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The Summary of the Cell Division 1. Significance of the Cell proliferation and its regulation a. For the growth and development of a multicellular organism, and for the generation of offspring; b. Produce new organisms in unicellular species; c. Renew the aging, apoptotic cells, and damaged tissue; 2. Definition of the cell cycle: The cell cycle is the process by which a cell grows, duplicates its DNA, and divides into identical daughter cells. 3. The cell cycle is divided into two main parts: interphase and mitosis . Interphase is subdivided into three phases: gap phase 1 (G1), synthesis (S), and gap phase 2 (G2). 4. Cells divides into 3 categories of : Cycling cells(Dividing continuously, constantly—Stem cells, skin cells, embryonic cells ;, G0 cells (Do not divide normally, cell never divides in adult; out of cell cycle in Go but divide when given an appropriate stimulus:liver cells, lymphocytes) and Terminal cells(Highly specialized, have lost the ability to divide until they die;muscle cells, red blood cells, nerve cells ) 5. Cell Cycle Interphase - the phase of a cell's life cycle in which DNA and histon are replicated. includes: G1 phase:Gap phase 1 begins at the completion of mitosis and cytokinesis and lasts until the beginning of S phase. This phase is generally the longest of the four cell cycle phases and is quite variable in length. During this phase, the cell chooses either to replicate its deoxyribonucleic acid (DNA) or to exit the cell cycle and enter a quiescent state (the G0 phase). S phase:Replication of the chromosomes is restricted to one specific portion of interphase, called S phase (DNA synthesis phase), which typically lasts about 6 h. In mammalian cells, the start of S phase—the actual initiation of DNA synthesis—takes place several hours after the cell has committed to carrying out DNA synthesis. During S phase, each chromosome replicates exactly once to form a pair of physically linked sister chromatids. In animal cells, a pair of centrioles is also duplicated during S phase. See also Chromosome; Genetics. G2 phase:The portion of interphase that follows S phase is called gap phase 2. Some cells can exit the cell cycle from G2 phase, just as they can from G1 phase. M phase M phase includes the overlapping processes of mitosis and cytokinesis. Mitosis is divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase. Cytokinesis usually begins during anaphase and ends at a point after the completion of mitosis. At the end of cytokinesis, the parent cell has formed its two G1 phase progeny and the cell is ready to repeat the cycle. See also Cytokinesis; Mitosis Prophase – a. the nuclear envelope dissolves and nucleolus disappear, b. the phase of mitosis in which the duplicated chromosomes condense, 1 c. centrioles divide and move to opposite ends of the cell. d. Spindle formed Metaphase - the phase of mitosis in which the chromosomes line up at the equator (the central plane) Golgi, ER reconstruct of the cell. Anaphase - the phase of mitosis in which the chromosomes begin to separate. Telophase - the last phase of mitosis, when the chromosomes migrate to opposite ends of the cell, two new nuclear envelopes and Nucleolus reassemble, and the chromosomes uncoil to chromatin. 6. Meiosis: reduces the number of chromosomes by half.Daughter cells differ from parent, and each other. Meiosis involves two divisions, both divide into prophase, metaphase, anaphase and telophase. Prophase futher divides into 5 subphases: Leptotene: The chromosomes strands are very elongated,thin,visible single strands,we call it “slender stage” too.The visible small dark granules are called CHROMOMERES. Zygotene: Homologous chromosomes pairs off in zipper fashion,starting at various points along this line,we call the phenomenon SYNAPSIS.The chromosome pairs is called BIVALENTS Pachytene: Chromosomes continue to shorten during this stage,each synapsis figure has four strands called TETRADS.Two closely related non-sister chromatids changed one or several parts by breaking and rejoining.We called it CROSSING OVER. Diplotene: The stage also lenown as “doubled thread”.tetrads can be observed microscopically,the doubled homologous repel each other.The points where crossing over had occurred between non-sister chromatids are called CHIASMATA. Diakinesis: Chromosomes shorten and thicken.The CHIASMATA moves toward the end of the chromatids at the same time homologous pairs continue to repel further.So we can see the shape like number “8”, “o” or “+” 7. There are 6 major take home lessons from the story of MEIOSIS.Let me summarize the major features: 1.Chromosomes duplicate only at INTERPHASEⅠ, not at INTERPHASEⅡ. 2.SYNAPSIS is the process that brings the homologous strands into intimate contact. 3.These homologous strands then CROSS OVER and RECOMBINE. 4.The four stranded BIVALENTS are separated by the two ANAPHASES. During ANAPHASESⅠ, the homologous chromosomes separate. During ANAPHASEⅡ , the sister chromosomes separate. 5.The four cell that result from the two division process carry only half the number of chromosomes from the original cell. 6.MEIOSIS is where increasing variation in the offspring take place such as: ← cross over ↑ random line up of chromosomes at METAPHASEⅠ so that paternal and maternal chromosomes are mixed by the time a cell enters MEIOSISⅡ → 2 random selection of cells with polar bodies to be the egg cells take place. 8. Comparison of Meiosis with Mitosis Before mitosis and meiosis, DNA replication occurs only once during the interphase prior to cell division. Mitosis requires one division, and Meiosis requires two divisions. Two diploid daughter cells result from mitosis, and Four haploid daughter cells result from meiosis. Daughter cells from mitosis are genetically identical to parental cells Daughter cells from meiosis are not genetically identical to parental cells. Mitosis occurs in all somatic cells for growth and repair. Meiosis occurs only in the reproductive organs for the production of gametes. Comparison of Meiosis I to Mitosis Meiosis I: Prophase I - pairing of homologous chromosomes Metaphase I – homologous pairs line up at metaphase plate Anaphase I – homologous chromosomes separate Telophase I – daughter cells are haploid Mitosis: Prophase has no such pairing Metaphase – chromosomes align at metaphase plate Anaphase – sister chromatids separate Telophase – diploid cells Comparison of Meiosis II to Mitosis The events of meiosis II are like those of mitosis except in meiosis II, the nuclei contain the haploid number of chromosomes. At the end of telophase II of meiosis II, there are four haploid daughter cells that are not genetically identical. At the end of mitosis, there are two diploid daughter cells that are identical. Genetic Recombination in Humans There are three ways in which meiosis and fertilization ensure that a child has a different combination of genes from that of either parent: 1. Independent assortment of chromosomes during metaphase I 2. Crossing-over during prophase I 3. Upon fertilization, recombination of chromosomes from different individuals (via their gametes) occurs. 3