Chapter 6 Section 2
THE CELL CYCLE
Grade 10 Biology
Fall 2010
Bell Ringer
A typical eukaryotic cell spends 90% of its time in:
1.
A.
B.
C.
D.
Mitosis
Anaphase
Interphase
Cytokinesis
2. What do you think a cell spends its life cycle doing?
Objectives
 Identify the major events that characterize each of
the 5 phases of the cell cycle
 Describe how the cell cycle is controlled in
eukaryotic cells
 Relate the role of the cell cycle to the onset of cancer
The Life of a Eukaryotic Cell
 Cell division in eukaryotes is more complex than in
prokaryotes
 Involves cytoplasm, chromosomes inside nucleus,
and internal organelles
The Cell Cycle
 Cell cycle: 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

Interphase
 Only enters the last two phases of cycle if it’s about to
divide
Cell Cycle
The Cell Cycle
 Phase #1: First growth (G1) phase
 Cell grows rapidly and caries out routine functions
 Occupies major portion of cells life
 Cells that are not dividing remain in G1 phase
 Most muscles and nerve cells never divide

If these cells die the body cannot replace them
The Cell Cycle
 Phase #2: Synthesis (S) phase
 Cell’s DNA is copied
 At the end of the phase, each chromosomes consists of two
chromatids attached at the centromere
The Cell Cycle
 Phase #3: Second growth (G2) phase
 Preparations are made for the nucleus to divide
 Hollow protein fibers called microtubules are rearranged
during G2 in preparation for mitosis
The Cell Cycle
 Phase #4: Mitosis
 Mitosis: 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
The Cell Cycle
 Phase #5: Cytokinesis
 Cytkinesis: the process during cell division in which the
cytoplasm divides
Cell Cycle
The Cell Cycle
 Mitosis and cytokinesis produce new cells that are
identical to the original cells
 Allows organisms to grow, replace damaged tissues,
and in some organisms reproduce asexually
Control of the Cell Cycle
 How does a cell know when to divide?
 Cells have a system that controls the phases of the
cell cycle
Control of the Cell Cycle
 Cell cycle as key checkpoints at which feedback
signals from the cell can trigger the next phase of he
cell cycle
 Other feedbacks can delay the next phase
 Cell cycle in eukaryotes us controlled by many
proteins
Control of The Cell Cycle
Three Checkpoints
 Checkpoint #1: Cell growth (G1) checkpoint
 Makes the decision of whether the cell will divide
 If conditions favorable and cell is healthy and large enough,
certain proteins will stimulate cell to begin synthesis (S) phase
 S phase, cell copies DNA
 If conditions are not favorable, cells can stop at this checkpoint
 Cells can also stop at this check point if cell needs to go into
resting period

Nerve, muscle cells
Control of the Cell Cycle
 Checkpoint #2: DNA synthesis (G2)
checkpoint


DNA replication is checked by DNA repair enzymes
If checkpoint passed, proteins trigger mitosis
Control of the Cell Cycle
 Checkpoint #3: Mitosis checkpoint
 Checkpoint triggers the exit of mitosis
 Signals the beginning of the G1 phase, the major growth period
of the cell
When Control is Lost
 Cancer
 If gene that controls the proteins that regulate cell growth and
division is mutated, protein may not function and regulation
of cell growth and division can be disrupted
 Cancer: the uncontrolled growth of cells, may result
Disorder of cell division
 Cancer cells do not respond normally to body’s control
mechanisms

When Control is Lost
 Some mutations
cause cancer


Over producing
growthpromoting
molecules
Inactivating the
control proteins
that normaly act
to slow or stop
the cell cycle
Review
Differentiate between the G1, G2, and S phases of
the eukaryotic cell cycle
2. Relate what occurs at each of the three principal
check points in the cell cycle
3. Why are individual chromosomes more difficult to
se during interphase than during mitosis ?
4. In the cell cycle of typical cancer cells, mutations
have caused:
1.
A.
B.
C.
Slower growth
A failure in mitosis
Uncontrolled growth
Answers
G1: growth and development; S: DNA is copied;
G2: preparations made for nucleus to divide
2. Checkpoint 1: makes decision if cell will divide;
Checkpoint 2: DNA replication is checked, triggers
mitosis; Checkpoint 3: checkpoint triggers exit
from mitosis
3. Individual chromosomes are more difficult o se
during interphae than in during mitosis because
they haven't condensed and divided
4. C) uncontrolled growth
1.