Protein Control of Cell Division Flashcards
What is the cytoskeleton?
Network of protein fibres that extend throughout the cytoplasm in all eukaryotic cells
Function of the cytoskeleton
Gives mechanical support and shape to cells
The cytoskeleton consists of…
Different proteins including microtubules
What are microtubules?
Hollow cylinders composed of the protein tubulin
Radiate from the microtubule organising centre (MTOC) or centrosome
Function of microtubules
Control the movement of membrane-bound organelles and chromosomes
Cell division requires the…
Remodelling of the cytoskeleton
What is the remodelling of the cytoskeleton?
Formation and breakdown of microtubules, involves polymerisation (growth) and depolymerisation (shrinkage) of tubulin
Microtubules form…
Spindle fibres that are active during cell division
What does the cell cycle regulate?
The growth and replacement of genetically identical cells throughout the life of the organism
2 stages of cell cycle…
Interphase
Mitotic (M) Phase
Interphase involves…
G1- a growth phase
S phase- DNA is replicated
G2- a further growth phase (of organelles and proteins)
(preparation for mitosis)
Mitotic phase involves…
Mitosis- chromosomal material is separated by the spindle microtubules
Cytokinesis- cytoplasm is separated into 2 daughter cells
4 stages of mitosis
Prophase
Metaphase
Anaphase
Telophase
What happens during prophase?
DNA condenses into chromosomes (consist of 2 sister chromatids)
Nuclear membrane breaks down, spindle microtubules extend from MTOC by polymerisation and attach to chromosomes via their kinetochores in the centromere region
What happens during metaphase?
Chromosomes are aligned at the metaphase plate (equator of the spindle)
What happens during anaphase?
As spindle microtubules shorten by depolymerisation, sister chromatids are separated and chromosomes are pulled to opposite poles
What happens during telophase?
Chromosomes decondense and nuclear membranes are formed around them (each set of chromosomes)
Progression through the cell cycle is controlled by…
Checkpoints
What are checkpoints?
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
What are the 3 checkpoints?
G1 checkpoint
G2 checkpoint
Metaphase checkpoint
G1 checkpoint…
Occurs towards end of G1
Sufficient cell growth must have occurred before cell can enter S phase
G2 checkpoint…
Occurs towards end of G2
Success of DNA replication and any damage to DNA is assessed
Metaphase checkpoint…
Occurs during metaphase and controls entry to anaphase
Progression is halted until the chromosomes are aligned correctly on the metaphase plate and attached to the spindle microtubules
What does retinoblastoma protein (Rb) do?
Acts as a tumour suppressor by inhibiting the transcription of genes that code for proteins needed for DNA replication
What do cyclin proteins do?
Accumulate during cell growth and are involved in regulating the cell cycle
Combine with and activate cyclin-dependent kinases (CDK’s)
Active cyclin-CDK complexes…
Phosphorylate proteins that regulate progression through the cycle, if sufficient phosphorylation is reached the progression occurs
Phosphorylation by G1 cyclin-CDK…
Inhibits retinoblastoma protein allowing transcription of genes that code for proteins needed for DNA replication (therefore cells can progress from G1 to S phase)
At G2 checkpoint, DNA damage triggers the activation of several proteins such as…
p53 which can…
Stimulate DNA repair
Arrest cell cycle
Cause cell death (initiate apoptosis) if DNA damage is too severe
What does an uncontrolled reduction of cell cycle rate lead to?
Degenerative disease e.g. Alzheimers
What does an uncontrolled increase in cell cycle rate lead to?
Tumour formation (cancer)
What is a photo-oncogene?
Normal gene usually involved in the control of cell growth or division
Can mutate to form tumour promoting oncogene
What triggers apoptosis?
Cell death signals
Why must destruction of cells in multicellular organisms be carefully controlled?
Excessive apoptosis leads to degenerative conditions
Inhibition of apoptosis leads to tumour formation
Where do external cell death signals originate?
Out with cell
Where do internal cell death signals originate?
Within cell
Example of external cell death signal
Production of death signal molecules from lymphocytes
External death signal molecules bind to…
A surface receptor protein, triggering a protein cascade within cytoplasms
Example of internal cell death signal
DNA damage
Internal death signal resulting from DNA damage results in…
Activation of p53 tumour-suppressor protein
Both types of cell death signal result in…
Activation of caspases
What are caspases?
Types of protease enzyme that cause destruction of cells
When else might cells initiate apoptosis ?
In the absence of growth factors
Apoptosis is essential during…
Development of am organism to remove cells no longer required as development progresses
Metamorphosis (when an animal changes from after hatching/birth)
