Cyclin dependent kinases (CDKs) play a key regulatory role in controlling the cell cycle. CDKs form complexes with cyclins and their activity varies throughout the cell cycle. There are checkpoints that ensure DNA replication and division occur accurately. CDK-cyclin complexes promote progression through the cell cycle phases (G1, S, G2, M) by phosphorylating target proteins. Cyclin levels oscillate while CDKs are constantly present. Together they activate enzymes and proteins required for DNA replication and mitosis. Precise regulation of CDK-cyclin complexes and degradation of cyclins through ubiquitination is crucial for orderly cell cycle progression.

1. ROLE OF CYCLIN DEPENDENT KINASES
IN CELL CYCLE CONTROL
SUBMITTED TO
DR. APARNA CHAUDHARI
PS AND HOD, FGB DIVISION
CIFE, MUMBAI.
SUBMITTED BY
SUBASHINI.V
FBT- MA- 09- 07

2. CONTENTS
• OVERVIEW OF CELL CYCLE
• CELL CYCLE CHECKPOINTS
• CYCLIN DEPENDENT KINASES (CDK)
• CYCLINS IN CELL CYCLE
• CYCLIN-CDK ACTIVATION
• ROLE OF CDK IN VARIOUS PHASES OF CELL CYCLE

3. OVERVIEW OF CELL CYCLE
• The cell cycle is the series of growth and development steps a cell undergoes
between its formation and division to make two new daughter cells.
• It consists of 2 stages. They are-
1. Interphase (period between two mitotic divisions)
2. Mitotic phase
• The interphase is again divided into-
1.G1 phase: Cell growth, RNA synthesis, transcription and translation takes place.
2. S phase: DNA is duplicated
3. G2 phase: protein synthesis and rapid cell growth

4. Continued…
• The Mitotic phase is divided into Karyokinesis and Cytokinesis.
• Karyokinesis (division of the nucleus) takes place in 4 steps-
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
• Cytokinesis(division of the cytoplasm)- produces two daughter cells with equal
chromosome number

5. Cell cycle

6. CELL CYCLE CHECKPOINTS
• The cell division is controlled by surveillance mechanisms called checkpoint
pathways that prevent initiation of each step in cell division until the earlier steps
on which it depends have been completed and mistakes that occurred during the
process have been corrected.
• There are 3 checkpoints in a cell cycle
• G1 checkpoint- cell checks for favourable environmental conditions
• G2-M checkpoint-check for damaged or unduplicated DNA
• Metaphase checkpoint- check for chromosome attachment to the mitotic spindle

8. CYCLIN DEPENDENT KINASES (CDK)
• Cyclin-dependent kinases are a family of small (30-40 kD)
serine/threonine kinases
• Catalytic subunit of the complex
• Kinase activity is not activated unless it is associated with cyclin
• Concentration does not vary and are always present in the cell
• CDKs are not only regulated by cyclin binding but also by both
activating and inhibitory phosphorylation.

9. CYCLINS IN CELL CYCLE
• Cyclin are the proteins that oscillates in abundance during the cell cycle.
• Regulatory subunit of the complex
• Have no enzyme activity
• Their concentration varies in the cell during cell cycle
• Cyclins bind to and activate CDKs.
• The activity and substrate specificity of any given CDK is primarily defined by
the particular cyclin to which it is bound.
• Cyclins are only present during the cell cycle stage that they trigger and are
absent in other cell cycle stages.
• Cyclins not only regulate a particular cell cycle stage but also set in motion
a series of events in preparation for the next cell cycle stage. In this way,
they propel the cell cycle

11. CYCLIN-CDK REGULATION
• The cyclin- CDK complexes are regulated by phosphorylating the Cdk.
• CDKs are phosphorylated at two distinct surfaces, one activating it
and the other inhibiting it.
• The activating phosphorylation occurs on a threonine residue which is
catalysed by the CDK activating kinase or the CAK protein.
• The inhibitory phosphorylation occurs at another region of the Cdk
either on a conserved tyrosine residue or on an adjacent threonine
residue.

