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3. Cells: The Living Units: Part D. Cell Cycle. Defines changes from formation of the cell until it reproduces Includes: Interphase Cell division (mitotic phase). Interphase. Period from cell formation to cell division Nuclear material called chromatin Four subphases:

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  1. 3 Cells: The Living Units: Part D

  2. Cell Cycle • Defines changes from formation of the cell until it reproduces • Includes: • Interphase • Cell division (mitotic phase)

  3. Interphase • Period from cell formation to cell division • Nuclear material called chromatin • Four subphases: • G1 (gap 1)—vigorous growth and metabolism • G0—gap phase in cells that permanently cease dividing • S (synthetic)—DNA replication • G2 (gap 2)—preparation for division

  4. G1 checkpoint (restriction point) S Growth and DNA synthesis G2 Growth and final preparations for division G1 Growth M G2 checkpoint Figure 3.31

  5. Centrosomes (each has 2 centrioles) Interphase Plasma membrane Nucleolus Chromatin Nuclear envelope Interphase Figure 3.33

  6. DNA Replication • DNA helices begin unwinding from the nucleosomes • Helicase untwists the double helix and exposes complementary chains • The Y-shaped site of replication is the replication fork • Each nucleotide strand serves as a template for building a new complementary strand

  7. DNA Replication • DNA polymerase only works in one direction • Continuous leading strand is synthesized • Discontinuous lagging strand is synthesized in segments • DNA ligase splices together short segments of discontinuous strand

  8. DNA Replication • End result: two DNA molecules formed from the original • This process is called semiconservative replication

  9. Old strand acts as a template for synthesis of new strand DNA polymerase Free nucleotides Chromosome Leading strand Two new strands (leading and lagging) synthesized in opposite directions Lagging strand Old DNA Helicase unwinds the double helix and exposes the bases Replication fork Adenine Thymine Cytosine DNA polymerase Old (template) strand Guanine Figure 3.32

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