L13 eukaryotic cell cycle

What is cell cycle control?

Systems that respond to intracellular and extracellular signals to regulate cell growth and division, coordinated by cyclin/CDK activities

What is the main role of cyclin/CDK complexes?

Ensure DNA replication occurs only once per cycle and chromosomes are equally segregated into daughter cells

Which cellular processes are coordinated with the cell cycle?

Centrosome duplication, mitotic spindle formation, polar movement of centrosomes, and nuclear envelope breakdown

Why is the G2/M transition important?

It ensures that all DNA has been replicated and the cell is ready for mitosis

What is MPF?

Mitosis

How was MPF activity first discovered?

By mixing M phase cell extracts with G2 cells, which caused the G2 cells to enter mitosis

What evidence suggested MPF is a diffusible factor?

M phase cell extracts could induce mitosis in cells at any cell cycle stage

Which phase

specific factor promotes S phase entry?

Which cells respond to the S phase

promoting factor?

Which model organisms were used to study cell cycle regulation?

Xenopus (frog) eggs and early embryos of sea urchins

Why are Xenopus and sea urchins ideal models for cell cycle studies?

They undergo rapid synchronous cell divisions after fertilisation and can be synchronised for experiments

How does MPF activity change during the cell cycle?

It rises to trigger mitosis and falls to allow mitotic exit

What happens to oocytes with high MPF activity?

They are arrested in meiosis until MPF levels decrease

How was cyclin discovered as a component of MPF?

By observing proteins synthesized after fertilisation in sea urchin oocytes using 35S

What is Cyclin B?

A regulatory protein that forms part of MPF and is required for mitotic entry

How do Cyclin B levels correlate with MPF activity?

Cyclin B levels rise and fall in parallel with MPF activity

What happens if cyclin B is not degraded?

Cells cannot exit mitosis because MPF remains active

How is cyclin B normally degraded?

By the proteasome, targeting it for destruction after mitotic entry

What experimental evidence shows cyclin B is required for mitotic entry?

Addition of cyclin B mRNA to extracts restores mitotic activation in RNAse

What happens when truncated, non

degradable cyclin B is expressed?

How do frog oocyte extracts help study mitosis?

Addition of sperm chromatin triggers spindle formation, chromosome condensation, and metaphase plate formation

What techniques were used to study cyclin synthesis?

35S

What is the relationship between MPF and cyclin B?

Cyclin B forms the regulatory subunit of MPF, controlling its activity

How does MPF regulate mitosis?

It triggers nuclear envelope breakdown, spindle formation, and chromosome condensation

Why must MPF be degraded for mitotic exit?

To inactivate CDK activity and allow cytokinesis and cell cycle progression

What are the four main biochemical phases of the eukaryotic cell cycle?

G1, S, G2, and M phases

Which biochemical entities control transitions between cell cycle phases?

Cyclins and cyclin

How was the timing of cell cycle events studied experimentally?

Using synchronized early embryos and oocyte extracts to follow protein synthesis and mitotic progression

What does MPF activation require besides cyclin B?

Proper phosphorylation/dephosphorylation of CDK and binding to cyclin B

How did researchers demonstrate that mitosis requires new protein synthesis?

RNAse

What is the significance of studying MPF in early embryos?

Early embryos provide large amounts of synchronised cells, allowing biochemical isolation of factors controlling mitosis

What is the role of sperm chromatin in cell

free experiments?

How is MPF an example of a biochemical switch?

Its activity is tightly regulated, rising to trigger mitosis and falling to permit mitotic exit

What are the main takeaways from cyclin and MPF studies?

Cyclin B is required for mitotic entry, MPF triggers mitosis, and cyclin degradation is essential for mitotic exit