SCIE1106 Lecture Notes - Lecture 25: Endonuclease, Centromere, Microtubule
Cell Cycle and Apoptosis
Monday, 3 October 2016
12:35 pm
Homeostasis:
• Adult (differentiated) cells - a steady state system
• DNA constantly reads out into a particular set of RNAs - proteins
• Rate of cell death = cell division
• Cell proliferation is controlled by the cell cycle - an ordered series of events leading to
replication of cells
• The cell cycle is a dynamic system
• Cell death because of trauma, toxins, infection - necrosis (rapid)
• Controlled cell death, achieved through apoptosis
Mitosis and Meiosis:
• Mitosis and meiosis enable maintenance of genetic continuity
• Mitosis - 2 cells with the same number of chromosomes as the parent cell are
produced - somatic cell division
• Meiosis - cells with 1/2 the number of chromosomes as the parent are produced,
gamete production
• Meiosis ensures genetic material is not doubled in new generations of sexually
reproducing organisms
The Eukaryotic Cell Cycle:
• Interphase
o Period between cell divisions
o Cell grows, develops and prepares for division
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o 3/4 phases
• Mitosis
o Process of nuclear division (karyokinesis)
o Process of cytoplasmic division (cytokinesis)
Interphase:
• 3 phases
o G1 - first gap phase
o S - synthesis phase
o G2 - second gap phase
• DNA and other cell constituents synthesised
• DNA not visible, decondensed state (chromatin)
• S, G2 and M are normally fixed duration
• G1 variable as call cycle can enter optional G0 resting phase
• Some cells, rapidly dividing (e.g. embryo) have no G0 phase
• Many differentiated cells are in G0 for remainder of their lives
• Stem cells fluctuate between G0 and the cell-division cycle
G1 Phase:
• Gap phase before S phase (DNA synthesis)
• Cell grows, undertakes normal metabolism and synthesises proteins needed for cell
division
• Contains G1/S checkpoint of cell division
• Passing G1/S checkpoint commits cell to cell division
• Cells can exit cell division prior to G1/S checkpoint
G0 Phase:
• During G1, cells may exit the cell cycle in response to regulatory signals
• Enter a non-dividing state
• Cell maintains a constant size
• Cells may stay in G0 for an extended period, or even indefinitely
• Cells may re-enter G1
• Not all cells enter G0
S Phase:
• DNA is replicated
• Amount of DNA doubles
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