The Cell Cycle

1
The Cell Cycle

• Chapter 12
• p. 218-228

2
Cell Division

• An important part of a cells life cycle is
  reproduction
• Cell Division division of an existing cell into
  2 genetically identical daughter cells
• 3 functions
• 1) reproduction,
• 2) growth and development
• 3) tissue repair/renewal

3
Reproduction, Growth Repair

• 1) Reproduction
• Asexual produces 2 identical cells (Mitosis)
• Sexual combines DNA from 2 parents to form new
  offspring (Meiosis Ch 13)
• 2) Growth
• Development of organism from 1 cell
• 3) Repair
• Occurs throughout life
• i.e. RBCs

4
Distribution of Chromosomes

• For most of cell cycle, DNA is packaged w/protein
  in chromatin (long, thin fiber)
• During Mitosis
• 1) Identical copy of DNA is made
• 2) DNA condenses into chromosomes (46)
• Composed of 2 identical sister chromatids,
  connected by centromere

5

• 3) Sister chromatids pull apart to opposite sides
  of cell
• 4) Cell splits, forming 2 new identical daughter
  cells (46 chromos. diploid)
• During Meiosis, each parent supplies1/2 chromos.
  needed (23 chromos. haploid))
• Occurs in reproductive organs (ovaries/testes)
• Cells must divide twice to cut chromosome number
  in half

6
The Cell Cycle an overview

• Most of cells life spent in Interphase (mitosis
  very brief)
• Grow, produce proteins organelles
• 3 Phases
• 1) G1 grows to full size
• 2) S replicates DNA
• 3) G2 prepares for mitosis

7
Phases of Mitosis an overview

• 1) Prophase
• DNA condenses
• Nucleolus disappears
• Spindle begins to form
• 2) Prometaphase
• Nuclear envelope breaks apart
• Spindles attach to kinetochore

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• 3) Metaphase
• Chromosomes line up on metaphase plate
• 4) Anaphase
• Sister chromatids pulled apart to opposite sides
  of cell
• 5) Telophase
• 2 new nuclear envelopes form
• Cytoplasm divides (cytokinesis), forming 2 new
  daughter cells

Mitosis in action!!!
9
The Mitotic Spindle

• Consists of fibers of microtubules proteins
• Assembled in centrosome at either end (pole) of
  cell
• Attach to kinetochore of each sister chromatid
• Align chromosomes along metaphase plate down
  center of cell

10
Cytokinesis

• Division of cytoplasm
• Cleavage pinching in of plasma membrane along
  cleavage furrow
• Plants develop cell plate ? new cell walls

11
Mitosis in Plant Cells
12
Binary Fission

• Prokaryotic cell division
• DNA replicates, each copy moves to cell poles
• Plasma membrane grows inward cell divides

13
Evolution of Mitosis
14
Regulation of the Cell Cycle

• Chapter 12
• p. 228-235

15
Regulation of Cell Cycle an overview

• Frequency of cell division depends on type of
  cell
• i.e. skin (fast), nerve (almost never)
• Cell Cycle Control System directs events of cell
  cycle
• Built-in molecular clock that triggers each
  phase
• Regulated at internal/external checkpoints

16
Cell Cycle Checkpoints

• Occur at G1, G2, M phases
• Stop cycle until requirements for next phase are
  met
• If not, cell exits cycle enters G0 phase
  (non-dividing state)
• G1 Checkpoint restriction point if clear, cell
  will usually complete cycle

17
Cell Cycle Clock

• Cycle is controlled by bonding of certain protein
  molecules in cell to each other
• 1) Cyclin-dependent kinases (Cdks)
• 2) Cyclin conc. fluctuates
• Rises until M phase, then drops off
• Together, make MPF complex
• Initiates mitosis by phosphorylating certain
  proteins
• Turns off by destroying cyclin

18
Internal Signals

• Occur at anaphase
• Will not proceed until all kinetochores are
  properly attached to spindles
• Anaphase Promoting Complex (APC) becomes active,
  cyclin breaks down, chromatids separate

19
External Signals

• Most cells depend on external signals to control
  cell division
• 1) Growth Factors proteins that stimulate other
  cells to divide
• i.e. PDGF stimulates platelet growth after
  injury (signal-transduction pthwy)
• 2) Density-Dependent Inhibition cells stop
  dividing if become crowded
• 3) Anchorage Dependence animal cells must be
  attached to substratum to divide

20
Cancer Cells Escape Cycle Controls

• Not affected by growth factors, density, or
  anchorage
• 1) Normal cell undergoes transformation ? cancer
  cell
• 2) Cancerous cell reproduces to form tumor
• 3) If remains in place ? benign
• If spreads impairs organs ? malignant
• May enter bloodstream, have abnormal
  chromosomes, affect metabolism
• Spread quickly (metastasis) may require
  radiation or chemotherapy