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Biochemical Control of the Cell Cycle. BNS230. Lecture programme. Three lectures Aims Describe the cell cycle Discuss the importance of the cell cycle Discuss how the cycle is regulated. Cell division. 15 hours. M phase. G 0 state. G 2 phase. G 1 phase. 12 hours. S-phase

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Biochemical Control of the Cell Cycle

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lecture programme
Lecture programme
  • Three lectures
  • Aims
    • Describe the cell cycle
    • Discuss the importance of the cell cycle
    • Discuss how the cycle is regulated

Cell division

15 hours



G0 state

G2 phase

G1 phase

12 hours


(DNA synthesis

5 hours

16 hour cell cycle

cell cycle definition
Cell cycle definition
  • A series of distinct biochemical and physiological events occurring during replication of a cell
  • Occurs in eukaryotes
  • Does not occur in prokaryotes
  • Time of cell cycle is variable
cell cycle timing
Cell cycle timing
  • Yeast 120 minutes (rich medium)
  • Insect embryos 15-30 minutes
  • Plant and mammals 15-20 hours
  • Some adults don’t divide
    • Terminally differentiated
    • e.g. Nerve cells, eye lens
  • Some quiescent unless activated
    • Fibroblasts in wound healing
components of the cell cycle
Components of the cell cycle
  • M phase
    • Cell division
    • Divided into six phases
      • Prophase
      • Prometaphase
      • Metaphase
      • Anaphase
      • Telophase
      • Cytokinesis
components of the cell cycle7
Components of the cell cycle
  • G1 phase
    • Cell checks everything OK for DNA replication
    • Accumulates signals that activate replication
    • Chloroplast and mitochondria division not linked to cell cycle
components of the cell cycle8
Components of the cell cycle
  • S-phase
    • The chromosomes replicate
    • Two daughter chromosomes are called chromatids
    • Joined at centromere
    • Number of chromosomes in diploid is four
components of the cell cycle9
Components of the cell cycle
  • G2-phase
    • Cell checks everything is OK for cell division
    • Accumulates proteins that activate cell division
why have a cell cycle
Why have a cell cycle?
  • Comprises gaps and distinct phases of DNA replication and cell division
  • If replicating DNA is forced to condense (as in mitosis) they fragment
  • Similarly if replication before mitosis
    • Unequal genetic seperation
  • I.e. Important to keep DNA replication and mitosis separate
why have a cell cycle11
Why have a cell cycle?
  • Important to have divisions in mitosis
  • e.g. Important metaphase complete before anaphase. Why?
  • If not segregation of chromosomes before attachment of chromatids to microtubles in opposite poles is possible
  • Down syndrome due to extra chromosome 21
why have a cell cycle12
Why have a cell cycle?
  • Gaps provide cell with chance to assess its status prior to DNA replication or cell division
  • During the cell cycle there are several checks to monitor status
  • These are called checkpoints
  • Checkpoint if G1 monitors size of cell in budding yeast (Saccharomyces cerevisae)
  • At certain size cell becomes committed to DNA replication
  • Called start or replication site
evidence of size checkpoint
Evidence of size checkpoint
  • Yeast cells (budding yeast) grown in rich medium
  • Switch to minimal medium
  • Cells recently entering G1 (buds) delayed in G1 (longer to enter S-phase)
  • Large cells above threshold size still go to S-phase at same time as in rich medium
evidence of size checkpoint15
Evidence of size checkpoint
  • Yeast in rich medium
    • 120 minute cell cycle
      • Short G1 phase
  • Yeast in minimal medium
    • Eight hour cell cycle primarily because of long G1 phase
  • Checkpoint 2 in G1 monitors DNA damage
  • Evidence?
    • Expose cells to mutagen or irradiation
    • Cell cycle arrest in either G1 phase or G2 phase
  • The protein p53 involved in cell cycle arrest
    • Tumour suppresser
  • Checkpoint in S-phase monitors completion of DNA replication
    • Cell does not enter M-phase until DNA synthesis is complete
  • Checkpoint in G2
    • DNA breaks cause arrest
    • Otherwise when chromosomes segregate in mitosis DNA distal to breaak won’t segregate
  • Checkpoint in mitosis
    • Senses when mitotic spindles have not formed
    • Arrests in M-phase
    • Otherwise unequal segregation of chromosomes into daughter cells
  • Described cell cycle, now I will talk about genes and proteins that control this process
molecular control of cell cycle
Molecular control of cell cycle
  • Two experimental approaches
    • Biochemical
      • Sea urchin fertilised eggs
      • Rapid
      • Synchronous division
      • Analyse proteins at various stages of cycle
    • Genetic analysis using
      • Budding yeast Saccharomyces cerevisae
      • Fission yeast Schizosaccharomyces pombe
using genetics to study the cell cycle
Using genetics to study the cell cycle
  • To study the genetic basis of a biological event
    • Make mutants defective in that event
    • Determine which genes have been mutated
    • Understand role of gene (and encoded protein) in the event
    • Problem: How do you make mutants that disrupt the cell cycle
    • Cells will not replicate
using genetics to study the cell cycle21
Using genetics to study the cell cycle
  • Isolate temperature sensitive mutants that have defect in cell cycle
  • At low temperature these mutants progress through cell cycle
  • Arrest in cell cycle at elevated temperature
  • Mutation causes gene product (protein) to be highly sensitive to temperature
using genetics to study the cell cycle22
Using genetics to study the cell cycle
  • Isolation of genes that regulate the cell cycle
  • Step 1: Create strains with mutations in cell cycle genes
isolating cell cycle mutants
Isolating cell cycle mutants

