CH. 11 THE CELL CYCLE AND CELL DIVISION

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CH. 11THE CELL CYCLE AND CELL DIVISION

• CHAPTER 11

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Lecture objectives

• Know difference between prokaryotic and
  eukaryotic cell division
• Know how eukaryotic cell division is controlled
• Be able to compare and contrast mitosis and
  meiosis
• Know what apoptosis is and understand how it
  occurs
• Know how unregulated cell division leads to
  cancer

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Why is cell division important?
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Events in cell division

• Signal
• DNA duplication
• Segregation
• Cytokinesis

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Binary fission (prokaryotes)
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Cell division in eukaryotes

• Signals for cell division are related to the
  needs of the entire organism.
• Many chromosomes replication and segregation are
  more intricate.
• DNA replication occurs between cell divisions
• Sister chromatidsnewly replicated chromosomes
  are closely associated.

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Mitosis meiosis

• Mitosis separates them into two new nuclei,
  identical to the parent cell.
• Meiosis is nuclear division in cells involved in
  sexual reproduction. The cells resulting from
  meiosis are not identical to the parent cells.

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The eukaryotic cell cycle
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Parts of the cell cycle

• G1
• S
• G2
• Restriction points
• M and Cytokinesis

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Signals that control cell division

• Cyclin-dependant kinases (cdks)
• Cdk is activated by binding to cyclin
• The G1-S cyclin-Cdk complex acts as a protein
  kinase and triggers transition from G1 to S.
• Other cyclin-Cdks act at different stages of the
  cell cycle.

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Cdks-cyclin complex
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retinoblastoma

• Progress past the restriction point in G1 depends
  on retinoblastoma protein (RB).
• RB normally inhibits the cell cycle, but when
  phosphorylated by G1-S cyclin-Cdk, RB becomes
  inactive and no longer blocks the cell cycle.

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How cdks-cyclin complex controls cell cycle
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Cyclins are transient during cell cycle
Cdks can be regulated by the presence or absence
of cyclins.
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• Cyclin-Cdks act at cell cycle checkpoints to
  regulate progress.
• Example If DNA is damaged during G1, p21 protein
  is made. p21 binds to G1 Cdks, preventing their
  activation. The cell cycle stops while DNA is
  repaired

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Factors affecting cell division

• Growth factors (platelet-derived growth factor)
• Interleukins
• Erythropoietin

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Chromatin, chromosomes, chromatid
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• DNA molecules are extensively packed even
  during interphase.
• Histones
• Nucleosomes.

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Dna densely packed
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Dna in mitosis
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segregation

• The centrosome determines the plane of cell
  division
• It doubles during S phase and will determine the
  spindle orientation
• Each centrosome can consist of two
  centrioleshollow tubes formed by microtubulesat
  right angles
• Plant cells lack centrosomes but have distinct
  microtubule organizing centers

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mitosis

• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase

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Mitotic spindle
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Phase of mitosis
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Sister chromatids
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Cytokinesis differs in animal plants
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Sexual reproduction

• Gametes
• Offspring genetically different from parents
• Somatic cells vs gametes
• Haploid vs Diploid
• Homologous chromosomes
• Fertilization (zygote)

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karyotype
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meiosis

• How diplontic life makes gametes
• Reduction in the number of chromosomes
• Genetic diversity

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Genetic recombination during cross over
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Crossing over
Important for Independent assortment
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• Prophase I may last a long time.
• Human males Prophase I lasts about 1 week, and 1
  month for entire meiotic cycle
• Human females Prophase I begins before birth,
  and ends up to decades later during the monthly
  ovarian cycle

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COMPARISON BETWEEN MITOSIS AND MEIOSIS
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MEIOTIC ERRORS

• NONDISJUNCTION
• TRANSLOCATION

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ANEUPLOIDY pOLYPLOIDY
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• Polyploidy (e.g., 3n) can prevent meiosis because
  not all chromosomes will have a homolog, and
  anaphase I will not take place.
• Polypolidy can enhance crop plants and triploidy
  produces sterile trout to stock rivers.

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Conditions of aneuploidy

• Down syndrome
• Miscarriages

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Cell death

• Necrosis
• Apoptosis

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Events in apoptosis

• Cell detaches from its neighbors
• Cuts up its chromatin into nucleosome-sized
  pieces
• Forms membranous lobes called blebs that break
  into fragments
• Surrounding living cells ingest the remains of
  the dead cell

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Cell death cycle is controlled by signals

• Lack of a mitotic signal (growth factor)
• Recognition of damaged DNA
• External signals cause membrane proteins to
  change shape and activate enzymes called
  caspaseshydrolyze proteins of membranes.

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apoptosis
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cancer

• Bening tumors
• Malignant tumors
• Metastasis

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A cancer cell and normal cells around it
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• There are factors that increase (positive) cell
  division (growth factors)
• Some inhibit (negative) cell division (RB)
• Oncogenes positive regulator
• Example HER2 gene in breast cancer
• Tumor Suppressors negative regulators (p21, p53,
  RB) may be blocked by the HPV

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Cancer treatment targets the cell cycle and tries
to inhibit division

• .
• Drugs such as 5-flourouracil block thymine, a
  base of DNA.
• Taxol interferes with the mitotic spindle.
• Herceptin targets only the HER2 growth factor
  receptor in some breast cancers.

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Cancer treatment interferes with cell cycle of
cancer cell