1.5 Flashcards
Function of the cytoskeleton
Gives mechanical support and shape to the cell
What does the cytoskeleton consist of
The cytoskeleton consist of different protein structures including microtubules, which are found in all eukaryotic cells
What are microtubules and where do they come from
Microtubules are hollow cylinder is composed of the protein tubulin. The radiate from the microtubule organising centre (MTOC) or centrosome.
Functions of microtubules
Microtubules control the movement of membrane bound organelles and chromosomes.
Other major functions
- Determine cell shape
- involved in cell movement
- form the spindle fibres
What does cell division require of the cytoskeleton
Cell division requires remodelling of the cytoskeleton
What does formation and breakdown of microtubules involve
Formation and breakdown of microtubules involves polymerisation and depolymerisation of tubulin
What forms the spindle fibres
Microtubules form the spindle fibres that are active during cell division
When is the centrosome/MTOC duplicated?
Duplicated in non-dividing (interphase) cells
Products of the cell cycle
Two genetically identical daughter cells
What are the two main phases of the cell cycle
The cell cycle consist of interphase and mitotic (M) phase
Mitotic = mitosis
List the phase of interphase
G1: initial growth phase. Protein synthesis occurs and new organelles are formed
Synthesis (S): replication of nuclear DNA
G2: second phase of growth prior to mitosis. By the end of this phase the centrosome/MTOC has been duplicated
Describe G1
G1: initial growth phase. Protein synthesis occurs and new organelles are formed
Describe S phase
Synthesis (S): replication of nuclear DNA
Describe G2
G2: second phase of growth prior to mitosis. By the end of this phase the centrosome/MTOC has been duplicated
What does the mitotic phase involve
Mitosis and cytokinesis
In mitosis do chromosomal material is separated by the spindle microtubules. This is followed by cytokinesis, in which the cytoplasm is separated into two daughter cells
What does mitosis (M) phase consist of
Prophase
Metaphase
Anaphase
Telophase
Describe prophase
DNA condenses into chromosomes each consisting of two sister chromatids. The nuclear membrane breaks down; spindle microtubules extends from the MTOC by polymerisation and attach to chromosomes via their kinetochores in the centromere region.
During prophase why do chromosomes condense
Chromosomes condense to become visible, to stop transcription and to prevent tangling during separation
Describe metaphase
The chromosomes are aligned at the metaphase plate (equator of the cell)
They are held in place by microtubules from each pole attached to each centromere
Describe anaphase
As spindle microtubules shorten by depolymerisation, sister chromatids are separated, and the chromosomes are pulled to opposite poles of the cell
Describe telophase
The chromosomes decondense and nuclear membranes are formed around them.
After what stage of mitosis does cytokinesis take place
Cytokinesis takes place at the end of telophase
What is cytokinesis
Cytokinesis occurs at the end of telophase and involves the separation of the cell into two daughter cells
Describe checkpoints in control of the cell cycle
Progression through the cell cycle is controlled by checkpoints at G1, G2 and M (metaphase in mitosis)
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
Describe the proteins that accumulate during cell growth
Cyclin proteins that accumulate during cell growth are involved in regulating the cell cycle.
Cyclins 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, progression occurs.
Describe the G1 checkpoint
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.
Phosphorylation by G1 cyclin-CDK inhibits the retinoblastoma protein (Rb). This allows transcription of the genes that code for proteins needed for DNA replication. Cells progress from G1 to S phase.
During the G1 checkpoint, describe what happens by phosphorylation
Phosphorylation by G1 cyclin-CDK inhibits the retinoblastoma protein (Rb). This allows transcription of the genes that code for proteins needed for DNA replication. Cells progress from G1 to S phase.
What is assessed during G1 checkpoint
Cell size
Describe the G0 (nondividing state (interphase))
If a go ahead signal is not reached at the G1 checkpoint the cell may switch to a nondividing state called the G0 phase.
Cyclin and cell size during G1
As the cell size increases during G1, Cyclin proteins accumulate and combine with regulatory proteins called CDKs and activate them.
Active CDKs cause the dephosphorylation of proteins that stimulate the cell cycle.
If a sufficient threshold of phosphorylation is reached the cell cycle moves on to the next stage. If an insufficient threshold is reached, the cell is held at G0.
What has happened by the end of G2
DNA replication has occurred
Describe the G2 checkpoint
At the G2 checkpoint, the success of DNA replication and any damage to DNA is assessed
What happens once the checks are completed at G2
Once the checks are completed at checkpoint G2 the cell is ready for mitosis.
The start of mitosis is triggered by the complex called mitosis promoting factor (MPF) which itself is controlled by a number of other cell cycle signals
How is the start of mitosis triggered
The start of mitosis is triggered by the complex called mitosis promoting factor (MPF) which itself is controlled by a number of other cell cycle signals
What does DNA damage trigger
DNA damage triggers the activation of several proteins including p53 that can stimulate DNA repair, arrest the cell cycle or cause cell death
2 main responses by active p53
Cell cycle arrest - DNA repair - cell cycle restart
Apoptosis - death and elimination of damaged cells
Describe checkpoint 3 (known as the M checkpoint occurring during metaphase)
A metaphase checkpoint controls progression from metaphase to anaphase.
At the metaphase checkpoint, progression is halted until the chromosomes are aligned correctly on the metaphase plate and attached to the spindle microtubules
What can an uncontrolled reduction in the rate of the cell cycle lead to
An uncontrolled reduction in the rate of the cell cycle may result in degenerative diseases such as Alzheimer’s or Parkinson’s disease.
What can an uncontrolled increase in the rate of the cell cycle lead to
An uncontrolled increase in the rate of the cell cycle may result in tumour formation (cancer)
What are proto-oncogenes
A proto-oncogene is a normal gene, usually involved in the control of cell growth or a division, which can mutate to form a tumour promoting oncogene.
These oncogenes can cause the cell to divide in an uncontrolled and unregulated manner.
What triggers apoptosis
Apoptosis is triggered by cell death signals that can be external or internal
Example of external death signal
The production of deaths signal molecules from lymphocytes
Example of internal death signal
DNA damage
What is apoptosis
Apoptosis is programmed cell death.
Describe the mitochondrion and the caspade cascade
During apoptosis dependability of the mitochondria membrane increases allowing cytochrome C to exit the inter-membrane space and into the cytoplasm.
Cytochrome C activate executioner caspase enzymes which have a controlled cascade of action. These act a DNAase, protease and enzymes that destroy the cell.
It can also activate other types of enzymes in the body like nucleases that can break down DNA
Describe how external death signal molecules work
External death signal molecules bind to a surface receptor protein and trigger a protein cascade within the cytoplasm
Describe how internal death signals work
An internal (intrinsic) death signal resulting from DNA damage causes activation of p53 tumour-suppressor protein
What do both types of death signal activate
Caspases (types of protease enzyme) that cause the destruction of the cell
Why and when is it apoptosis essential
Apoptosis is essential during development of an organism to remove cells no longer required as development progresses or during metamorphosis.
When may cells initiate apoptosis
Cells may initiate apoptosis in the absence of growth factors
