Protein Control of Cell Division Flashcards
What is the cytoskeleton ?
The cytoskeleton gives mechanical support and shape to cells
It consists of different protein structures including microtubules, which are found in all eukaryotic cells, which extend through the cytoplasm
Eukaryotic cells have a membrane-bound nuecleus (DNA contained within a membrane)
What are microtubules ?
Microtubules are hollow cylinders composed of the protein tubulin.
They radiate from the microtubule organising centre (MTOC) or centrosome.
MTOC and centrosome same thing
What do microtubules do ?
Microtubules control the movement of membrane-bound organelles and chromosomes and the distribution of organelles.
How are microtubules made and broken down ?
Formation and breakdown of microtubules involves polymerisation and depolymerisation of tubulin
What is the role of microtubules in cell division ?
-Microtubules form the spindle fibres that are active during cell division and pull chromatids apart
-Cell division requires remodelling of the cytoskeleton and
What are the two parts of the cell cycle ?
Interphase and mitotic (M) phase
What is interphase ?
The cell grows and DNA is synthesised in preparation for cell division
-Interphase involves growth and DNA synthesis including G1, a growth phase (where proteins and organelles are synthesised); S phase, during which the DNA is replicated; and G2, a further growth phase.
Is an active period of growth. The cell will still continue to grow during S phase
What is mitotic (M) phase ?
Mitotic phase involves mitosis and cytokinesis
In mitosis the chromosomal material is separated by the spindle microtubules. This is followed by cytokinesis, in which the cytoplasm is separated into two daughter cells.
What are the stages of mitosis ?
Mitosis consists of prophase, metaphase, anaphase and telophase
Describe the stages of mitosis ?
Prophase
-DNA condenses into chromosomes each consisting of two sister chromatids. Nuclear membrane breaks down; spindle microtubules extend from the MTOC by polymerisation and attach to chromosomes via their kinetochores in the centromere region.
Metaphase
-Chromosomes are aligned at the metaphase plate (equator of the spindle).
Anaphase
-As spindle microtubules shorten by depolymerisation, sister chromatids are separated, and the chromosomes are pulled to opposite poles.
Telophase
-The chromosomes decondense and nuclear membranes are formed around them.
Cytokineses after
What is the mitotic index ?
Used to determine the percentage of cells undergoing mitosis and is determines by the tissue type
P + M + A + T
———————– x 100
Total cells
Where P is the number of cells in prophase
What controls the cell cycle ?
Progression through the cell cycle is controlled by checkpoints
What are checkpoints ?
Checkpoints are mechanisms within the cell that assess the condition of the cell during the cell cycle and halt progression to the next phase until certain requirements are met.
Which proteins are involved in regulating the cell cycle
Cyclin proteins that accumulate during cell growth (interphase) are involved in regulating the cell cycle
Cyclins combine with and activate cyclin-dependent kinases (CDKs). Active cyclin-CDK complexes phosphorylate proteins that regulate progression through the cycle. If sufficient phosphorylation is reached, progression occurs.
If an insufficient phosphorylation is reached, the cell is held at a checkpoint.
What are the three major checkpoints ?
-G1 checkpoint
-G2 checkpoint
-M checkpoint
What is the G1 checkpoint ?
-Occurs towards the end of G1
-Sufficient cell growth and other conditions must have occurred and be satisfied before the cell can enter S phase
Which protein regulated the G1 checkpoint ?
-Retinoblastoma (Rb) protein is cell cycle regulatory protein which restricts progression from G1 into S phase, when non-phosphorylated.
-At the G1 checkpoint, retinoblastoma protein (Rb) acts as a tumour suppressor by inhibiting the transcription of genes that code for proteins needed for DNA replication (in S phase, so cell stays in G1)
-Phosphorylation by G1 cyclin-CDK inhibits the retinoblastoma protein (Rb) - This allows transcription of the genes that code for proteins needed for DNA replication so cells progress from G1 to S phase.
Is a cell cycle regulatory regulatory protein which, when non-phosphorylated, acts as a tumour suppressor
What is the G2 checkpoint ?
-Occurs at the end of G2
-At the G2 checkpoint, the success of DNA replication and any damage to DNA is assessed.
DNA damage triggers the activation of several proteins including p53 that can;
-Stimulate DNA repair
-Arrest the cell cycle
-Cause cell death (apoptosis - must be carefully controlled in a multicellular organism)
p53 is The Guardian of the Genome
What is a metaphase checkpoint
-A metaphase checkpoint controls progression from metaphase to anaphase, and occurs during metaphase.
-At the metaphase checkpoint, progression is halted until the chromosomes are aligned correctly on the metaphase plate and attached to the spindle microtubules.
-Ensures that each daughter cells receives the correct number of chromosomes.
What can uncontrolled reductions and increases in the rate of the cell cycle lead to ?
-An uncontrolled reduction in the rate of the cell cycle may result in degenerative disease (such as, Alzheimer’s)
-An uncontrolled increase in the rate of the cell cycle may result in tumour formation
Which proteins are usually involved in the control of cell growth or division.
A proto-oncogene is a normal gene, usually involved in the control of cell growth or division, which can mutate to form a tumour-promoting oncogene
What is apoptosis ?
Programmed cell death
What triggers apoptosis ?
Apoptosis is triggered by cell death signals that can be external or internal
What is an example of an external death signal
The production of death signal molecules from lymphocytes is an example of an external death signal.
Cells may initiate apoptosis in the absence of growth factors
What is an example of an internal death signal
DNA damage is an example of an internal death signal.
How do external death signal work ?
External death signal molecules bind to a surface receptor protein and trigger a protein cascade within the cytoplasm
Result in the activation of caspases (types of protease enzyme) that cause the destruction of the cell
How does an internal death signal work ?
An internal death signal resulting from DNA damage causes activation of p53 tumour-suppressor protein
Result in the activation of caspases (types of protease enzyme) that cause the destruction of the cell
Why is apoptosis important
Apoptosis is essential during development of an organism to remove cells no longer required as development progresses or during metamorphosis