Yeast culture

(S. pombe)

Mutagenise and plate

out at high and low temperature



Colonies 4 and 10 are possible cell cycle mutants. Called

cell division cycle (cdc) mutants >70 cdc mutants isolated

are the temperature sensitive mutants cdc mutants
Are the temperature sensitive mutants cdc mutants?

Grow colonies at 30°C

Shift temperature to 37°C

Look under a microscope

Colony 4: Too small; enters mitosis too

early (Wee 1 mutant)

Colony 10: very long stuck in G2

(cdc25 mutant)

Wild type cells

using genetics to study the cell cycle25
Using genetics to study the cell cycle
  • Step 2: Insert plasmids containing fragments of wild type DNA
  • Step 3: Look for plasmid that corrects genetic defects
  • Step 4: Plasmid contains a cell cycle control gene
what do we do with the mutants
What do we do with the mutants?

Use mutants to isolate cdc genes and

then study what the proteins do



S. pombe

Extract DNA



Cut with restriction enzyme

and ligate into vector

Yeast vector

take recombinant vectors and transform into cdc mutants
Take recombinant vectors and transform into cdc mutants
  • Wee mutant with normal gene wee1 gene in plasmid will grow at 37
  • cdc25 mutant with normal cdc25 gene in plasmid will grow at 37
  • I.e gene in recombinant plasmid is complementing the mutation
biochemical studies
Biochemical studies
  • 1st evidence proteins regulate cell cycle
    • Fuse interphase cells (G1, S or G2) withM-phase cells
    • Cell membranes breakdown and chromosomes condense
    • I.e Mitotic cells produce proteins that cause mitotic changes in other cells
microinjection with frog oocyte
Microinjection with frog oocyte
  • Oocyte stays in G2-phase
  • Male gets busy and female produces progesterone
  • Oocyte enters mitosis
  • Purify proteins from oocyte cells treated with progesterone
  • Inject into G2 arrested cells and see which protein causes mitosis (1971)
  • Protein identified that causes mitosis
  • Called maturation promoting factor
  • MPF in all mitotic cells from yeast to humans
  • Renamed mitosis-promoting factor
properties of mpf
Properties of MPF
  • MPF activity changes through the cell cycle
  • MPF activity appears at the G2/M interphase
  • and then rapidly decrease
how does mpf cause mitosis
How does MPF cause mitosis?
  • It’s a protein kinase
    • Phosphorylates proteins
  • Phosphorylates proteins involved in mitosis
  • Phosphorylates histones causing chromatin condensation
  • Phosphorylates nuclear membrane proteins (lamins) causing membrane disruption
characterisation of mpf
Characterisation of MPF
  • Consists of two subunits; A and B
  • Subunit A: Protein kinase
  • Subunit B: Regulatory polypeptide called cyclin B
  • Protein kinase present throughout cell cycle
  • Cyclin B gradually increases during interphase (G1, S, G2)
  • Cyclin B falls abruptly in anaphase (mid-mitosis)

