Unit 7 bio test 3
Do all cells go through the cell cycle continuously?
- depends on cell type, developmental stage, external signals
- some cells divide continuously at high rate
- some cells do not divide at all once matured
- early embryos divide more quickly
Do some cells opt out of the cell cycle?
- some cells divide very slowly, but can be induced to re-enter cell cycles
- some cells become highly specialized and can no longer divide (terminally differentiated)
how to control cell cycle
- injection of cytoplasm from M-phase eggs pushs the injected egg into M-phase (negative control)
- in interphase a cytoplasm sample does nothing
What are hetero diamteric proteins
- cyclin dependent kinase (Cdk)
- Cyclin
What is a Cdk (4)
- cyclin dependent kinase
- catalytic subunit, transfers phosphate from ATP to certain amino acids on target proteins
- not active unless bound to cyclin partner
- levels are consistent
What is cyclin
- cyclin is a regulatory subunit
- levels oscilate thoughout cell walls
Rise and Fall of M-Cdk activity through cell cycle
- as Cdk activity occurs, cyclin concentration rises
- high in mitosis, low in interphase
What are the different classes of cyclin-cdk complexes
1. G1-Cdk: help passage through restriction point
2. G1/S-Cdk: commit cells to DNA replication
3. S-Cdk: carry out replication
4. M-Cdk: promote events of mitosis
where is the restriction point and sprinkle checkpoint in a cell cycle
1. G1/S
2. G2/M
What is the targeted desctruction of cyclin used for
1. It inactivates Cdk
2. Drives cell from anaphase -> completion of M phase -> G1
how does destruction of cyclin occur (3)
1. Increased transcription of M-cyclin gene through M-phase
2. The targeted destruction of M-cyclin protein by anaphase promoting complex tags M-cyclin with ubiquitin and marks it for destruction
3. Sudden drop in M-Cdk activity -> cell enters anaphase -> completes M phase
Addition / removal of inhibitory phosphate groups
- increase - remove phosphate (activating phosphatase
- decrease - inhibit the phosphatase
How does removal of inhibitory phosphates activate M-Cdk
- when a inhibitory phosphate is present, inactive M-Cdk and M cyclin can not bind. When phosphate is removed, phosphatase is activated and M-Cdk and M-cyclin can bind and activate
how is cell able to be paused at key points (3)
- Cdk inhibitors block entry to S phase
- Inhibition of activating phosphatase blockes entry to mitosis
- inhibition of APC activation delays exit from mitosis
How does the cell decide whether to proceed through the cell cycle
- animal cells need an external signal to divide (G0)
- if DNA has issues halt cell and fix it, if it cant be fixed cell death mus occur in G1
- All chromosomes must be attached to spindles, if not halt mitosis and fix it (M)
What are the three major checkpoints in cell cycle
1. Enters phase
- is environment favourable? G1/S checkpoint, restriction point
2. G2/M checkpoint
- is all DNA replicated?
- Is all DNA damage repaired?
3. Spindle checkpoint
- checks if all chromosomes are properly attached to the mitotic apind
- pulls duplicated chromosomes apart
- meta -> ana
What is the G1/S checkpoint
- ensures the cell has the resources needed to support cell division
- makes sure the cell is large enough to split
- DNA must be undamaged
- if there is no signal to divide from extra cellular signal molecules, cell stays in G0
What is the G2/M checkpoint
- Checks for internal issues:
- is DNA properly replicated, is the DNA undamaged, is the activated MPF present
- if any are answered no, blocks activity of M-Cdk
what is the spindle checkpoint
- makes sure all chromosomes moved to the metaphase plate
- makes sure they are all alligned and all attached to the mitotic spindle
- spindle checkpoint is where chromosomes gathered at metaphase plate and are turninf into sister chromatids pulled apart