The cell cycle Flashcards
(37 cards)
Define the cell cycle
The process by which a cell duplicates its contents in order to produce two daughter cells
What 2 major processes does the cell cycle involve?
- DNA replication
- Accurate chromosome segragation
What occurs during the S phase
Replication of chromosomes
What occurs during the M phase
Segregation of chromosomes (mitosis) before the cell divides (cytokinesis)
Name the 5 stages of mitosis in the order that they occur
- Prophase
- Prometaphase
- Metaphase
- Anaphase
- Telophase
What key change occurs at the metaphase to anaphase transition?
Sister chromatid cohesion (formed during S phase) is lost and the sister chromatids are pulled appart
What are the 4 main stages of the cell cycle in order?
G1 phase
S phase
G2 phase
M phase
What is the function of the gap (G) stages?
- allow cell to grow
- prepare for S and M phases
- monitoring of internal and external cell environment
What occurs when conditions are not favourable at the G1-S transition?
Cells enter specialised resting state referred to as Gzero
How is cell cycle progression controlled?
- complex network of regulatory interactions
- include ordered biochemical switches and checkpoints that ensure the next stage has not started before all earlier things are completed
- monitoring of outside signals in response to developmental/environmental cues
Why is budding yeast a useful model for the study of the cell cycle?
- Generation time of under 2 hours
- Simple to grow in the lab
- Very amenable to genetic manipulation
- Can be maintained in haploid/diploid form
What is the advantage of using haploid organisms for studies?
- Genes can be mutated to investigate function without interference from a second copy
- However can only use conditional lethal mutants, as inactivation of essential genes would cause organism death
Why is xenopus laevis a good model for cell cycle investigations?
- Egg extracts and sperm can be mixed to recapitulate early cell cycle events
- Egg extracts can be analysed biochemically by depletion of individual factors
What are the 3 main regulatory points in the cell cycle?
G1-S
G2-M
Metaphase-anaphase
How is the progression through the cell cycle regulated?
CDK-cyclin complexes
What are CDK-cyclin complexes composed of?
- catalytic subunit containing protein kinases (CDKs) which are present at a constant level
- regulatory subunits - cyclins which undergo synthesis and destruction
What are the 3 essential classes of cyclins and their roles?
- G1/S cyclins which activate in late G1, committing to S-phase entry (start)
- S phase cyclins interact with CDKs after the ‘start’ to initiate DNA replication
- M phase cyclins activate CDKs to drive entry into mitosis
Appart from activating CDK enzymes, what is the other role of cyclins?
Confer substrate specificity
Describe an additional method of cell cycle regulation
Some substrates are only available at some points during the cell cycle
How are cyclin dependant kinases activated?
- Cyclin binds to T-loop on ATP molecules in kinase, causing partial activation
- kinase CAK then phosphorylates serine/threonine in the T-loop in order to further enhance activity
How is M-CDK activity regulated?
- Wee1 protein kinase phosphorylates Cdk1 on two neighbouring amino acids to the active site causing inhibition
- Cdc25 phosphoprotein phosphatase removes phosphates in order to relieve inhibition
How is M-CDK activity amplified during the G2-M transition?
Through positive feedback loops
How do CDK inhibitor proteins work?
e.g p27
distorts active site and partially blocks the ATP-binding site
How is the metaphase-anaphase transition triggered?
Via regulated proteolysis
- Protein destruction triggered by catalysed by a ubiquitin ligase known as the anaphase promoting complex or cyclosome (APC/C)
- S- and M-cyclins are a major target of APC/C