This slideshow contains various stages of cell cycle regulation, cell cycle checkpoints and their proteins involved in regulation. Cell cycle checkpoints transition phases.

1. The cell concept is the axis around which the
whole of the modern science of life revolves.
- Paul Ehrlich

2. Cell Cycle Regulation
Speaker:
Himakara Datta Mandalapu
M.Sc.(Ag.) previous year
Department of Genetics and Plant breeding
CoA, IGKV, Raipur.
GP 591 (0+1)
Master’s Seminar

3. Cell Cycle: The circle of life.

5. Stages of Cell Cycle
Cell Cycle
Interphase
G1
S
G2Mitosis

6. Cell cycle regulation by cell growth and
extracellular signals

7. Cell cycle regulation in Yeasts
(S.cerevisiae)
• Regulation occurs at a point in late G1 phase.
• Called as START point.
• Controls progression from G1 to S phase
• The passage through START is influenced by external signals such as
nutrient availability, mating factors, cell size etc.
• Analogous to restriction point in animal cells.

8. Regulation by START point
• If all the conditions are favorable – Proceeds to S phase.
• Low nutrient availability – enters a resting state till the optimum
availability of nutrients occur.
• Mating factors present – cell cycle arrested at START, and allows
haploid yeast cells to fuse to become diploid.
• Cell size of daughter cell not adequate – arrested at G1 till the
daughter cells become uniform in size.

9. • In animals cells, if appropriate growth factors are not present the
cells enter a quiescent stage called G₀ phase.
• The cells have arrested development in aforementioned phase but
are still metabolically active.

10. • Even though most of the cell division control takes place in G₁ phase,
some cell cycles are controlled in G₂ phase.
• Control takes place at G₂ to M phase
• Examples:
The cell cycle of fission yeast Schizosaccharomyces pombe.
• Oocytes of vertebrates remain arrested in G₂ for long periods of time
until their progression into M phase is triggered by hormonal
stimulation.

11. Cell Cycle Checkpoints

12. • The current concepts of cell‐cycle regulation focus on the idea of a
biochemical clock whose progression is regulated by a set of fail‐safe
monitors called checkpoints.
• Checkpoints are signaling pathways that detect cellular defects, stop
cell‐cycle progression, or initiate specific repair pathways.

13. • The coordination between the different phases of the cell cycle is
dependent on a series of cell cycle checkpoints.
• They prevent the entry of cell cycle into the next phase until the
preceding phase events have been completed.

14. Three major checkpoints:
G1 checkpoint
G2/M checkpoint
Metaphase/Anaphase checkpoint

15. • Size checkpoints at the molecular level is based on regulation of the
proteins involved in G1 and G2/M progression.
• Control of the G1 cell size checkpoint has been studied most
extensively in budding yeast, where the cyclin Cln3, which activates
Start, regulates cell size
• Control of the G2/M cell size checkpoint has been studied most
extensively in fission yeast, where Cdc25 and Wee1 respond to cell
size and nutritional status in their control of the Cdc2-cyclinB complex

16. Key proteins involved

17. Cyclin dependent kinases (Cdks)
• The central machines that drive cell cycle progression are the cyclin-
dependent kinases (CDKs).
• These are serine/threonine protein kinases that phosphorylate key
substrates to promote DNA synthesis and mitotic progression.
• The catalytic subunits are in molar excess, but lack activity until bound
by their cognate cyclin subunits.
• Cyclin subunits are tightly regulated at both the levels of synthesis and
ubiquitin-dependent proteolysis.
• All CDKs exist in similar amounts throughout the entire cell cycle.

18. • Cyclin-binding allows inactive CDKs to adopt an active
configuration akin to monomeric and active kinases.
• Layered on top of this regulation, CDK activity can also be
negatively regulated by the binding of small inhibitory
proteins, the CKIs, or by inhibitory tyrosine
phosphorylation which blocks phosphate transfer to
substrates.

