Download Control of Cell Cycle

Title: The cell cycle and apoptosis
15th January 2014
Learning question: What happens in interphase? What is
apoptosis?
Homework:
Read pages 64-67 and answer textbook questions for Friday
17th January. Make sure you mark the questions please!
Starter:
Can you arrange these images in sequence?
1
5
3
2
4
3,1,4,5,2
Learning outcomes
(a) describe the cell cycle, with reference to interphase (G1,S and G2),
mitosis and cytokinesis leading to diploid cells;
(b) describe the structure of DNA and explain the importance of
complementary base pairing and hydrogen bonding;
(c) explain how DNA replicates semi- conservatively during the S phase
of the cell cycle (HSW1);
(d) outline the processes taking place at the G1 and G2 points in the
cycle;
(e) describe the appearance of the components of the nucleus and cell
during mitosis with reference to: nuclear envelope, centrioles, spindle
fibres, centromere, chromatids and chromosomes;
(f) define the term apoptosis;
(g) state that cell deletion by apoptosis and cell addition by mitosis are
essential for normal growth and repair;
The Cell Cycle
The majority of the cell
cycle is interphase and it is
divided into the following
parts:
• G1
• S
• G2
Mitosis only accounts for a
small part of the total cell
cycle.
G1 and S
• G1 - First growth phase
– Proteins are made
– Proteins are used to build organelles
– More cytoplasm is made
• S – synthesis
– DNA is replicated
G2 & M
• G2 – second growth phase
– Cell organelles grow and divide
– Cell builds up energy stores
• M – mitosis phase
– Nucleus undergoes mitosis and then cytokinesis
(cell division
Control of Cell Cycle
• To know when one phase has
ended and when to start the
next, there are a series of
checkpoints.
• In a cell, these checkpoints are
proteins and enzymes.
• Proteins that signals the start
of the next phase are call
cyclins
• Enzymes that activate cyclins
are called cyclin-dependent
kinases (CDKs)
Cyclins + CDKs = next phase activation
Important checkpoints
• G1 checkpoint
– Late G1 triggers initiation of
S phase
– Replication of organelles
• G2 checkpoint
– Late G2 triggers mitosis
• M(etaphase) checkpoint
– Controls entry to anaphase
Signal Control of Cell Cycle
• Internal and external
signals cue the assembly
or disassembly of the
cyclin-CDK completes
Signal Control of Cell Cycle
• What would happen to
the cell cycle if a defective
mitotic CDK was
produced, resulting in a
deformed active site?
• Mitotic cyclin would not
be able to bind to the
active site
• The initiation of the M
phase would not result
• Mitosis phase may not
occur / occurs at a very
slow rate
Signal Control of Cell Cycle
• Stopping the cell cycle can be helpful – why?
• Allows DNA repair to take place
• In some forms of breast cancer, over-stimulation of Ck4
(a CDK) leads to increased cell division
• The protein p53 in a helpful protein that inhibits cell
division.
• In over 50% of cancers contain a defective p53 gene
Apoptosis
• Apoptosis is programmed cell death in development
– Series of biochemical events leading to an orderly and tidy
cell death
• Hayflick Constant
– Cells undergo about 50 mitotic divisions before apoptosis
• Necrosis
– Untidy and damaging cell death occurring after trauma
Sequence of Apoptosis
•
•
•
•
•
•
•
Enzymes breakdown cell cytoplasm
Cytoplasm becomes dense
Organelles are tightly packed
Cell surface membrane changes and blebs form
Chromatin condenses, nuclear envelope breaks
Cell breaks into vesicles
phagocytosis
Apoptosis
Apoptosis
Control of Apoptosis
• Apoptosis is controlled by cell signalling
– Cytokines from the immune system
– Hormones and growth factors
– Nitric oxide
• Makes inner mitochondrial membrane more permeable
to hydrogen ions
Apoptosis and tissue development
• The rate of cells dying should balance the rate
of cells produced by mitosis
– Not enough apoptosis leads to the formation of
tumours
– Too much leads to cell loss and degeneration
• Cell signalling plays a role in maintaining the
correct balance
Apoptosis in Development
• The formation of the
digits (fingers and toes)
occurs due to apoptosis
during the development
of the embryo.
Apoptosis and metamorphosis
• As tadpoles grow they
develop legs, change
their body shape and
lose their tails
• The tail is lost by
apoptosis
APOPTOSIS
L/O: What is it?
Why is it important?
How is it controlled?
What is its role in age-related disease?
Apoptotic and necrotic cells,
respectively
APOPTOSIS
Programmed cell death
Orderly cellular self destruction
Process: as crucial for survival of multi-cellular
organisms as cell division
Quick Questions
1. What word means “programmed cell death”?
2. Give an example of apoptosis in the human body
3. What would happen if the rate of apoptosis
exceeded the rate of mitosis?
Use
whiteboards to
show your
answers
Tasks
1. Create a flow diagram of the stages that a cell goes
through during apoptosis.
2. Make a list of key words for apoptosis then write a
description or definition for each.
3. Summarise apoptosis and its role in development.
– Be sure to include any examples;
• The consequences of too much/too little apoptosis;
• Explain why cells commit apoptosis.