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Biology Chapter 11.docx

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Lovaye Kajiura

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tyBiology Chapter 11: The Cell Cycle Bernard Ho October 15, 2010 − Sperm and eggs are male and female reproductive cells (gametes) − All other cell types are referred to as somatic cells − Mitosis is the division of genetic material that produces daughter cells that are genetically identical to their parent cell − Mitosis is usually accompanied by cytokinesis (the division of cytoplasm into two daughter cells) − Together, mitosis and cytokinesis are responsible for three key events o Growth o Wound repair o Reproduction Mitosis and the Cell Cycle − Mitosis occurs in somatic (non-sex) cells − Daughter cells are genetically identical to parent cell − Chromosomes consists of a single, long DNA strand that is wrapped our proteins − A gene is a length of DNA that codes for a particular protein or RNA found in the cell − M Phase and Interphase o Growing cells cycle between a dividing phase called the mitotic (M) phase and a nondividing phase called interphase o During M phase the chromosomes are condensed into compact structures o During interphase, the chromosomes are uncoiled into extremely long, thin structures o Researchers also discovered a phase where DNA was synthesized (S phase) o Interphase includes G1, S and G2phase − During cell cycle, two key events are o Replication of hereditary material in chromosomes o Partitioning of copied chromosomes to the two daughter cells during M phase o Hereditary material is duplicated with one copy going to each daughter cell during mitosis o As a result, daughter cells contain genetic information identical to that of the parent cell − Discovery of Gap Phases o G 1hase occurs after M phase, but before S phase  In G1phase, cell growth and duplication of organelles happen o Lag in cell cycle between S phase and M phase cam to be known as the G Phase (second gap) 2  In G2phase, cell growth and duplication of organelle continue to build the protein “machinery” o The two gap phases provide the time required to replicate organelles, manufacture additional cytoplasm o Also, parent cell must grow large enough and synthesize enough organelles that its daughter cells will be normal in size and function How Mitosis Takes Place − How bacteria divide o Bacterial cells divide into two genetically identical daughters through binary fission o Chromosome is located mid-cell o Chromosome replicates o Chromosomes pull apart, ring of FtsZ protein forms o FtsZ ring constricts, membrane and cell wall infold o Fission complete − Events in mitosis o Eukaryotic chromosomes normally exist as extremely long, threadlike strands consisting of DNA associated with globular proteins called histones o In eukaryotes DNA-protein material is called chromatin o Each of the DNA copies in a replicated chromosome is called a chromatid o The two chromatids are joined together along their entire length as well as at a specialized region of the chromosome called the centromere o Chromatids from the same chromosome are referred to as sister chromatids o Sister chromatids represent exact copies of the same genetic information o Each chromatid contains one long DNA double helix o At the start of M phase, each chromosome consists of two sister chromatids that are attached to one another at the centromere o During mitosis, the two sister chromatids separate to form independent chromosomes and one copy of each chromosome goes to each of the two daughter cells o As a result, each daughter cell receives a copy of the genetic information that is contained in each chromosome o Every daughter cell ends up with exactly the same complement of chromosomes as the parent cell had prior to replication and thus every daughter cell receives the same genetic information o Proteins needed for Mitosis: Cohesins, microtubules, nuclear lamins and condensins o Prior to mitosis  Interphase • After chromosome replication,each chromosome is composed of two sister chromatids and centrosomes have replicated o Prophase  Marked by the formation of mitotic spindle (structure that produces mechanical forces that pull chromosomes into the daughter cells during mitosis)  Mitotic spindle consists of an array of microtubules in centrosome (animals) or MOC (plants)  Groups of microtubules attach to the chromosomes and are called spindle fibres  In all eukaryotes, spindle fibres originate from a microtubule organizing centre  Mitotic spindles either begin moving to opposite sides of the cell or form on opposite sides  Nucleoli