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Section 6-2
The Cell Cycle
The Cell Cycle Describes the Life
of a Eukaryotic Cell
Cell division in eukaryotic cells is more
complex than in prokaryotic cells because
it involves dividing both the cytoplasm and
the chromosomes inside the nucleus.
 Also have to correctly rearrange
organelles to form two properly functioning
cells.
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The Cell Cycle
The cell cycle is a repeating sequence of
cellular growth and division during the life
of an organism
 A cell spends 90% of its time in the first
three phases of the cycle – called
interphase
 The cell only enters the last two phases of
the cell cycle if it is about to divide.
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Five Phases of the Cell Cycle
First Growth Phase or G1
1.
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During this phase, a cell grows rapidly and
carries out its routine functions.
This phase occupies the majority of most
cell’s life.
Five Phases of the Cell Cycle
Synthesis Phase or S
2.
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The cell’s DNA is copied during this phase.
At the end of this phase, each chromosome
consists of two chromatids attached to the
centromere.
Five Phases of the Cell Cycle
Second Growth Phase or G2
3.
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Preparations are made for the nucleus to
divide.
Microtubules are assembled and used to
move the chromosomes during mitosis.
Five Phases of the Cell Cycle
Mitosis
4.
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Process during cell division in which the
nucleus of a cell is divided into two nuclei.
Each nucleus ends up with the same
number and kinds of chromosomes as the
original cell.
Five Phases of the Cell Cycle
5.
Cytokinesis
The process during cell division in which
the cytoplasm divides.
The Cell Cycle
The Cell Cycle is Carefully
Controlled
How do cells know when to divide? How is
the cell cycle controlled?
Cells have a system that controls the
phases of the cell cycle.
 Similar to a set of “red-light-green-light”
switches that are regulated by feedback
information from the cell.

Has key checkpoints or inspection points
at which feedback signals from the cell
can trigger the next phase of the cell cycle
(green light).
 Other feedback signals can delay the next
phase to allow for completion of the
current phase (yellow or red light).
 Controlled by proteins

Principle Checkpoints of the Cell
Cycle
1.
Cell Growth Checkpoint or G1 Checkpoint
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Makes the key decision of whether the cell will divide.
If the cell is healthy and large enough, proteins will
stimulate the cell to begin the synthesis or S phase to
copy the cell’s DNA.
If conditions aren’t favorable, cell can stop cycle at this
phase.
Also will stop at this phase if cell needs to pass into a
rest period. Some cells (muscle and nerve cells)
remain in rest period permanently.
Principle Checkpoints of the Cell
Cycle
2.
DNA Synthesis Checkpoint or G2
Checkpoint
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DNA replication is checked at this point by
DNA repair enzymes.
If checkpoint is passed, will trigger mitosis
and cell begins many molecular processes
needed to proceed into mitosis.
Principle Checkpoints of the Cell
Cycle
3.
Mitosis Checkpoint
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This checkpoint triggers the exit from
mitosis.
Signals the G1 phase (major growth period
of cell cycle)
When Control is Lost: Cancer
Certain genes contain information
necessary to make the proteins that
regulate cell growth and division.
 If one of those genes are mutated, the
protein may or may not function , and
regulation of cell growth and division may
be disrupted.

When Control is Lost: Cancer
Cancer – the uncontrolled growth of cells
 When proteins are disrupted, cancer may
occur.
 Essentially a cell division disorder
 Cells do not respond to the body’s normal
control mechanisms

When Control is Lost: Cancer
Some mutations cause over-production of
growth-promoting molecules, speeding up
cell cycle
 Other mutations inactivate the proteins
that slow or stop the cell cycle.

When Control is Lost: Cancer
Mutations may occur spontaneously, but
many occur due to environmental
influences.
 Many causes of cancer linked to lifestyle
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Tobacco
UV radiation
Viruses
Diet?
Section 6-2 Summary
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There are 5 phases of the cell cycle: G1 phase,
S phase, G2 phase, Mitosis, and Cytokinesis.
There are three checkpoints to determine the
phase the cell will enter. If the cell does not
pass the checkpoint, it does not proceed into the
next phase.
Cancer is uncontrolled cell growth caused by
mutations of the genes that control the cell cycle.
Section 6-2 Homework
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Review Questions
page 127 (1-4)
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STUDY FOR CHAPTER 6
VOCABULARY TEST!!!!