chap_16_outline - Chapter 16 - Eukaryotic Cell Cycle Human...

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Chapter 16 - Eukaryotic Cell Cycle Human cells divide once ~24 hours. Yeast cells can bud in 90 minutes Fertilized cells can divide in 30 minutes Divided into two parts Mitosis (followed by cytokinesis ) – M phase ~5% in this cycle Interphase –G1 phase, S phase, G2 phase ~95% of time Chromosomes decondensed and distributed in nucleus Cell growth and DNA replication occur G1 (gap 1) – cell grows and is metabolically active (protein and RNA synthesis), no DNA rep. S phase (synthesis) – DNA replication takes place G2 (gap 2) – cell growth continues and proteins for mitosis are synthesized – often brief Fertilized cells do not grow during division; there is no G1 or G2 phase, only short S phase and M phase Some cells only divide occasionally to replace cells from injury (skin fibroblasts, liver cells) and some don’t divide at all (neurons). These cells exit G1 and enter G0 phase, where they do not proliferated until signaled to do so Mitotic cells can be distinguished microscopically; cells in S phase can be identified because of incorporated radioactive thymidine. G1 stage cells are diploid (2n), S phase has from 2n to 4n, G2 phase has 4n and mitotic cells are 2n Regulation of cell cycle is by cell growth and extracellular signals. In many cells, a regulatory point occurs in late G1 and controls progression from G1 to S First studied from budding yeast cells, known as START -Once cells pass START, they must enter S phase and undergo one cell cycle division -Highly regulated – if cells face shortage of nutrients, cycle stops at START and enters resting state instead of S phase -Polypeptide factors that signal yeast mating can stop cycle at START, allowing haploid cells to fuse with one another instead of progressing to S phase. -START is where cell growth is coordinated with DNA replication and cell division. Cell size must be appropriate before division. Daughter cell spends more time in G1 and grows more than mother cell (in budding yeast cells). Animal cells are similarly regulated by the restriction point in late G1, analogous to START Entrance to cycle is regulated by extracellular growth factors, if present, S phase begins. Once passed through restriction point, cell must go through whole cycle. If growth factors not present, cell enters G0, cease growth until growth factor signals Some cell cycles are controlled by G2; example is fission yeast Schizosaccharomyces pombe. Vertebrate oocytes also are arrested in G2 until M phase is triggered by hormonal stimulation. Cell cycle checkpoints – coordination of cell cycles to stop entry into next phase until previous one is complete Sense unreplicated/damaged DNA and repair until progression into next stage. Checkpoint in G2 prevents mitosis until replication is complete. G1 checkpoint allows repair of damaged DNA before it’s replicated in S phase
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This note was uploaded on 03/19/2008 for the course MMG 409 taught by Professor Arvidson during the Spring '08 term at Michigan State University.

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chap_16_outline - Chapter 16 - Eukaryotic Cell Cycle Human...

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