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Biology Chapter 8

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Western University
Biology 1201A

Biology Notes Chapter 8 Cell Cycles Pages 161-180 Why It Matters Cells are able to coordinate their growth and division by 1. DNA replication 2. a dynamically changing cytoskeletion 3. cell cycle checkpoints 8.1 The Cycle of Cell Growth and Division: An Overview Cell replication is important because progeny cells (new cells) are needed for expanding population size, growth, coordinating, and regenerating. TooSlow:may grow inefficiently large TooFast:daughter cells may be to small or lacking cytoplasm or other materials. 8.2 The Cell Cycle in Prokaryotic Organisms A prokaryotic organism doubles in size, replicates its circular chromosome, and moves into two daughter cells (binary fission). Threeperiodsofbinaryfission: 1. Cells have a growing time after birth (B) 2. Cells commence DNA synthesis by replicating the chromosomes and moving the to opposite ends of the cell © 3. The membrane pinches together between them and two daughter cells are formed (D) 8.2A Replication Occupies Most of the Cell Cycle in Rapidly Dividing Prokaryotic Cells Bacteria and Archaea contain their hereditary material in a single, circular chromosome of double stranded DNA. The DNA is kept in the nucleoid of a prokaryotic cell. When there is a high amount of nutrients, prokaryotic cells do not have a B period because their cytoplasm can divide as soon as replication is complete. 8.2B Replicated Chromosomes are Distributed Actively to the Halves of the Prokaryotic Cell FrancoisJacob:proposed a model for the segregation of bacterial chromosomes to the daughter cells by the fact that the two chromosomes that come to stick to the plasma membrane and separate as new plasma membrane is added during elongation, proposing that chromosome separation is passive. OriginofReplication: the specific region that replication commences (ori). This will be the middle of the cell where the enzymes for DNA replication are located. CytokineticRing: causes cytoplasmic division by its construction dividing it into two equal parts. 8.2C Mitosis has Evolved from an Early Form of Binary Fission Prokaryotic cells will divide easily into halves of the cell because it has one chromosome and if two daughter cells both contain one in the end it is likely to be successful. Eukaryotic cells (Mitosis) contain many more chromosomes, with each containing more length of DNA. Missing a single chromosome can be lethal. They also are contained in a nuclear membrane, making replication mechanisms between prokaryotic cells and eukaryotic cells very different. CentromereFunction: not just to keep the twin/sister chromatids together, but to also allow for orientation relative to the cytoskeleton, in time, for proper distribution of chromosomes for the daughter cells. In higher eukaryotic cells, the nuclear membrane disintegrates and reforms in the daughter cells. In yeast the cell does not disintegrate the nuclear membrane but instead uses a spindle of microtubule made of polymerized tubulin protein. 8.3 Mitosis and the Eukaryotic Cell Cycle The3InterrelatedSystemsthatContributetotheEukaryioticCell Cylce: 1. A complex program of specific checks and balances that ensures orderly and timely progression throughout the cycle 2. The process of DNA Synthesis replicates each DNA chromosome into two copies almost perfectly. 3. The structural and mechanical web of interwoven "cables" and "motors" of the cytoskeleton that separates the DNA copies precisely into the daughter cells. 8.3A Chromosomes Are the Genetic Units Divided by Mitosis DNA molecules are combined with proteins that 1. stabilize the DNA molecules, 2. assist in packaging DNA during cell division, 3. influence expression of individual gene Most eukaryotic cells have 2n which is a diploid cell containing two copies of every unique chromosome. Ploidy:the number of chromosome sets (haploid n 1, diploid 2n two, etc) Chromosome: the double strand of DNA molecules 9before replication, or the twin chromatids held together by a cetromere. Chromatid: The replicated chromosome, held to its sister/twin during replication ChromosomeSegregation: the equal distribution of daughter chromosomes. DNA replication increases the amount of DNA in the cell but not the number of chromosomes. When a cell replicates, its an identical clone, unless it has a mutation, and every organism came from a single zygote, that is why we are able to have genetic identities for forensic studies. 8.3B Interphase Extends from the End of One Mitosis to the Beginning go the Next Mitosis Cells go through growth and division and it is categorized into phases although the real process is more smooth and continuous. Internal regulatory controls trigger each phase only when the previous is complete. Other internal regulatory controls say how many cycles a cell can live, these internal controls can be influenced by external triggers from viruses, cells, signal molecules, hormones, growth factors, death factors. Interphase:first and the longest, where the cell grows and replicates its DNA in preparation for mitosis, and cytokinesis. -G1phaseinitial phase from daughter cell, time of cytoplasmic growth. This phase varies in length for different species (rate determining step) -Sphasewhere the cell duplicates the DNA and the proteins. -G2phaseis when the cell continues to synthesize proteins and grow, especially those needed for mitosis, there is no DNA synthesis. -G0phase: is a state of division arrest, mature cells Chromosomes are relatively loose, but organized in the nucleus. Transition from G1 phase to S phase is very important and where cancer research lies. 8.3C After Interphase, Mitosis Proceeds in Five Stages Prophase: - the twin chromatids, or the single chromosome condenses into chromatin - the nucleolus becomes smaller and eventually disappears (indicates end of prophase) - nucleolus disappearance stops RNA replication - in the cytoplasm mitotic spindle begins to form the centrosomes as they start migrating toward the opposite ends of the cell to form the spindle poles - the spindle develops as the bundles of microtubules that radiate from the spindle poles Prometaphase: - spindle fibres continue to grow from the opposite ended centrosomes and attach to the chromosomes - chromosomes seldom seen as a double structure at this point - a complex of several proteins, a kinetochore, has formed on each chromatid at the centromere - kinetochore microtubules bind to the kinetochores - this determines the outcome of the daughter cells receiving the correct number of chromosomes Metaphase: - the spindle reaches its final form and moves all the chromosomes to midpoint (metaphase plate) - chromosomes complete their condensation and assume their characteristic shape - only when perfectly lined up do the chromatids begin to separate - this is the same as a karyotype, a collection of twin chromatids in a cell arranged to determine problems Anaphase: - sister chromatids separate and move to opposite poles - first signs of movement are seen at the centromere which may not always be in the middle - chromosome segregation is completed Telophase: - spindle disassembles - chromosomes at each spindle pole decondenses and returns to the extended state typical of interphase - nucleolus reforms, nuclear envelope forms around the chromosomes at each pole, forming a new nucleus, - the cell has two nuclei at this point 8.3D Cytokinesis Completes Cell Division by Dividing the Cytoplasm between Daughter Cells Cytokinesis: the division of cytoplasm, starts in telophase of a dividing cell. By the time it is complete the daughter cell is already entering the G1 phase of the next cell cycle. it is divided by a layer of microtubules that persist at the former spindle midpoint. Furrowing: - In animals, protists, and many fungi, the furrow girdles the cell and gradually deepens until it cuts the cytoplasm into two parts - the layer of microtubules that remains at the former spindle midpoint expands laterally it stretches entirely across the dividing cell - the layer develops and the band of microfilaments forms just the inside of the plasma membrane - this creates a belt that follows the inside boundary of the cell in the plane of the microtubule layer - this is powered by motor proteins and the microfilaments slide together tightening and constricting the cell - furrowing deepens and two daughter cells are separated CellPlateFormation: - in plant
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