Cell Cycle

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Cell Cycle Cell Division
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Cell Cycle

• Interphase
• consists of Gap1, Synthesis, and Gap2 phases
• cells spend most of time here

• Mitosis
• also called Mphase
• set of steps responsisble for division of nuclear
  material and cytoplasm to two, identical daughter
  cells
• prophase, metaphase, anaphase, telophase

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Control of cell cycle- interphase

• specialized cells divide at different rates
• developing embryos? quickly
• red blood cells, muscle cells, nerve cells ? lose
  capacity to divide as they mature
• cortical cells in plants ? divide only when
  necessary

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Control of cell cycle- interphase

• commitment to enter mitosis occurs at the G1 ? S
  phase transition
• cyclin-dependent kinase (Cdk) enzyme that
  controls the transition from G1 ? S, and G2?
  Mphase
• kinase removes Pi from ATP to another protein
  (phosphorylation)
• allosteric regulation require the bonding of the
  protein cyclin to become active (exposes the
  active site of the enzyme)

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Cyclin-Cdk complexes

• act at checkpoints, or points at which a cell
  cycles progress can be monitored to see if the
  next step can continue
• if DNA is damaged by radiation during G1, the
  protein p21 (21,000D molecular weight) is made
  binds to 2 G1 Cdks blocking the bonding of the
  cyclins
• when DNA is repaired, the p21 diassociates from
  Cdks

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Involvement of different cyclins

• Cyclin D-Cdk4 acts during the middle of G1 at the
  restriction point (R)
• retinoblastoma protein (RB) inhibits the cell
  cycle unless it is inhibited by phosphorylation
  (enzymes Cdk4 Cdk2)
• Cyclin E-Cdk2 middle of G1
• Cyclin A- Cdk2 acts during S phase ? stimulates
  DNA replication
• Cyclin B-Cdk1 acts at the G2-M boundary,
  initiating mitosis

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Control of cell cycle- growth factors

• growth factors proteins that externally signal
  cells that divide infrequently
• ex. cut your skin platelets gather at the wound
  to stop bleading produce a platelet-derived
  growth factor diffuses to adjacent skin cells to
  stimulate division.
• interleukins made by WBC and promote cell
  division in immune cells
• erythropoietin made by kidney stimulate
  division in bone marrow of red blood cells.

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Cancer no cell cycle control

• cyclin-Cdk controls are disrupted
• fast growing breast cancer too much cyclin D is
  made overstimulates Cdk4

• some cancer cells make their own growth factors
• some cells become cancerous b/c they no longer
  require growth hormones

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DNA in cell cycle

• during interphase, DNA is in the form of
  chromatin
• DNA is unwound, but packed together around
  nucleosomes
• between nucleosomes is unwound DNA linkers

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DNA in cell cycle

• During prophase in mitosis and meiosis, the DNA
  binds more tightly around the nucleosomes
  nucleosomes condense, forming visible chromosomes
• continues until chromosomes begin to move apart
  in metaphase

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Interphase 3 parts

• Gap1 phase
• cell biochemically prepares for S phase
• varies in length
• Synthesis phase
• replication of DNA
• Gap2 phase
• cell makes preparations for mitosis
• synthesizes components of microtubules to move
  chromosomes
• chromosomes now consist of connected, identical
  sister chromatids

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Cytokinesis division of cytoplasm

• Plane of cell division
• once commitment to enter S phase has been made
  DNA is replicated, centrosomes are replicated
• cetrosomes organelles that lie near the nucleus
• each may contain 2 centrioles (animal cells)
  hollow tubes lined with 9 microtubules
• G2-M transition two centrosomes separate moving
  to opposite ends of nuclear membrane ? determines
  plane at which cell will divide

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Mitotic spindles
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Cytokinesis animal vs. plant

• Animal cells
• cytoplasm begins to furrow by the contraction
  of actin and myosin microfilaments
• cell pinches into two

• Plant cells
• when spindle breaks down after mitosis, vessicles
  derived from Golgi apparatus appear at the
  equatorial plane
• vessicles are propelled along plane by protein
  kinesin ? plasma membrane ? cell plate ? cell
  wall

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