BIOL 111 Chapter 12

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Cell Division and the Cell Cycle

Why do cells need to divide?

growth and development; cell replacement

Where do cells divide?

everywhere! (particularly in gut, hair, skin)

What is the end result of cell division?

two genetically identical sister cells

Prokaryotes undergo binary fission — no nucleus to divide; just replicate the genetic material and pull apart

Eukaryotes undergo mitosis and cytokinesis to take care of all of the membrane-bound organelles

Eukaryotic Division Components

Chromosome

a tightly-wound strand/packet of DNA

Sister Chromatids

two identical chromosomes joined by a centromere

Centromere

The joining mechanism between sister chromatids

Centrioles

Origin of microtubules during cell division

Centrosome

the mass of microtubules containing the centrioles and their spindle fibers

Kinetochores

attach to centromere between sister chromatids to pull the chromosomes apart

Cell Cycle

• Interphase
  • G₁ (first growth phase)
  • S (DNA Synthesis)
  • G₂ (second growth phase)
• Mitotic (M) Phase
  • Mitosis (division of the nucleus)
  • Cytokinesis (division of everything else)

Stages Mitosis

G₂ of Interphase

Chromatin has been duplicated

centrioles divide

Prophase

Chromatin condenses into Chromosomes

Centrosomes begin to move apart

Prometaphase

Nuclear envelope disintegrates

Spindle fibers (kinetochores) attach to centromeres

Metaphase

Chromosomes line up along the metaphase plate

Anaphase

Chromosomes are pulled apart into daughter chromosomes

Proteins in kinetochore take apart the microtubule

Telophase

Chromatin starts to decondense

nuclear envelope reforms

Cytokinesis

In animal cells, a cleavage furrow begins to pinch cell in half. The furrow is a contractile ring made of microfilaments actin and myosin that ratchet past each other.

In plant cells, vesicles form beginning of cell plate, they fuse and divide the cell in half.

Control of the Cell Cycle

In order to avoid cancer (when things go dreadfully wrong in cell division), the cells have special checkpoints (similar to border crossings: in order to go from one country to another, there are certain requirements and inspections)

Special signals or cyclins: different cyclins for different phases

G₂ Checkpoint

• Cyclin protein accumulates
• Cyclin interacts with Cdk enzyme to form...
• Mitosis Promoting Factor (MPF)
• Concentration of MPF initiates Mitosis

Cancer

• Uncontrolled cell division

Causes

• No Checkpoints
• Non-stop mitosis causes tumors
• Tumors can spread to other parts of the body: Metastasis (Mĕ•tăs•tə•sĭs)

Treatments and cures affect all cells undergoing division (in the gut, hair, skin, etc.), which are exactly the side-effects of chemotherapy.