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CELL CYCLE
LIFE OF A CELL
 Most cells in the body are called somatic cells.
 These cells go through a series of stages
called the cell cycle.
 Most of the cell’s life is spent in
interphase (a phase of growth and
normal functioning).
 A portion of its life is spent in cell division.
 Only happens in eukaryotes
THE CELL CYCLE
 The four stages of the
cell cycle are:
 G1 - Growth
 S - DNA replicates
 G2 - Growth
 M - Mitosis (cell divides)
CHECKPOINTS
 The cell cycle has a series of checkpoints- cell cycle
control system.
 Checkpoints are controlled by proteins.
 3 Checkpoints:
 G1- Checks for normal growth
 G2- Checks for proper DNA synthesis
 M- Checks for proper attachment in mitosis before cells
are pulled apart.
 If any of these checkpoints fail apoptosis
(programmed cell death)
HOW DO CHECKPOINTS WORK?
 There are a series of
proteins called cyclindependent kinase.
 These enzymes wait for
a signal from the cell
and then activate the
cell cycle.
 Signal is in the form of a
protein (usually)
WHAT CAN GO WRONG?
 At any point in the cycle, the cell could malfunction.
 If it does not pass a checkpoint, it either dies or
goes into a permanent resting state.
 If the checkpoint system malfunctions, cells divide
uncontrollably Tumors (Cancer)!