Chapter 8.docx

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University of Toronto Scarborough
Biological Sciences
Mark Fitzpatrick

Chapter 8: Cell Cycles Three cellular processes:  Dna replication  A dynamically changing cytoskeleton  Cell cycle checkpoints 8.1 Cycle of Cell Growth and Division: An Overview New progency cells are needed:  Expand population size  Multicellular tissue growth  Asexual reproduction  Replacement of cell lost to wear and tear 8.2 Cell Cycle in Prokaryotic Cells  Binary fission is the mechanism of prokaryotic cell division; that is splitting or dividing into two parts.  Three periods: Initiating DNA synthesis, chromosome separation to opposite ends, and membrane that pinches together and forms the two daughter cells. 8.2a Replication Occupies Most of the Cell Cycle in Rapidly Dividing Prokaryotic Cells  DNA is hereditary material is bacteria an archaea  Single circular chromosome of double stranded DNA, packed in a nucleoid 8.2b Replicated Chromosomes are Distributed Actively to the Halves of the Prokaryotic Cell  Chromosome separation used to be thought of as passive, but now known to be active:  Model seen on page 163 Figure 8.3  Origin of Replication (ori)- the region of a bacterial chromosome where replication begins (Middle of cell where enzymes are)  Once ori duplicates, the two new origins migrate towards the opposite ends of the cell as replication continues for the rest of the chromosomes.  Cytoplasmic division is associated with an inward constriction of a cytokinetic ring of cytoskeletal proteins.  Two divide the cell into two parts a plasma membrane and cell wall material is assembled 8.2c Mitosis has Evolved from an Early Form of Binary Fission  Work efficiently because most prokaryotic organisms have only a single chromosome  However genetic info of eukaryotes is among several chromosomes with each chromosome containing a much greater length of DNA that a prokaryotic one. If a daughter cell fails to receive a copy of even one chromosome, the result is disastrous.  Because eukaryotic chromosomes are within a nuclear membrane for most of the cycle, mitosis is the mechanism used instead of binary fission  Mitosis can hold two new double stranded DNA (chromatids) together following DNA synthesis enabling to keep track so such a long replicated chromosomes and orienting them to proper places at proper time.  Nuclear membrane disintegrate when chromosomes are distributes and reform around them in daughter cells.  In more advanced, there are spindles of microtubules made of polymerized tubulin protein that are form and chromosome segregate without disintegrating 8.3 Mitosis and the Eukaryotic Cell Cycle  Three elegantly interrelated system contribute to mitosis:  Elaborate master program of molecular check and balances  Process of DNA syntehssi replicates each DNA chromosome into two copies  Cables and motors of cytoskeleton separates the DNA copies precisely into the daughter cells 8.3a Chromosomes are the Genetic Units Divided by Mitosis  Eukaryotic hereditary info is distributed in several linear double stranded DNA molecules  Chromosome in a cell is composed of one of these DNA molecule along with its protein.  Most eukaryotes have two copies of each type of chromosome and so they are known as diploid (2n). Humans have 23 different pairs of chromosomes for a total of 46.  Other eukaryotes are haploid, as they only have one copy of each type of chromosome.  Some can grow as haploid or diploid.  Ploidy is the number of chromosome sets of a cell or species  Replication of the DNA of each individual chromosome create two new identical molecules called sister chromatids.  Sister chromatids are held at centromere until mitosis seperates them, placing one in each daughter nuclei.  Chromosome segregation is the equal distribution of daughter chromosomes to each of the two cell that result from cell division.  EXAMPLE: Typical fruitfly has 4 different chromosome and each body cell is diploid so the fly has a total of 8 chromosome. Each of the 8 are composed of one double stranded DNA molecule. Two identical DNA molecules are made from 8 originals. Two sister chromatids are attached right now but still referred to as one chromosome.  Thus before replication, one chromosome has one DNA but after one chromosome has 2 DNAs. Amount of DNA increases but not chromosome.  Eight DNA molecules before synthesis and eight pairs after. (16).  Eight chromosome before DNA synthesis and 8 replicated chromosomes after.  During division each daughter cell receives one sister chromatid. 8.3b Interphase Extends from the End of One Mitosis to the Beginning of the Next Mitosis  First and longest phase of mitosis is interphase.  During interphase, the cell grows and replicates its DNA in preparation for mitosis (M phase)  Begins as a daughter cell from a previous division cycle and enters an initial period of cytoplasmic
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