Cell Division Flashcards
What are the phases of the cell cycle
Interphase (G1 phase, S phase and G2 phase) and the M phase (Mitosis and Cytokinesis)
The cell cycle is critically controlled by what?
Cyclin dependant Kinase. The activity of these enzymes are cyclical
How are cyclin dependant kinases activated?
Inactive cyclin-Cdk complex forms
and the complex is phosphorylated by protein kinases (still inactive). Phosphate is removed by activating phosphotase (Cdc 25) activating the cylin-Cdk complex
How are active cyclin-Cdk complexes inactivated
First there is the Ubiquitination of cyclin and this leads to the destruction of cyclin resulting in the inactive Cdk.
Describe the process of ubiquitination
Ubiquitin (large protein) binds to E1 enzyme and then is transfered to the E2 enzyme. The E2 enzyme (Ubiquitin conjugating enzyme) is joined by an E3 enzyme (ubiquitin ligases) allowing the ubiquitin to transfer to the substrate. The presence of ubiquitin on the substrate signals for destruction of substrate
Explain how proteasomes work
The protein complex recognises the polyubiquitin signal attached to the protein that is wants to degrade. The protein is unfolded and fed through the core particle where is gets broken up
What can occur at checkpoints of the cell cycle?
If the environment is not right then there will be a suppression of Cdk by inducing transcription of Cdk inhibitors
Name an example of a Cdk inhibitor and how it works
p21 - works by holding the Cdk in an inactive position
What is the function of Wee1?
It is an inhibitory kinase so prevents cell division
What is the R-point
It is where the cell transitions from being in the G1 phase to the S phase
At the R-point how can the cell be prevented from proliferating?
Cdk is held in an inactive position meaning Rb (retinoblastoma) protein is activated and the expression of S-phase genes is inhibited.
At the R-point how can the cell continue to proliferate?
p16 is absent or inactive meaning the cyclin-Cdk complex is activated allowing it to phosphorylate the Rb protein inactivating it. Therefore it cannot prevent the transcription of genes necessary for the S phase.
Describe how DNA replication is allowed when transitioning from the G1 phase to the S phase
In the G1 phase there is a pre-replicative complex sat on the origin of replication, preventing DNA synthesis occuring. In the S phase, the Cdc6 molecule is a target for Cdk and becomes phosphroylated and then destroyed, leaving the pre replicative complex. The assembily of the replication fork can begin and DNA replication can then be completed.
What occurs if DNA becomes damages?
The response depends on the stage of cell cycle. p53 causes the transcription of cdk-inhibitors. CHK2 is activated causing the inhibition of cdc25
What are the stages of mitosis?
Prophase, Prometaphase, metaphase, anaphase, telophase and cytokinesis.
What occurs in the prophase?
DNA is condensed and held together by condensins. Spindle fibres form from the centrosomes. Phosphorylated lamins trigger the breakdown of the nuclear envelope and microtubules attach to the kinetochore.
Describe what occurs in the metaphase?
Chromosomes align on the metaphase plate. Mitosis cannot proceed until chromosomes are properly attached and under tension - spindle check point ensures this.
What holds sister chromatids together and when is this important
Cohesion complexes. This is very important during the transition between metaphase and anaphase
Defects in cohesion complexs can cause what?
A very rare disease called Roberts Syndrome. In this disease the chromosomes are not held tightly together so they move away from eachother.
What allows the cell cycle to move from metaphase to anaphase?
Separase as they cut the cohesion complexes allowing the chromosomes to move to opposite polls
How are separases activated?
Separases are kept in the inactive form by securin. APC (anaphase promotor complex) becomes activated and targets the securin molecule for Ubiquitin, securin is then degraded activating separase
Defects in the spindle check point can cause what?
Aneuploidy, this is a cell with abnormal chromosomes.
What is essential for mitotic exit?
Degredation of cyclins and securins
In cytokinesis what allows for the cell to separate into two daughter cells?
A ring of actin and myosin