19. • A Cdks is an enzyme that adds negatively charged
phosphate groups to other molecules in a process
called phosphorylation.
• Through phosphorylation, Cdks signal the cell that it is
ready to pass into the next stage of the cell cycle.

21. Phylogenic tree of Cdk genes

22. Cyclins
• Cyclins are named such because they undergo a constant cycle of
synthesis and degradation during cell division.
• When cyclins are synthesized, they act as an activating protein and
bind to Cdks forming a cyclin-Cdk complex.
• This complex then acts as a signal to the cell to pass to the next cell
cycle phase.
• Eventually, the cyclin degrades, deactivating the Cdk, thus signaling
exit from a particular phase.

23. • Common classes of cyclins include G1-phase cyclins, G1/S-phase
cyclins, S-phase cyclins, and M-phase cyclins.
• G1/S cyclins – essential for the control of the cell cycle at the G1/S
transition,
• Cyclin A / CDK2 – active in S phase.
• Cyclin D / CDK4, Cyclin D / CDK6, and Cyclin E / CDK2 – regulates
transition from G1 to S phase.
• G2/M cyclins – essential for the control of the cell cycle at the
G2/M transition (mitosis). Cyclin B / CDK1 – regulates progression
from G2 to M phase.

27. Cell cycle stage Cyclins CDKs Comments
G1 Cyclin D CDK4&6
Can react to outside signals such
as growth factors or mitogens.
G1/S Cyclins E & A CDK2
Regulate centrosome duplication;
important for reaching START
S Cyclins E & A CDK2
Targets are helicases and
polymerases
M Cyclins A & B CDK1
Regulate G2/M checkpoint. The
cyclins are synthesized during S
but not active until synthesis is
complete. Phosphorylate lots of
downstream targets.

28. p53 – The Guardian of the genome
• p53, also known as TP53 or tumor protein is
a gene that codes for a protein that regulates the
cell cycle and hence functions as a tumor
suppression.
• Described as "the guardian of the genome",
referring to its role in conserving stability by
preventing genome mutation
• Activated p53 is stabilized through protection
from its E3 ubiquitin ligase Mdm2.
• p53 can direct the alternative cell fates of
apoptosis or senescence

29. • Trans activates the expression of a large number of genes, including
the cyclin-dependent kinase inhibitor (CKI) p21.
• Through this mechanism, G1 CDKs are inhibited, and DNA damage is
repaired prior to DNA replication.
• p53 can also repress the expression of genes, and is required for
prolonged G2 arrest in the face of persistent DNA damage.
• Defective p53 could allow abnormal cells to proliferate, resulting in
cancer.

30. Wee 1
• Wee1 is a nuclear kinase belonging to the Ser/Thr family of protein
kinases in the fission yeast Schizosaccharomyces pombe (S. pombe).
• Wee1 is a key regulator of cell cycle progression. It influences cell size
by inhibiting the entry into mitosis, through inhibiting Cdk1.
• Wee1 acts as a dosage-dependent inhibitor of mitosis.
• Thus, the amount of Wee1 protein correlates with the size of the
cells

31. Anaphase promoting complex or
cyclosome (APC/C).
• E3 ubiquitin ligase that marks target cell cycle proteins for
degradation by the 26S proteasome.
• E3s mediate the transfer of one or several ubiquitin monomers
on a protein substrate in a two-step reaction involving at least
three partners.
• First, an ubiquitin-activating enzyme (E1) activates and transfers
ubiquitin to an ubiquitin-conjugating enzyme (E2).
• Next, E3 mediates the transfer of ubiquitin from E2 to a lysine
residue of the target protein.
• Activated by the phosphorylation by Cdk1/cyclin B complex.