13. • This inhibitory phosphorylation is catalysed by the kinases of Wee 1
family.
• The inhibitory phosphates are then removed by the Cdc25 family of
phosphatases after receiving signals to whether the cell cycle should
go forward and hence the complex is activated.

15. DEGRADATION OF CYCLIN
• The abrupt instability of cyclins is due to activation of Ubiquitin ligases that
target cyclins for proteasome mediated degradation.
• The Ubiquitin attaches to itself to the cyclin protein at the lysine residue of
the protein via an enzyme cascade that consists of-
Ubiquitin activating enzyme(E1)
Ubiquitin conjugating enzyme(E2)
Ubiquitin ligating enzyme(E3)
• The ubiquitin protein after conjugating with the cyclin targets it to
proteasome.
• After the attachment of multiple ubiquitins to the cyclin protein, the poly-
ubiquitinated protein is then degraded into peptides by the proteasomes.

17. CELL CYCLE REGULATION
• The G1 cyclins are the lynchpin in coordinating the cell cycle with
extracellular events.
• In metazoans, G1 cyclins are known as Cyclin D and they bind to CDK4
and CDK6
• START defines a stage in G1 after which cells are irreversibly
committed to the cell cycle.

18. G1 PHASE CDK
• During G1, E2Fs are held inactive through their association with the
retinoblastoma protein (Rb).
• G1 CDKs activate E2Fs by phosphorylating and inactivating Rb.
• E2Fs then activate genes encoding many of the proteins involved in
DNA synthesis.
• They also stimulate transcription of genes encoding the G1/S phase
cyclins and the S phase cyclins.

20. S PHASE CDK
• Loading and activation of the MCM helicase occur in mutually
exclusive cell cycle states, MCM helicase loading can only occur when
CDK activity is low (during early G1), MCM helicases are activated
when CDK activity is high.
• S phase CDKs and DDK trigger the initiation of DNA replication by the
recruitment of MCM helicase activators to origins.

21. • Initiation of DNA replication in
S phase due to the activity of S
phase cyclin–CDK.

22. MITOTIC CDK
• Activation of Mitotic CDKs is the consequence of rapid inactivation of
Wee1 kinase and activation of Cdc25 phosphatase.
• At the onset of Anaphase, the APC/C is directed by Cdc20 to
ubiquitinylate securin, which is subsequently degraded by
proteasomes. This activates separase.
• Exit from mitosis is triggered by mitotic CDK inactivation mainly
brought about by mitotic cyclin degradation.
• Exit from mitosis requires the activity of protein phosphatases such as
Cdc14 to remove mitotic phosphorylations from many different
proteins, permitting mitotic spindle disassembly, the decondensation
of chromosomes and the reassembly of the nuclear envelope.

23. ENTRY INTO
MITOSIS

24. SISTER CHROMATID SEPARATION
• Sister chromatids are held together by cohesin complexes.
• Plk 1 phosphorylates cohesin subunits of chromosomal arms early in
prophase so cohesins are found only at centromeric positions after
prophase.
• Separase, a site specific protease is required for removal of cohesins
from centromeric positions.
• Securin is the inhibitor of separase.

25. • A E3 ubiquitin ligase called
anaphase promoting
complex/cyclosome (APC/C)
after binding with its
activator Cdc20, mediates
degradation of securin.

26. EXIT FROM MITOSIS
Activation of Cdc14 phosphatase
leads to dephosphorylation of
Cdh1 (APC/C activator).
Cdh1 binds to APC and then APC
mediates degradation of several
substrate for proper exit from
mitosis.
Coordination of sister chromatid
separation and Cdk1 inactivation is
critical cell cycle timing issue.

27. DNA DAMAGE RESPONSE SYSTEM

28. REFERENCE
• Lodish,h., Berk,A., Kaiser,C.A., Krieger,M., Bretscher,A., Ploegh,H.,
Amon, A., Scott,M.P., (2013). The Eukaryotic Cell Cycle. Molecular Cell
Biology, 7th Edition, p 873-910.

29. THANK YOU