How Cells Divide

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How Cells Divide

• Chapter 11

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Outline

• Cell Division in Prokaryotes
• Discovery of Chromosomes
• Structure of Chromosomes
• Phases of the Cell Cycle
• Interphase
• Mitosis
• Cytokinesis
• Cell Cycle Control
• Cancer

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Cell Division in Prokaryotes

• Prokaryotic cell division occurs as binary
  fission in which cell divides into two halves.
• Genetic information exists as a single, circular
  double-stranded DNA molecule.
• Copying begins at replication origin, and
  proceeds bi-directionally.
• One genome ends up in each daughter cell.

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Binary Fission
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Discovery of Chromosomes

• All eukaryotic cells store genetic information in
  chromosomes.
• Most eukaryotes have between 10 and 50
  chromosomes in their body cells.
• Human cells have 46 chromosomes.
• 23 nearly-identical pairs

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Structure of Chromosomes

• Chromosomes are composed of a complex of DNA and
  protein, chromatin.
• heterochromatin - not expressed
• euchromatin - expressed
• DNA exists as a single, long, double-stranded
  fiber extending chromosomes entire length.
• forms nucleosome every 200 nucleotides
• DNA coiled around histone proteins

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Eukaryotic Chromosomal Organization
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Structure of Chromosomes

• Karyotype - Individuals particular array of
  chromosomes.
• diploid - A cell possessing two copies of each
  chromosome (human body cells).
• Homologous chromosomes are made up of sister
  chromatids joined at the centromere.
• haploid - A cell possessing a single copy of each
  chromosome (human sex cells).

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Chromosomes
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Phases of the Cell Cycle

• Five phases of cell division
• G1 - primary growth phase
• S - genome replicated
• G2 - secondary growth phase
• collectively called interphase
• M - mitosis
• C - cytokinesis

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Cell Cycle
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Interphase

• G1 - cells undergo majority of growth
• S - each chromosome replicates to produce sister
  chromatids
• attached at centromere
• contains attachment site (kinetochore)
• G2 - chromosomes condense
• assemble machinery such as centrioles

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Mitosis

• Prophase
• spindle apparatus assembled
• Microtubules connect kinetochores on each pair of
  sister chromatids to the spindle poles.
• nuclear envelope breaks

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Mitosis

• Metaphase
• chromosomes align in cells center
• metaphase plate
• Spindle forms with spindle fibres

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Mitosis

• Anaphase
• sister chromatids pulled toward poles
• poles move apart
• centromeres move toward poles
• microtubules shorten
• Telophase
• spindle disassembles
• nuclear envelope forms around each set of sister
  chromatids

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Cytokinesis

• Cleavage of cell into two halves
• animal cells
• constriction belt of actin filaments
• plant cells
• cell plate
• fungi and protists
• mitosis occurs within the nucleus

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Cytokinesis
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Cell Cycle Control

• Two irreversible points in cell cycle
• replication of genetic material
• separation of sister chromatids
• Cell can be put on hold at specific checkpoints.

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Cell Control Cycle

• G1 / S - primary division decision point
• G2 / M - commitment to mitosis
• Spindle checkpoint - all chromosomes are attached
  to spindle

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Growth Factors and the Cell Cycle

• Each growing cell binds minute amounts of
  positive regulatory signals (growth factors) that
  stimulate cell division.
• If neighboring cells use up too much growth
  factor, there is not enough left to trigger cell
  division.
• Growth factors trigger intercellular signaling
  systems.

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Cancer and Cell Proliferation

• Cancer is essentially a failure of cell division
  control (unrestrained, uncontrolled cellular
  growth).
• gene p53 plays a role in G1 checkpoint
• halts cell division if detects damaged DNA and
  stimulates activity of repair enzymes
• appears to be absent in many cancerous cells

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Growth Factors and Cancer

• Growth factors influence cell cycle
• proto-oncogenes - normal cellular genes that
  become oncogenes when mutated
• positive approach that stimulates cell growth
• tumor-suppressor genes - negative approach that
  inhibits cell division

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