begin to disappear  Chromatin fibers contract by tightly coiling  Chromosomes are visible and consist of two identical sister chromatids joined together at the centromere o Prometaphase  Once chromosomes have condensed, the nucleolus disappears and the nuclear envelope fragments  After disintegration of nuclear envelope, spindle fibres from each mitotic spindle attach to one of two sister chromatids of each chromosome  Attachment between spindle fibres and each chromatid is made at a structure called the kinetochore (located at the centromere region of chromosome, where sister chromatids are attached to each other)  Each chromosome has two kinetochores where spindle fibres attach (one on each side)  Kinetochore microtubules are polymerized, randomly oriented at first and then they become aligned parallel with microtubules  Centrosomes continue their movement to opposite poles of cell  Microtubules that are attached to the kinetochores being moving the chromosomes to middle of cell  Non-kinetochore microtubules radiate from each centrosome towards metaphase plate without attaching to chromosomes • May overlap with those from different poles • Assist in elongating entire cell during anaphase • Form a cage-like network, which facilitates the activities of cell cycle components o Metaphase  Animal centrosomes complete migration to opposite poles of cell  In all eukaryotes, kinetochore microtubules finish moving the chromosomes to the middle of cell  When metaphase is over, chromosomes are lined up along imaginary plane called metaphase plate  At this point, mitotic spindle is complete  Each chromatid is attached to spindle fibres that run from its kinetochore to one of two poles in cell  Each chromosome is held by kinetochore spindle fibres reaching to opposite poles and exerting same amount of tension or pull o Anaphase  Centromeres that hold sister chromatids together split, a process called disjunctional segregation  Sister chromatids are pulled apart equally to create independent chromosomes  Kinetochore spindle fibres begin to shorten  As they do, motor proteins pull chromosomes to opposite poles of cell  Two poles of cell are also pushed away from each other by motor proteins associated with microtubules that are not attached to chromosomes  Replicated chromosomes split into two identical sets of unreplicated chromosomes (ensures that each daughter cell receives same complement of chromosomes)  Cell elongates  When anaphase is complete, each pole of cell has an equivalent and complete collection of chromosomes that are identical to those present in parent cell prior to chromosome replication  As soon as sister chromatids separate, they are no longer chromatids, they are chromosomes o Telophase  Nuclear envelope beings to form around each set of chromosomes  Mitotic spindle disintegrates and chromosomes being to de- condense  Non-kinetochore microtubules further elongate the cell  Once two independent nuclei have formed, mitosis is complete o Cytokinesis  Cytoplasm divides to form daughter cells, each with own nuclei and set of organelles  In animals, fungi and slime moulds, cytokinesis begins with formation of cleavage furrow that pinches to parent cell in two • Due to actin filaments and myosin  In plants, cytoplasm is divided by cell plate that forms in the middle of parent cell − How chromosomes move during mitosis o Motor protein causes chromosome to move along mictrotubule o As that happens, microtubule excretes α and β tubulin subunits o Tubulin subunits of kinetochore microtubules are depolymerised from kinetochore ends o Motor proteins (dynein) attach and detach along kinetochore microtubules length (chromosome moves) o Experiment with bleached dye show that microtubules shorten at kinetochores Control of Cell Cycle − In rapidly dividing cells1 G is essentially eliminated − Most non-dividing cells in contrast, are permanently stuck in 1 − Researchers refer to this as the G0state − Cells that are in G 0ave effectively exited the cell cycle and are sometimes referred to as post-mitotic o Non-dividing state o Stimulated by environmental factors so they do divide  Ex. Alcohol and liver  Can go back from G to 0ell cycle  Triggered by external cues (growth factors released upon injury) − Nerve cells, muscle cells and many other cell types enter G once th0y have matured − Discovery of cell-cycle regulatory molecules o Heterokaryon (different nuclei)  Can use electric charge or chemicals to fuse heterokaryon o Cell fusion experiments (Johnson and Rao)  When investigators fused cells that were in different stages of the cell
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