Unit 4: Cell Cycle and Division

Interphase

• longest phase of cell’s life w/ 3 stages:

  • G1

  • S

  • G2

• GROWTH + cellular functions present at all times

• diagram

G₁ Phase

• 1st stage of interphase

• Actions:

  • growth

    • must get to full size

  • cellular functions

    • can start these before reaching full size

• length:

  • cell can spend YEARS here

    • where it resides until it receives growth signal

  • transition to G0?

S Phase

• 2nd stage of interphase

• actions:

  • growth

    • now growing to prepare to DIVIDE in 2

  • cellular functions

  • copying DNA

    • need to make 2 sets of DNA → 2 nuclei

    • copy EXACT chromosomes

• length:

  • amount of time it takes to copy DNA

    • irreversible; w/ 2 sets of DNA, cell MUST now divide

• copying microtubule organizing centers

  • = protein complexes

    • long, stiff, hollow tubes that organize cell’s contents

  • = centrosomes in animal cells

G₂ Phase

• last stage of interphase

• Actions:

  • growth

  • cellular functions

  • preparation for mitosis

    • produce necessary machinery

      • split 2 nuclei in half

M Phase

• time when everything divides (period of cell division)

• 2 stages:

  • Mitosis

  • Cytokinesis

• results separate daughter cells

  • # copies DNA = # daughter cells

Mitosis

• 1st stage of M Phase

• division of DNA + nucleus (genetic info)

  • must be 2 EXACT copies for daughter cells to function

    • 50% of DNA each

• diagram

Cytokinesis

• 2nd stage of M Phase

• division of other organelles + cytoplasm

  • less exact than mitosis

• results in separate cells with separate copies of DNA

  • repeat cycle

• diagram

Checkpoint

• processes that control whether a cell progresses to the next cell cycle phase

  • collect information about cell’s external environment + internal conditions of cell

• G1/S → notes on conditions

  • most stringent

  • growth = permitted

• S

  • DNA = undamaged

• G2

  • all DNA accurately replicated

• M

  • daughter cells receive correct # chromatids

• FAILURE TO PASS:

  • normal options:

    • 1. wait to fix problem

    • 2. apoptosis

  • abnormal options:

    • 3. cancer (BYPASS checkpoint)

Density-Dependent Inhibition

• crowded cells stop dividing

  • division stops when come in contact w/ neighbors

• checkpoint verifies: “is there room for more cells?”

• *LIMITS CELL GROWTH

Anchorage Dependence

• cells attach to each other via a web of proteins called the extracellular matrix (EM)

  • if not stuck can freely float in body upon division

    • cancer metastasis

  • STOP dividing if detach

• checkpoint verifies: “is the cell firmly attached to other cells in a tissue?”

• *LIMITS CELL GROWTH

Apoptosis

• programmed cell death of abnormal or unneeded cells

  • abnormal = ex: mutations

  • unneeded = ex: excess in development

• way body eliminates cells

  • release contents to be recycled + reused by other cells

• orderly pathway

  • body eliminates cells stuck at cell cycle checkpoint + disposes of cells no longer needed

• cell fragments engulfed + recycled by neighbors

*specific cell effects

• chromosomes break into small pieces

• mitochondria stop producing ATP

• cell collapses