Introduction to cell biology Flashcards
(29 cards)
What is the total human genome and what is it made up of?
3.2 billion base pairs long
- formed of DNA sequences
- 1 chromosome from each of 22 autosome pairs and both sex chromosomes
Also mitochondrial DNA (only 16,000 base pairs long)
How are cell numbers controlled?
- controlled by cell-cell communication
- growth factor signals and cell-cell contacts positively regulate cell proliferation - controlling if and when cells divide
What processes are in place to ensure new cells are identical?
- processes of growth and division must be tightly controlled
- cell cycle control involved checkpoints and feedback control
- maintains genome integrity which is crucial
What are the phases of the cell cycle and their timings?
Interphase:
- G1 phase (about 11 hours)
- S phase (about 8 hours)
- G2 (about 4 hours)
M phase (about 1 hour) - varies based on cell type
- this phase includes mitosis and cytokinesis
For most cells the cycle is 24 hours
What are the phases of mitosis?
Prophase: nuclear envelope dissembles and chromosomes condense, becoming visible, centrosomes start forming mitotic spindle
Metaphase: Centrosomes migrate to opposite poles of the cell, chromosomes line up along metaphase plate (equatorial plate)
Anaphase: paired chromatids pulled apart to opposite poles by mitotic spindle attached to kinetochores at the centromeres of the chromosomes
Telophase: nuclear envelope reforms, mitotic spindle disappears and cytokinesis is completed
What is Go phase?
quiescent phase- cell is neither dividing or preparing to divide
e.g. lymphocytes are typically not in cycle so are in the Go phase
What is happening in the G1 phase?
Period during which cells are responsible to mitogenic growth factors and to TGF-beta
Later in this stage is the restriction point - this checkpoint is responsive to external growth factors however beyond this point the cells is unresponsive
What are the cell cycle checkpoints?
G1 restriction point: responds of extracellular signals. Cell irreversibly commits to cell division. Check the cell has sufficient nutrients/reserves and its big enough. Checks for any DNA damage G1 checkpoint: Checks for any DNA damage Intra-S checkpoint:Checks for any DNA damage G2/M checkpoint: Nutrients/ reserves and size are checked. Checks replication is complete and for any DNA damage M checkpoint (spindle checkpoint): checks all the spindle are correctly attached at the kinetochores of the centromeres
What signals induce a cell to divide?
- death of a nearby cell
- release of growth-promoting hormones, such as human growth hormone (HGH)
- the size of the cell; as a cell grows, it becomes inefficient due to its decreasing surface-to-volume ratio. The solution to this problem is to divide.
What enzymes regulate the cell cycle?
Cyclin dependent kinases
- regulate cell cycle checkpoint transitions- they are regulated by feedback
(kinases phosphorylate)
What do cycling dependent kinases do to the cell cycle?
they fluctuate in abundance and activity to control the pace of events through the cell cycle
- Cyclin D to Cyclin E to Cyclin A to Cyclin B
What do kinase enzymes do and how do CDKs do it?
Kinases activate or deactivate proteins via phosphorylation
- in the cell cycle they provide the signals to drive the checkpoints
- they are activated by CYCLINs which are proteins named due to their cyclical functioning concentrations
What cyclin and CDK is involved in the S phase?
Cyclin A and CDK2
- binding of cyclin A to CDK2 increases its enzymatic activity by 400,000 fold
What are the pairs of cyclin and CDKs?
Cyclin D and CDK 4+ 6- CYCLIN D is an extracellular signal and is controlled by colony stimulating factor 1 Cyclin E and CDK2 Cyclin A and CDK2 Cyclin A and CDK1 Cyclin B and CDK1
The cyclin and CDK complexes in one phase lead to activation of the next pair - APART from cyclin D which is controlled by extracellular signals
What does the G1 restriction point do?
-ensures cell is large enough to divide and that enough nutrients are available to support the resulting daughter cell
If a cell receives a go-ahead signal at the G1 checkpoint it will usually continue with the cell cycle
If the cells does not receive the go-ahead signal it will exit the cell cycle and switch to a non-dividing state called Go
What is the purpose of the G2 checkpoint?
It ensure that DNA replication in the S phase has been completely successfully
What is the purpose of the M checkpoint?
ensures all the chromosomes are attached to the mitotic spindle by kinetochore
What is the purpose of the intra-S checkpoint?
check for any DNA damage
In the absence of growth factors what regulates the G1 restriction point?
retinoblastoma protein (pRB) - its job is to stop the cell cycle
It binds to E2F transcription factor and this stops it being induced and promoting growth
- pRB is a negative regulator of growth
How does the cell stop pRB inhibiting cell division?
growth factors which activate cyclin D and CDK4+6 complex phosphorylate pRB causing it to release E2F transcription factor - therefore allowing cell proliferation
What does p53 regulate?
Regulates G1/s phase, responding to damaged DNA and shutting the cell cycle down if damage is detected
What is p53 mechanism of action?
DNA damage activates p53 which subsequently activates p21 which is an inhibitor of CDK therefore preventing continuation into S phase
What are pRB and p53?
Tumour suppressor genes and are most commonly lost genes in cancer
- primary negative regulators of cell growth
What does p53 do if the DNA damage is too severe?
induces the cell to kill itself by apoptosis - can also make the cell senescent
