YEAST CELL CYCLE + START + G0 + MATING/SPORULATION

Which CDK drives the entire S. cerevisiae cell cycle?

Cdc28

How does one CDK control different stages in yeast?

By binding different cyclins at different times

Which cyclins function at START?

Cln1

Which cyclins drive DNA replication?

Clb5 and Clb6

Which cyclins drive G2/M and mitosis?

Clb1

What is START?

A decision point in late G1 where cells commit to division

Where does START occur?

In yeast G1 phase

Is START a single moment or a window?

A series of diminishing choices rather than a single point

What major event is decided at START?

Whether the cell has sufficient nutrients and signals to divide

What happens if conditions are unfavourable at START?

Cell enters G0 instead of progressing

What is G0 in yeast?

A quiescent state where cells are metabolically quiet and do not divide

Why do yeast enter G0?

Nutrient limitation or environmental stress

What happens to haploid yeast in G0?

They arrest and can mate with cells of opposite mating type

What happens to diploid yeast in G0?

They may undergo meiosis and form spores

Why does yeast mating occur?

To combine genetic material when growth conditions are poor

What do diploids produce during meiosis?

Spores

What is required for mating to occur?

Haploid cells of opposite mating type and pheromone signalling

What decides whether diploid cells sporulate?

Starvation

What role does Cdc28 play in G0 entry?

Temperature-sensitive cdc28(ts) mutants allow G0 entry even though G1 cannot proceed

What happens in cdc7(ts) mutants?

Cells cannot enter G0 and cannot mate/sporulate appropriately

Why does cdc7(ts) block G0 entry?

cdc7 functions downstream of START

How does the cdc7(ts) mutant behave in haploids?

Cells lack mating factors and cannot enter G0

How does the cdc28(ts) mutant behave in diploids?

Meiosis and sporulation can still occur despite G1 arrest

What happens in cdc7(ts) diploids?

Cells cannot enter G0 but meiosis can occur

What do these mutant behaviours demonstrate?

START is upstream of the decision to arrest or differentiate

What is the major checkpoint equivalent to START in mammals?

The Restriction Point (R point)

What is the key environmental cue for START?

Nutrient availability

How do cells detect nutrient conditions?

Through signalling pathways that regulate G1 cyclin transcription

Why do multiple cyclins exist at START?

They allow graded activation of Cdc28 activity

Which cyclins appear first at START?

Cln3 followed by Cln1/Cln2

What does Cln3 do?

Initiates the first activation of Cdc28 to trigger START gene expression

Which cyclins amplify Cdc28 activity?

Cln1 and Cln2

What transcriptional programme is triggered at START?

Genes required for budding

Why is START irreversible?

It triggers CDK activity that commits cells to replication

Can a cell reverse START?

No — once past START

What happens to cells lacking Cln1

2

Which phase do cells arrest if START is not passed?

G1 phase

Why must budding occur before S phase?

Morphogenesis and cell wall expansion need to precede DNA replication

Which cyclins support budding?

G1 cyclins Cln1/2

Which cyclins support spindle formation?

Clb1–4

How does START relate to DNA replication licensing?

Cdc28–Clb cyclins promote replication origin firing

How do yeast prevent relicensing?

High CDK activity prevents new origin loading until mitotic exit

Why is Cdc28 essential?

Without it

What is the impact of nutrient starvation on cell cycle?

Cells downregulate CDK activity

Is yeast G0 permanent?

No — cells can re-enter the cycle when nutrients return

Why can haploid cells mate while diploids cannot?

Diploids express both mating alleles and cannot respond to pheromone in the same way

What determines fate after G0?

Ploidy and environmental triggers (mating vs sporulation)

What is sporulation?

Meiosis producing four spores enclosed in an ascus

Why is sporulation beneficial?

Increases survival during adverse conditions

What causes spores to germinate?

Return of nutrients

What cellular structures change during G0?

Ribosome production declines and metabolism slows

Why is G0 important in nature?

Helps yeast survive feast–famine environments

What happens to cdc28(ts) if restrictive temperature is removed?

Cell cycle resumes — mutant is reversible

Why are temperature-sensitive mutants useful?

They allow synchronised arrest at a chosen stage

Why is synchronisation important experimentally?

Permits study of fluctuations in CDK and cyclin levels

What does the START decision integrate?

Internal cell mass and external environment

How is START controlled genetically?

Cdc28 activity and cyclin accumulation

Why must cells reach critical size?

To ensure sufficient resources for genome duplication and division

What causes small daughter cells to delay START?

Lower cyclin accumulation rates than mother cells

Why do fast-growing cells pass START earlier?

Rapid nutrient uptake accelerates cyclin transcription

What is the connection between START and mating pheromones?

Pheromone signalling arrests cells before START

What happens to signalling pathways at START?

They trigger transcription of over 200 G1/S genes

Why are START decisions layered?

Later steps lock in commitment while earlier choices remain reversible