Cell cycle and mitosis

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  • Created by: Jess
  • Created on: 31-03-15 15:00
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  • Cell cycle and mitosis
    • Prokaryotes reproduce by binary fission, caused by environmental reproductive signal.
      • Usually contain one circular, compacted and folded chromosome.
        • Two important regions for replication
          • Ori - Origin of replication
            • Replication occurs through a "replication complex" of proteins
              • After replication, the ori regions move towards opposite ends of the cell, aided by special proteins
                • Pinching of the plasma membrane and synthesis of new cell wall materials result in separation of the two cells
          • Ter - Termination of replication
            • Replication occurs through a "replication complex" of proteins
              • After replication, the ori regions move towards opposite ends of the cell, aided by special proteins
                • Pinching of the plasma membrane and synthesis of new cell wall materials result in separation of the two cells
      • Two important regions for replication
        • Ori - Origin of replication
          • Ter - Termination of replication
        • Eukaryote reproduction is related to the needs of an entire organism rather than the single cell.
          • Eukaryotes have many chromosomes thus DNA replication and segregation is far more intricate than that of prokaryotes
          • The Cell Cycle: The period from one cell division to the next
            • Interphase (G1>S>G2)
              • G1: Chromosomes are single, unreplicated structures. Variable time period (Some cells enter a resting phase G0)
              • S: DNA replicates and sister chromatids remain together
              • G2: Cell prepares for mitosis E.g. producing spindle fibres
              • Signals that trigger the transitions between the phases act through cyclin-dependant kinases (Cdk's)
                • Cdk is activated by binding to cyclin; altering its shape, exposing its active site by phosphorylation
                  • E.g. Progress past the G1-S point is normally inhibited by retinoblastoma protein, but when phosphorylated by the G1-S cyclin-Cdk, RB becomes inactive and no longer blocks the cycle
            • MITOSIS and DNA structure
              • Chromosomes condense to make separation easier
                • DNA molecules normally associate with proteins (Histones) to form nucleosomes which are bound tightly to form chromatin
              • Phases of mitosis
                • PROPHASE: Chromatin condenses and chromatids become visible
                  • METAPHASE: Chromosomes line up at the midline
                    • ANAPHASE: Chromatids separate and move to opposite poles
                      • TELOPHASE: Nuclear envelope reform, spindle disappears and chromosomes become less compact.
                      • NB Chromatids become daughter CHROMOSOMES that have their own centromere as opposed to sharing one.
                      • Seperation of sister chromatids is controlled by cyclin- Cdk: it activates anaphase promoting complex.
                  • Late in Prophase kinetochores form in the centromere region of each chromosome. Later the kinetochore microtubules attach to them and ensure that the chromatids will move to opposite poles.
                    • Kinetochores also contain motor proteins that alongside the shortening of the kinetochore microtubules cause the chromosome to move along the microtubules towards the poles
      • SPINDLE APPARATUS: Moves the sister chromatids apart. Made of microtubules and are controlled by centrosomes.
        • Centrosomes identify the poles toward which the chromosomes move, formed by centrioles.
        • Late in Prophase kinetochores form in the centromere region of each chromosome. Later the kinetochore microtubules attach to them and ensure that the chromatids will move to opposite poles.
          • Kinetochores also contain motor proteins that alongside the shortening of the kinetochore microtubules cause the chromosome to move along the microtubules towards the poles

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