Lesson 4 – Control of the Cell
Cycle
1.2 Cell Growth and the cell
cycle (continued)
How is the cell cycle controlled?
As you have seen the cell cycle is a sequence
of very precise events with exact outcomes.
Any variation from this could be disastrous!
Think of ways that things could go wrong if it
wasn’t carefully controlled.
Cells keep dividing - cancer
Cytokinesis happens too
soon – cell dies
Cells don’t start dividing –
no growth
Cells enter G2 without proper
checks on replication –
mutations arise
Timing
Timing is essential because:
1. Different cell types need to replicate
at different times and in different
quantities.
2.Each event within the cell cycle must be
complete before the start of the next
event.
Growth Factors
During the various stages of the cell cycle
growth (in may forms) occurs. Over 50
different growth factors have been identified
as important to the cell cycle.
Without these growth factors the cell will stop
growing and enter a phase know as G0 – a nongrowing phase.
Checkpoints
A checkpoint is when the cycle gets
regulated by stop and start signals.
There are 3 main checkpoints.
Checkpoint 1 – Towards the
end of G1 phase
Remember that in G1 growth and
synthesis of organelles occurs.
What do you think this checkpoint will be
for?
Assessment of cell size – if it is large
enough and the environment is
favourable to undergo cell division it
moves onto the S phase for DNA
replication
Checkpoint 2 – at the end of the
G2 phase
By the end of G2 DNA replication has occurred.
Success of Replication - This checkpoint is
essential in making sure that replication has
been successful.
Assessment of Cell Size - The size of the cell is
also checked and the environmental conditions
assessed.
Checkpoint 2 and Mitosis
Promoting Factor (MPF)
Once the checks are completed at Checkpoint 2
the cell is ready for mitosis.
The start of mitosis is triggered by a complex
called mitosis promoting factor (MPF) which
itself is controlled by a number of other cell
cycle signals.
Checkpoint 3 – during metaphase
This is known as the M Checkpoint and occurs
during metaphase. The main checks here are:
Correct spindle formation – is the spindle fibre
attached to the choromosome?
Correct chromosomal alignment chromosomes
arranged so that each daughter cell receives
one of each chromosome?
Cell Cycle Animation
What happens when there is a
problem?
We know that the cell is checking –
so what if an error is found? There
are 2 possible outcomes:
1. The cell cycle is stopped while the
problem is repaired OR
2. The cell is instructed to kill itself
(apoptosis).
Your Tasks
1. Revise the stages of mitosis by listing the names of
each stage along with the activities that distinguish
it. You will have a quiz next week where you will be
given images and need to decide what stage they are.
2. Research Mitotic Index. What is it? How is it used?
How is it measured? How would you measure it using
equipment in the school?
3. Begin a glossary using all the terms you have learned
so far. (You can have a jotter for this if you like).
4. Try the quiz here: http://highered.mcgrawhill.com/novella/MixQuizProcessingServlet