32. Cell division cycle protein 20 (Cdc20)
• Activator protein that regulates the ubiquitin ligase activity and
substrate specificity of the anaphase promoting
complex/cyclosome (APC/C).
• Required for sister chromatid separation and disassembly of the
mitotic spindle. Target of the spindle checkpoint pathway through
participation in the mitotic checkpoint complex (MCC) and the
MAD2-CDC20 sub complex.
•

33. G1-S transition
• The primary G1/S cell cycle checkpoint controls the
commitment of eukaryotic cells to transition through the G1
phase to enter into the DNA synthesis S phase.
• Two cell cycle kinase complexes, CDK4/6-Cyclin D and CDK2-
Cyclin E, work in concert to relieve inhibition of a dynamic
transcription complex that contains the retinoblastoma protein
(Rb) and E2F.

34. Rb-E2F regulation
• In G1-phase uncommitted cells, hypo-
phosphorylated Rb binds to the E2F-DP1
transcription factors forming an inhibitory
complex
• Commitment to enter S-phase occurs through
sequential phosphorylation of Rb by Cyclin D-
CDK4/6 and Cyclin E-CDK2, permitting
transcription of genes required for DNA
replication.

35. G1-S transition pathway
in plants

36. G2/M transition
• Brought about by the Maturation Promoting Factor (MPF)
• MPF is made up of two subunits: Cdk1(protein kinase) and Cyclin
B(catalytic activity).
• Phosphorylation of Cdk1 at Thr161, Thr14, Tyr15 by Wee1
• Phosphorylation at Thr14, Tyr15 inhibits the Cdk1 activity.
• Dephosphorylation of Thr14, Tyr15 by a protein phosphatase Cdc25.
• Thus, the activated cdk1 phosphorylates a variety of proteins that
initiate the events of M phase.

37. Entry into Mitosis
• Regulated by Cdk1/Cyclin B complex along with two other kinases i.e.
Polo-like kinase and Aurora Kinase (A&B).
• Cdk1, Polo-like kinase and Aurora Kinase (A&B) as Mitotic Protein
Kinases (MPK).
• They are activated in a positively controlled feedback loop at the
onset of M phase.
• These interactions bring about multiple nuclear and cytoplasmic
changes during mitosis by phosphorylation of different proteins.

38. Positive feed back loop of MPKs
Aurora
Kinase
Polo-like
Kinase
Cdk1

39. MPK interaction during Mitosis
MPKs Phosphorylating Protein Result
Cdk1 + Aurora kinase B Condensins
Chromatin condensationAurora B + Polo-like Kinase Cohesins
Aurora B kinase Histone H3 serine-10
Cdk1/Cyclin Lamins
Nuclear envelop breakdownCdk1 Proteins of inner nuclear
membrane & nuclear pore complex
Cdk1 and Polo like Kinases Golgi matrix proteins Breakdown of Golgi apparatus into
vesicles
Cdk1, Aurora A & Polo-like Kinases Microtubule associated proteins Centrosome maturation, separation
& spindle assembly

40. DNA damage checkpoints
• DNA damage checkpoints ensure that damaged DNA is not passed
on to the daughter cells.
• Function in G1, S, G2, phases of the cell cycle.
• Checkpoint at G2 phase – prevents the initiation of mitosis if the
DNA has not been completely replicated.
• Checkpoint at G1 phase – allows repair of damaged DNA before
entering S phase.
• S phase Checkpoint – continuous monitoring of the integrity of DNA
to ensure any damage of DNA to be repaired before replication.

41. • DNA damage checkpoints can be separated into those controlled by
the tumor suppressor and transcription factor p53, and those
ultimately under the control of the checkpoint kinase Chk1, Chk2
• Two protein kinases ATR and ATM are activated in response to DNA
damage.
• ATR acts on single strand breaks or unreplicated DNA.
• ATM acts on double strand breaks.
• Chk1,2 act by phosphorylating and inhibition of Cdc25.

43. Spindle Assembly Checkpoints
• Also called as the ‘wait anaphase’ checkpoint, or the mitotic
checkpoint.
• Monitors the alignment of chromosomes on the metaphase spindle.
• Presence of even a single unaligned chromosome is enough to
prevent the activation APC/C.

45. Conclusion

46. Thank You