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Chapter 9

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BIOL 1500
Gerry Goldberg

2012 BIOL1000 Jasmine Nathaniel Chapter 9LES] The Cycle of Cell Growth and Division: An Overview The Products of Mitosis Are Genetic Duplicates of the Dividing Cell o Three elegantly interrelated systems o One component is an elaborate master program of molecular checks and balances that ensures an orderly and timely progression through the cell cycle o Process of DNA synthesis replicates each DNA chromosome into two copies o Final system is a structural and mechanical web of interwoven cables and motors of the mitotic cytoskeleton that separates the DNA copies precisely into the daughter cells o Meiosis produces the daughter nuclei o Cells that are the products of meiosis may function as gametes in animals and spores in plants Chromosomes Are the Genetic Units Divided by Mitosis o In eukaryotes, the hereditary information of the nucleus is distributed among several linear DNA molecules o These molecules are combined with proteins that stabilize the DNA, assist in packaging DNA during cell division, and influence the expression of individual genes o Most eukaryotes have two copies of each type of chromosome (diploid) o Other eukaryotes have one copy of each type of chromosome (haploid) o Some have more than two copies (ploidy) o Replication of the DNA of each individual chromosome creates two identical molecules (sister chromatids) o Sister chromatids are held together until mitosis separates them o Chromosome segregation: equal distribution of daughter chromosomes to each of the two cells that result from cell division The Mitotic Cycle Interphase Extends from the End of One Mitosis to the Beginning of the Next Mitosis o G1Phase: initial growth stage o Cell makes various RNAs, proteins, and other types of cellular molecules but not nuclear DNA o If cell divides: DNA replication begins, initiating the S phase o Stage where many cells stop dividin0 (G phase) o S Phase: cell duplicates chromosomal proteins and DNA and continues synthesis of other cellular molecules o G2Phase o Cell synthesizes RNAs and proteins and cell continues to grow o At the end of this phase, mitosis begins After Interphase, Mitosis Proceeds in Five Stages o Prophase o The replicated chromosomes start to condense into compact, rod-like structures o Condensation packs the DNA molecules into units small enough to be divided successfully during mitosis o Nucleolus becomes smaller and eventually disappears o In the cytoplasm the mitotic spindle begins to form between the two centrosomes as they start migrating toward the opposite ends of the cell to form spindle poles o Prometaphase o Bundles of spindle microtubules grow from centrosomes at the opposing spindle poles toward the centre of the cell o Some of the spindle attaches to the chromosomes o A kinetochore (complex of several proteins) has formed on each chromatid at the centromere o Metaphase o Spindle microtubules move chromosomes into alignment at the spindle midpoint (metaphase plate) o Two sister chromatids attached to microtubules leading to opposite spindle poles o Anaphase o Sister chromatids separate and move to opposite spindle poles o Movement continues until separated chromatids (now daughter chromosomes) have reached the two poles o Telophase o Spindle disassembles o Chromosomes at each spindle pole decondense and return to the extended state typical of interphase o Nucleolus reappears, RNA transcription resumes, new nuclear envelope forms around chromosomes, producing two daughter nuclei Cytokinesis Completes Cell Division by Dividing the Cytoplasm between Daughter Cells o Cytokinesis: division of the cytoplasm o Produces two daughter cells o Begins during telophase or late anaphase o Animals, protists, fungi: the furrow (a groove) girdles the cell and gradually deepens until it cuts the cytoplasm into two parts o Plants: cell plate (new cell wall) forms between the daughter nuclei and grows laterally until it divides the cytoplasm o Furrowing: layer of microtubules that remains at former spindle midpoint expands laterally until it stretches entirely across the dividing cell o Band of microfilaments forms inside plasma membrane o Microfilaments slide together, tightening the band and constricting the cell o Constriction forms a groove (furrow) in the plasma membrane o Furrow gradually deepens until daughter cells are completely separated o Cell plate formation o Layer of microtubules serves as an organizing site for vesicles produced by the ER and Golgi complex o As vesicles collect, layer expands until it spreads entirely across the dividing cell o During expansion, vesicles fuse together, assembling the cell plate (new cell wall) o Cytoplasm and its organelles is separated into two parts a
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