intestinal epithelium, have a cycle time as short as 9 to 10 hours. Stem cells in resting mouse skin may have a cycle time of more than 200 hours.
Most of this difference is due to the varying length of G1, the most variable phase of the cycle. M and S do not vary much. In general, cells are most radiosensitive in late M and G2 phases and most resistant in late S. For cells with a longer cell cycle time and a significantly long G1 phase, there is a second peak of resistance late in G1. The pattern of resistance and sensitivity correlates with the level of sulfhydryl compounds in the cell. Sulfhydryls are natural radioprotectors and tend to be at their highest levels in S and at their lowest near mitosis. See also