Summary 7 of unit 5 PDF

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Dr. Kooyman. Summary of the Cell Cycle and the different phases....

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Ashlyn Martin! March 20, 2014! Dr. David Kooyman! PDBio 120 Sec 3!

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Summary 7: Unit 5!

! Both prokaryote and eukaryote cells go through cell cycles, part of which is cell division. Prokaryotes divide by binary fission, and eukaryotes divide by a process called mitosis. The eukaryotic cell cycle has four stages: G1, S, G2 and M. The first three stages are called interphase and are preparatory for the last, mitosis. During the G1 phase, the cell grows, becomes committed to a divide and condenses it’s genetic material. During S phase, each chromosome is replicated to form a pair of sister chromatids. During G2 phase, the cell synthesizes proteins necessary for chromosome sorting and cell division. M phase consists of mitosis and cytokinesis.! ! Mitosis has 5 stages, prophase, prometaphase, metaphase, anaphase, and telephase, ends with cytokinesis. The purpose is to divide one cell nucleus into two nuclei, distributing the duplicated chromosomes, so each daughter cell receives the same complement of chromosomes. In humans, right before mitosis, a cell has 46 chromosomes, but 92 sister chromatids. After mitosis, human cells have 46 chromosomes/46 chromatids. To divide, the cell creates centrioles, protein complexes that orient the centrosome, two of which make up a centrosome, an organelle from which spindle fibers originate. During division, the centrioles move to opposite poles of the cell and release spindle fibers which attach to the chromosomes’ kinetochore, line them up in a metaphase plate and pull the chromosomes apart at the centromere, separating the two sister chromatids. Then, during cytokinesis, the cell membrane folds in on itself and pinches off to form two new cells, each of which have half of the genetic material.! ! In G1 phase, each chromosome has only one chromatid, but in G2, each chromosome has two sister chromatids attached at the centromere. A cell that has two sets of chromosomes (46) is called diploid (most of our cells are diploid) and the pair of chromosomes is homologous. Sex cells, or gametes, are haploid, meaning that they only have one set of chromosomes (23).! ! Sex cells go through a different division process called meiosis. Meiosis has two phases, Meiosis I and II. During meiosis I, the homologous chromosome pairs separate in reductional division (the chromosome number is halved, the genetic material is halved). During meiosis II, the sister chromatids of the single chromosome divide in equational division, just like in equational division (the chromosome number stays the same, the genetic material is halved). The resulting cells are all haploid.! ! It is during Meiosis I, specifically prophase I, that crossing over can occur between the homologous chromosomes and result in DNA mutations and genetic diversity. This can occur because the homologous chromosomes (4 sister chromatids) come together and pair in a process called synapsis to form tetrads. When they are in tetrad form, crossing over can occur.! ! The more complex an organism, the longer it stays in the diploid phase. Humans spend about 9-12 years in the diploid phase, until puberty....