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Cell Cycle
• Repeating cycle of growth and division
• Interphase—takes up ~ 80% of cell’s life; 3 phases
• G1—cell is rapidly growing and carrying out routine functions (longest)
• S—synthesis phase when DNA replicates from a single chromatid into a full chromosome
• G2—organelles replicate and chemicals needed for mitosis are produced and sent to proper locations within the cell
Cell Division and Mitosis• When a cell becomes too large
to function properly due to surface area-to-volume ratio it will be stimulated to divide
• Prokaryotic cells (bacteria) undergo binary fission: asexual reproduction that produces identical daughter cells
• Can occur every 20 minutes!!• First the single circular strand
of DNA replicates, then the cell divides into 2 cells that are identical in every way—think “cloning”!
Eukaryotic Mitosis and Cell Division
• Due to all the organelles in eukaryotes, process more complex
• Mitosis—nuclear division—nucleus divides into 2 identical nuclei; made of 4 phases
• Replaces old cells in an organism
• Replaces damaged cells• Remembered by the
acronym: PMAT• Occurs only in somatic
cells—gametes are a whole another process!
P = Prophase
• Chromosomes begin to condense into fuzzy X’s and are visible under a light microscope
• Nuclear membrane starts to dissolve
• Centrioles start to move apart to opposite sides of cell
• Spindle fibers (protein microtubules) form between centrioles—these fibers help pull the chromosomes apart later
• Plants do not have centrioles, but they do have spindle fibers
M = Metaphase
• Second set of spindle fibers extend out from each chromosome’s centromere—2, one in each centriole direction
• Grow until they reach the first set of spindle fibers
• Spindle fibers line the chromosomes up at the center (equator) of the cell
A= Anaphase
• Centromeres divide and the 2 chromatids separate
• Spindle fibers shorten and the chromatids move toward opposite sides of the cell
• Looks like centrioles are reeling them in as if fishing!
T = Telophase
• Chromatids undergo name change and are now called chromosomes again
• Chromosomes start to uncoil at opposite sides of cell
• New nuclear membranes form around them
• Spindle fibers break and dissolve
Cytokinesis • Cell then starts to divide: cytoplasm is split into 2 new cells
• In animals a cleavage furrow forms between the nuclei—cytoplasm is pinched in half
• In plants vesicles (created by Golgi bodies) fuse at center of the cell and form a cell plate
• New cell wall is created on either side of the plate
• In both plants and animals, the 2 new cells each have their own organelles (formed during G2)