What does this profile tell you?

MPF not just due to association of subunits A and B

other factors involved

Protein kinase (subunit A)

Cyclin B levels (subunit B)

MPF activity

G1 S G2 M


Cyclin B (subunit B)


Protein kinase (subunit A)









cyclin b
Cyclin B
  • How do Cyclin B levels decrease abruptly
  • Proteolytic degradation
  • Degraded in a protease complex present in eukaryotic cells called “The Proteosome”
  • Specific proteins degraded by complex when tagged by a small peptide called ubiquitin
cyclin b37
Cyclin B
  • Cyclin B is tagged for Proteosome degradation at anaphase
    • Tagged at N-terminus at sequence called
    • Destruction box
    • DBRP binds to Destruction box
      • Guides Ubiquitin ligase to add ubiquitin molecules to Cyclin B
  • Why is Cyclin B only degraded in anaphase








Ubiquitin ligase adds ubiquitin

when DBRP binds to the

destruction box





DBRP = Destruction box recognition protein

cyclin b39
Cyclin B
  • DBRP is normally inactive and is only activated in anaphase via phosphorylation
  • Possible MPF phosphorylates DBRP causingCyclin B destruction
    • Binds to the destruction box
    • Activates ubiquitin ligase to add ubiquitin to Cyclin B
    • Cyclin B then targeted to the Proteosome for degradation
cyclin b40
Cyclin B
  • When this causes MPF inactivation
    • DBRP dephosphorylated by constitutive phosphorylase
  • Other proteins also control MPF
    • Activity doesn’t increase as Cyclin B increases
  • Proteins discovered in yeast by cdc mutant complementation

Cyclin B (subunit B)

Y15 T161

Y15 T161

Y15 T161

Y15 T161


Protein kinase (subunit A)





Inactive MPF



Inactive MPF




Active MPF


mpf activity
MPF activity
  • Wee mutant small: Enters mitosis prematurely
  • cdc 25 mutant long: Stays in G2 for longer
  • Wee phosphorylates Y15 and inactivates MPF
  • CAK (cdc2 [MPF]-activating kinase) phosphorylates T161
  • cdc25 dephosphorylates Y15 and activates MPF
cell cycle
Cell cycle
  • How is entry into S-phase controlled?
  • Throughout cell cycle the protein kinase (cdc28 in sc and cdc2 in sp) binds to specific cyclins
  • This changes the specificity of the protein kinase
activity of protein kinase
Activity of Protein Kinase
  • Cdc28-cyclins B1-4: Protein kinase activates proteins involved in early mitosis by phorphorylating them
  • Cdc28-cyclins 1-3: Protein kinase activates proteins involved in initiation of DNA replication by phosphorylating them
  • cdc28-cyclin 5: Phorphorylates and thus activates proteins that maintain DNA replication
how many protein kinases
How many protein kinases?
  • In both yeasts only one protein kinase
  • In higher eukaryotes multiple protein kinases
    • Active at different stages of the cell cycle
  • As with yeast different cyclins