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Lecture 8

DEV2011: Lecture 8 summary

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Department
Medicine
Course
DEV2011
Professor
Various
Semester
Spring

Description
LECTURE 8 Cell Cycle: The cell cycle, or cell-division cycle, is the series of events that take place in a cell leading to its division and duplication (replication). In eukaryotes, the cell cycle can be divided in two periods: interphase—during which the cell grows, accumulating nutrients needed for mitosis and duplicating its DNA—and the mitotic (M) 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. Phases: The cell cycle consists of four distinct phases1 G phase, S phase (synthesis2, G phase (interphase) and M phase (mitosis). M phase is itself composed of two tightly coupled processes: mitosis, in which the cell's chromosomes are divided between the two sister cells, and cytokinesis, in which the cell's cytoplasm divides in half forming distinct cells. Activation of each phase is dependent on the proper progression and completion of the previous one. Cells that have temporarily or reversibly stopped dividing are said to have entered a state of dormancy called G 0hase. After cell division, each of the daughter cells begin the interphase of a new cycle. State Description Abbreviation Dormancy Gap 0 G0 A resting phase where the cell has left the cycle and has stopped dividing Interphase Gap 1 G1 Cells increase in size in Gap 1. The G1 checkpoint control mechanism ensures that everything is ready for DNA synthesis Synthesis S DNA replication occurs during this phase Gap 2 G2 During the gap between DNA synthesis and mitosis, the cell will continue to grow. The G2 checkpoint mechanism ensures that everything is ready to enter the M phase and divide. Cell Division Mitosis M Cell growth stops at this stage and cellular energy is focused on the orderly division into two daughter cells. A checkpoint in the middle of mitosis ensures that the cell is ready to complete cell division G0 Phase: Nonproliferative cells in multicellular eukaryotes generally enter the dormant G 0 state from G 1nd may remain dormant for long periods of time, possibly indefinitely. 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 G ph0se semi- permanentally e.g., some liver and kidney cells. Interphase: Before a cell can enter cell division, it needs to take in nutrients. All of the preparations are done during the interphase. Interphase proceeds in three stages, G1, S, and G2. Cell division operates in a cycle. Therefore, interphase is preceded by the previous cycle of mitosis and cytokinesis. Interphase is also known as preparatory phase. In this stage nucleus and cytosol division does not occur. The cell prepares for division. G1 Phase: The first phase within interphase, from the end of the previous M phase until the beginning of DNA synthesis is called G1 (G indicating gap). It is also called the growth phase. During this phase the biosynthetic activities of the cell, which had been 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 G1is highly variable, even among different cells of the same species. It is under the control of the p53 gene. 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 (sister) 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 external factors such as any drugs taken or any mutagens G2 Phase: During the gap between DNA synthesis and mitosis, the cell will continue to grow. The G2 checkpoint control mechanism ensures that everything is ready to enter the M (mitosis) phase and divides Mitosis: The M phase has been broken down into several distinct phases: - Prophase - Metaphase - Anaphase - Telophase - Cytokineses Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two nuclei. 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 (M) 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. The process of mitosis 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 stages are prophase, prometaphase, metaphase, anaphase and telophase. 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. The cell then divides in cytokinesis, to produce two identical daughter cells. Because cytokinesis usually occurs in conjunction with mitosis, "mitosis" is often used interchangeably with "M phase". Errors in mitosis can either kill a cell through apoptosis or cause mutations that may lead to cancer. Role of Cyclins and CDKs: G1/S Cyclins rise in late G1 and fall in early S phase. The Cdk- G1/S cyclin complex begins to induce the initial processes of DNA replication, primarily by arresting systems that prevent S phase Cdk activity in G1. The cyclins also promote other activities to progress the cell cycle, like centrosome duplication in vertebrates or spindle pole body in yeast. The rise in presence of G1/S cyclins is paralleled by a rise in S cyclins. S cyclins bind to Cdk and the complex directly induces DNA replication. The levels of S cyclins remain high, not only throughout S phase, but through G2 and early mitosis as well to promote early events in mitosis. M cyclin concentrations rise as the cell begins to enter mitosis and the concentrations peak at metaphase. Cell changes in the cell cycle like the assembly of mitotic spindles and alignment of sister-chromatids along the spindles
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