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Chap 12 - Cell Cycle.docx

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University of Lethbridge
BIOL 1010
Brent Sellinger

th Reece et al., 9 Ed 1 Chapter 12 The Cell Cycle I. The Roles of Cell Division A. Unicellular organisms • reproduction B. Complex multicellular organisms • reproduction • growth • renewal and repair Cell division • requires some preparation • copies genome (all of an organism’s genetic material - DNA) and distributes “identical” genetic material to daughter cells II Prokaryotes Binary fission (Figure 12.12)  single circular chromosome replicates  Initiates replication of the chromosomal DNA at the origin of replication on the chromosome  As chromosome is replicated, each origin moves towards opposite ends of the cell  copies of the chromosomes move apart as cell grows  membrane pinches inward when cell reaches approximately twice initial size  cell wall forms between the two daughter cells. Plasmids are also replicated and partitioned between daughter cells • It's a numbers game for multicopy plasmids - random partitioning between two cells • lower copy number plasmids often have replication tied to cell division III. Eukaryotes • genome size is much larger than prokaryotes • must still duplicate genome prior to cell division • genomes are parceled out in multiple linear chromosomes, the number of chromosomes is characteristic of the species e.g., Human - 46 chromosomes in somatic cells and 23 in reproductive cells or gametes. Fruit fly - 8 chromosomes in somatic cells and 4 in gametes Eukaryotic chromosomes • Composed of chromatin - long linear DNA molecules associated with specific proteins. • Each chromosome carries several hundred to several thousand genes Reece et al., 9 Ed 2 Chapter 12 • Proteins function in maintaining chromosome structure and control activity of genes (i.e., when expressed) • Duplicated chromosome ▯ two sister chromatids held together initially along their lengths by protein complexes (cohesions). In the condensed form, there is a narrow region called the centromere, where the two sister chromatids are most closely attached. The sister chromatids contain an identical linear dsDNA molecule Mitosis = nuclear division - separates the sister chromatids into separate nuclei. Cytokinesis = cytoplasmic division - usually follows mitosis and separates the cytoplasm in two resulting in two cells A. Cell cycle (Figure 12.6) i) Mitotic phase (M) • mitosis and cytokinesis • shortest part of cell cycle • alternates with interphase ii) Interphase • up to 90% of the cell cycle • cell growth and replication of genetic material occurs during interphase • continued cell growth (protein synthesis and production of organelles) during all subphases Three Subphases of Interphase 1. G1phase - first "gap" 2. S phase - synthesis - chromosomes are duplicated (DNA replication) 3. G2phase - second "gap" Cells in late interphase (G2) have • well defined nuclei • two centrosomes (in animal cells - each with a pair of centrioles) formed in early interphase • nucleoli present • chromosomes duplicated during S phase • microtubules extending from centrosomes B. Mitosis 1. prophase - cytoplasmic and nuclear changes - nucleoli disappear - chromatin condenses to form discrete chromosomes composed of 2 sister chromatids Reece et al., 9 Ed 3 Chapter 12 - mitotic spindle formation begins - microtubules arranged between centrosomes - centrosomes move away from each other Mitotic spindle - forms in cytoplasm during prophase - consists of fibre made of microtubules and associated proteins - elongates by incorporation of tubulin subunits initiated at the centrosome (microtubule- organizing centre - MOC) 2. prometaphase - nuclear envelope fragments - microtubules interact and attach to chromosomes - centrosomes move to opposite poles - bundles of microtubules extend from each pole to the equator of the cell - each chromosome has a kinetochore kinetochore - protein structure which is the attachment site for microtubules; found in the centromere region of each sister chromatid ♦ believed equipped with a motor mechanism that walks the chromosome along the shortening microtubules. ♦ kinetochore microtubules attach to kinetochores – leads to uneven movement of chromosomes ♦ There is also evidence of a second mechanism by which kinetochore microtubules are reeled in by motor proteins at the spindle poles 3. metaphase - centrosomes are now at opposite poles - chromosomes arranged on plate running through cell equator or metaphase plate - chromosomes aligned and chromatids straddle the metaphase plate. - spindle apparatus - entire arrangement of kinetochore and nonkinetochore microtubules - longest stage of mitosis, lasting up to 20 min. 4. anaphase - paired centromeres of each chromosome sepa
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