The Cell Cycle

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The Cell Cycle

• Chapter 12

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• Cell division - process cells reproduce
  necessary to living things.
• Cell division due to cell cycle (life of cell
  from origin in division of parent cell until own
  division into 2)
• Unicellular organisms - results in many new
  members.

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• Multicellular organisms - division helps in
  development of organism and repair and renew
  preexisting cells
• Requires distribution of identical genetic
  material (DNA) to 2 daughter cells.

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• Genome - cells genetic information packaged as
  DNA.
• DNA molecules packaged into chromosomes.
• Body cells - somatic cells sex cells - gametes.
• DNA has proteins maintains structure helps
  control gene activity.

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• Duplicated chromosome - 2 sister chromatids
  (identical copies of chromosomes DNA)
• Region where strands connect shrinks to narrow
  area (centromere)

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• Processes continue every day to replace dead and
  damaged cells.
• Produce clones - cells with same genetic
  information.

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http//www.s8int.com/images2/cloned.jpg
Cloned cells
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• Mitotic (M) phase of cell cycle alternates with
  much longer interphase.
• M phase includes mitosis, cytokinesis.
• Interphase - 90 of cell cycle.

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• Interphase - cell grows by producing proteins and
  cytoplasmic organelles, copies chromosomes,
  prepares for cell division 3 subphases.
• 1G1 phase (first gap) - growth.
• 2S phase (synthesis) -chromosomes copied.
• 3G2 phase (second gap) - cell completes
  preparations for cell division.

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http//www.fhcrc.org/science/labs/fero/RL_gifs/cyc
le.jpg
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• Mitosis 5 subphases.
• End interphase - centrosomes duplicated, begin to
  organize microtubules into aster (star).

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• 1Prophase - chromosomes tightly coiled, with
  sister chromatids joined together.
• Nucleoli disappear mitotic spindle forms,
  appears to push centrosomes away toward opposite
  ends (poles) of cell.

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• 2Prometaphase - nuclear envelope fragments and
  microtubules from spindle interact with
  chromosomes.
• Microtubules from 1 pole attach to 1 of 2
  kinetochores (special regions of centromere),
  microtubules from other pole attach to other
  kinetochore.

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• 3Metaphase - spindle fibers push sister
  chromatids until all arranged at metaphase plate
  (imaginary plane equidistant between poles)

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• 4Anaphase - centromeres divide, separating sister
  chromatids.
• Each pulled toward pole to which it is attached
  by spindle fibers.
• 2 poles have equivalent collections of
  chromosomes.

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• 5Telophase - cell elongates free spindle fibers
  from each centrosome push off each other.
• 2 nuclei form, surrounded by fragments of
  parents nuclear envelope.
• Cytokinesis (division of cytoplasm) begins.

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• Animals - cytokinesis (cleavage) - appearance of
  cleavage furrow in cell surface near old
  metaphase plate.
• Cytoplasmic side of cleavage furrow contractile
  ring of actin microfilaments and motor protein
  myosin form.
• Contraction of ring pinches cell in 2.

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• Plants, cytokinesis - cell plate between dividing
  cells.
• Plate enlarges until membranes fuse with plasma
  membrane at perimeter contents vesicles forming
  new wall material in between.

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Bacteria

• Prokaryotes - binary fission.
• DNA of bacteria coiled, highly packed.
• Binary fission - chromosome replication begins at
  1 point in circular chromosome, (origin of
  replication).
• Copied regions move to opposite ends of cell.

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• As chromosome replicates and copied regions move
  to opposite ends of cell, bacterium grows until
  it reaches 2x original size.
• Cell division involves inward growth of plasma
  membrane, dividing parent cell into 2 daughter
  cells with complete genome.

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Regulation of cell cycle

• Some cells divide frequently in life (skin
  cells), others can divide (reserve - liver cells)
  mature nerve, muscle cells do not divide at all.
• Some control over when cells divide/how often
  they divide in lifetime.

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http//www.ii.bham.ac.uk/webs/shuttleworth/bbsrc1.
jpg
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• Cycle driven by specific chemical signals in
  cytoplasm.
• Events of cell cycle directed by cell cycle
  control system.
• Checkpoint in cell cycle is critical control
  point where stop/go signals regulate cycle.
• 3 major checkpoints found in G1, G2, and M
  phases.

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• G1 checkpoint (most important), cell either get
  go ahead to finish cycle and divide, or receive
  stop signal.
• If stop signal - goes into G0 phase (remains in
  limbo waiting to start).
• Most human cells in this mode.

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http//www.microscopy-uk.org.uk/mag/imgaug99/01.jp
g
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• Proteins, kinases, can activate/deactivate other
  proteins.
• Kinases always present in cell need cyclins
  (protein) to activate.
• Complex of kinases and cyclin - cyclin-dependent
  kinases (Cdks).

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http//www.mie.utoronto.ca/labs/lcdlab/biopic/fig/
9.5.jpg
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• MPF (maturation-promoting factor) triggers
  cells passage past G2 checkpoint to M phase.
• G1 checkpoint regulated by at least 3 Cdk
  proteins and several cyclins.

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http//www.uic.edu/classes/bios/bios100/summer2002
/cdk02.gif
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• M phase checkpoint makes sure chromosomes are
  attached to spindle so each cell ends up with
  right amount of chromosomes.
• Cell division influenced by growth factors,
  proteins released by 1 group of cells that
  stimulate other cells to divide.

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http//www.fhcrc.org/science/education/courses/can
cer_course/basic/img/growth_factors.gif
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• Platelet growth driven by growth factors.
• Presence of injury - released to stimulate
  division of platelet cells to seal wound.
• Density of cells too high - cell division
  inhibited.

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Cancer

• Cancer cells divide out of control - no
  regulation.
• Can either produce own growth factors or have
  problem in signaling pathway.
• Can divide indefinitely if they have continual
  supply of nutrients.

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http//www.sandia.gov/news/resources/releases/2005
/images/mitopic.jpg
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• Starts when single cell undergoes transformation
  to change it into cancer cell.
• If immune system does not destroy, can form tumor
  (gathering of cells).
• If tumor does not invade other areas - benign.
• If it does - malignant.
• If cells get into blood stream, travel throughout
  body (metastasis).

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http//www.livercancer.com/images/metastasis.gif
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• http//www.teachersdomain.org/resources/tdc02/sci/
  life/stru/dnadivide/index.html