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CELL CYCLE (FIG. 1), 1. Definition. All those changes which occur during, cell-growth and cell-division are collectively called, , Cell cycle. It is the sum of all those events by which a, cell duplicates its genome and synthesizes cell contents, and eventually divides into two daughter cells. These, events are themselves under genetic control., 2. Two parts, cycle:, , of Cell, , A. Interphase., , B. M-phase., A. Interphase. It is the period between the end of, one cell division to the, beginning of next cell division. It, is misnomerly called resting phase (no visible, changes, Occur) but is actually highly metabolically active phase, or energy, phase in which cell prepares itself for next, cell division by a number of biosynthetic activities, so, is also called preparatory phase., lasts foor, , Interphase, , about 95% (90 96%) of cell cycle., On the basis of synthetic activities,, is, interphase, divided into three sub-phases, (a) G, or Gap - I or Post-mitotic or Pre-synthetic, -, , phase, , First growth phase, It is the period between mitosis and initiation of, DNA, or, , replication. It is characterized by increase in cell size and, nucleus due to high rate of, biosynthesis. It involves:
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) Pooling of amino acids for protein synthesis., (i) Pooling of, , cleotides for the synthesis of RNA, , and DNA., , anii)Synthesis of enzymes (proteins) and energy, molecule, , to unzip and synthesize DNA molecules., , G2, , SYNTHESPROTEIIS-SPINNNSDLE, , NTHES/s, , S, , ATP, RNA, , NOILYO, , PROPHASE, , METAPHASE, , ZANAPHASsE, , TELOPHAS`E, , YNTHESIS, , OTEIN AND RNA, INCREASE IN NUCLE, , G1, , SPECIALIZATION, CELL, , Fig. 1. Mitotic cell cycle., , (iv) Synthesis of macromolecules like RNAS,, carbohydrates, lipids and proteins as the cell is, metabolically active., (v) Chromosome, , duplication occurs., (b) S or Synthetic, , (i), , fully, , are, , phase., , DNA replicates, , so, , of two sister-chromatids, , extended. But, It is, , no, , DNNA, , characterized by:, , each chromosome is formed, , joined, , at centromere., , of histones (basic proteins)., the amount (4C) of DNA, Each cell is with double, , (ii) Synthesis, , but the chromosome, , Gap-II, , number remains, or, , Post-synthetic or Pre-mitotic, phase. It is characterized by, , (c) G,, phase or Second growth, or, , volume., increased nuclear, , (i), , same., , It involves, , proteins., Synthesis of spindle, , of three types of RNA molecules., and storage of ATP molecules., , (ii) Synthesis, (ii) Synthesis
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607, , (iv) Duplication, , centrioles., , (v), B., , of, , mitochondria, plastids, , and, , Damaged DNA is repaired in G,., , M-Phase or Mitotic phase or D-phase. It follows, the interphase and involves the, of sister, separation, chromatids and their redistribution into daughter cells., It also involves the, orderly distribution of cell organelles, and, , macrobiomolecules to the daughter cells. So after, M-phase, exactly same hereditary informations are, distributed in the daughter cells., Karyokinesis and Cytokinesis., , It consists of 2, , phases:, , 3. Duration of Cell-cycle. It varies from, organism, to organism and also with cell type. It depends on the, type of cell and external factors such as temperature, food, and oxygen. Time-period for cell cycle is species specific, under specific environmental conditions e.g. 20 minutes, , for bacterial cell, 90 minutes for yeast cells, 8-10 hours for, intestinal epithelial cell, 20 hours for onion root-tip cell, 24, , hours forhuman cells in culture, 29 hours for Trillium etc., 4., , Regulation of cells cycle. Stage of regulation of, , cell cycle is G, phase during which a cell may follow, one of the three options :, , (a) It may start a new cell cycle, enter the S-phase, and finally divide., (b) It may be arrested at a specific point of G, phase., (c)It may stop division and enter G or quiscent, , stage so G, stage is called as the state of arrested, condition. But depending upon the requirement of, organism, cell in G, phase can resume the growth and, re-enter the G, phase, otherwise it may undergo cell, , differentiation and specialization.e.g. nerve cells, muscle, cells, red blood cells and white blood cells., Once this G, check point has been passed and cell, entered the S-phase and has begun DNA replication,, cell goes on to complete new cell cycle. So initiation, of cell division is triggered by the doubling of DNA, in S-Phase (even before the start of M-phase).
