Two key classes of regulatory molecules, cyclins and cyclin-dependent kinases (CDKs), determine a cell’s progress through the cell cycle.
Many of the genes encoding cyclins and CDKs are conserved among all eukaryotes, but in general more complex organisms have more elaborate cell cycle control systems that incorporate more individual components.
Many of the relevant genes were first identified by studying yeast, especially Saccharomyces cerevisiae; genetic nomenclature in yeast dubs many of these genes cdc (for “cell division cycle”) followed by an identifying number, e.g., cdc25.
Cyclins form the regulatory subunits and CDKs the catalytic subunits of an activated heterodimer; cyclins have no catalytic activity and CDKs are inactive in the absence of a partner cyclin.
When activated by a bound cyclin, CDKs perform a common biochemical reaction called phosphorylation that activates or inactivates target proteins to orchestrate coordinated entry into the next phase of the cell cycle.
Different cyclin-CDK combinations determine the downstream proteins targeted. CDKs are constitutively expressed in cells whereas cyclins are synthesized at specific stages of the cell cycle, in response to various molecular signals.
Cyclin-dependent kinases (CDKs) are typical serine/threonine kinases that display the 11 subdomains shared by all kinases.
Among the 19,099 predicted genes, there are 14 CDKs and 34 cyclins; nine CDKs and 11 cyclins have been identified in man, referred to as CDK1-CDK9.
This also allows the activating phosphorylation on Thr160 (by CDK7/cyclin H/MAT1).
The second conformational change induced by cyclin binding is found within the ATP-binding site where a reorientation of the amino acid side chains induces the alignment of the triphosphate of ATP necessary for phosphate transfer.
The strong sequence homology between the catalytic domains of different CDKs suggests that their tridimensional structures will be similar.
Progression through the G1, S, G2 and M phases of the cell division cycle is directly controlled by CDKs.
In early-mid G1, extracellular signals modulate the activation of CDK4 and CDK6 associated with D-type cyclins.
These complexes phosphorylate and inactivate the retinoblastoma protein pRb, resulting in the release of the E2F and DP1 transcription factors which control the expression of genes required for the G1/S transition and S phase progression.
The CDK2/cyclin E complex, which is responsible for the G1/S transition, also regulates centrosome duplication.
During S phase, CDK2/cyclin A phosphorylates different substrates allowing DNA replication and the inactivation of G1 transcription factors.
Around the S/G2 transition, CDK1 associates with cyclin A.
Later, CDK1/cyclin B appears and triggers the G2/M transition by phosphorylating a large set of substrates.
Phosphorylation of the anaphase promoting complex (APC) by CDK1/cyclin B is required for transition to anaphase and completion of mitosis
General mechanism of cyclin-CDK interaction
Upon receiving a pro-mitotic extracellular signal, G1 cyclin-CDK complexes become active to prepare the cell for S phase, promoting the expression of transcription factors that in turn promote the expression of S cyclins and of enzymes required for DNA replication.
The G1 cyclin-CDK complexes also promote the degradation of molecules that function as S phase inhibitors by targeting them for ubiquitination.
Once a protein has been ubiquitinated, it is targeted for proteolytic degradation by the proteasome.
Active S cyclin-CDK complexes phosphorylate proteins that make up the pre-replication complexes assembled during G1 phase on DNA replication origins.
The phosphorylation serves two purposes: to activate each already-assembled pre-replication complex, and to prevent new complexes from forming.
This ensures that every portion of the cell’s genome will be replicated once and only once.
The reason for prevention of gaps in replication is fairly clear, because daughter cells that are missing all or part of crucial genes will die.
However, for reasons related to gene copy number effects, possession of extra copies of certain genes would also prove deleterious to the daughter cells.
Mitotic cyclin-CDK complexes, which are synthesized but inactivated during S and G2 phases, promote the initiation of mitosis by stimulating downstream proteins involved in chromosome condensation and mitotic spindle assembly.
A critical complex activated during this process is a ubiquitin ligase known as the anaphase-promoting complex (APC), which promotes degradation of structural proteins associated with the chromosomal kinetophore.
APC also targets the mitotic cyclins for degradation, ensuring that telophase and cytokinesis can proceed.
Specific action of cyclin-CDK complexes
Cyclin D is the first cyclin produced in the cell cycle, in response to extracellular signals (eg. growth factors).
Cyclin D binds to existing CDK4, forming the active cyclin D-CDK4 complex.
Cyclin D-CDK4 complex in turn phosphorylates the retinoblastoma susceptibility protein (RB).
The hyperphosphorylated RB dissociates from the E2F/DP1/RB complex (which was bound to the E2F responsive genes, effectively “blocking” them from transcription), activating E2F.
Activation of E2F results in transcription of various genes like cyclin E, cyclin A, DNA polymerase, thymidine kinase, etc.
Cyclin E thus produced binds to CDK2, forming the cyclin E-CDK2 complex, which pushes the cell from G1 to S phase (G1/S transition).
Cyclin A along with CDK2 forms the cyclin A-CDK2 complex, which initiates the G2/M transition.
Cyclin B-CDK1 complex activation causes breakdown of nuclear envelope and initiation of prophase, and subsequently, its deactivation causes the cell to exit mitosis.
🔬 Section 1: Basics of Cyclins and CDKs
What type of molecules are cyclins and CDKs? A) Hormones B) Enzymes C) Regulatory molecules D) Antibodies Answer: C) Regulatory molecules
Which of the following acts as the catalytic subunit in the cyclin-CDK complex? A) Cyclin B) CDK C) p53 D) E2F Answer: B) CDK
Which subunit of the cyclin-CDK complex has no catalytic activity? A) CDK B) Cyclin C) E2F D) RB Answer: B) Cyclin
Which model organism helped identify many CDK-related genes? A) Drosophila B) Escherichia coli C) Saccharomyces cerevisiae D) Arabidopsis Answer: C) Saccharomyces cerevisiae
What does the gene prefix “cdc” stand for? A) Cell differentiation code B) Cell division cycle C) Cyclin degradation complex D) Cytoplasmic DNA control Answer: B) Cell division cycle
What is the main function of activated CDKs? A) DNA replication B) Protein degradation C) Phosphorylation of target proteins D) Translation Answer: C) Phosphorylation of target proteins
CDKs are generally expressed in what manner in the cell? A) Only in G2 phase B) In response to DNA damage C) Constitutively D) Only after cyclin synthesis Answer: C) Constitutively
Cyclins are produced: A) All the time B) Only after mitosis C) In response to extracellular and intracellular signals D) Randomly during G2 Answer: C) In response to extracellular and intracellular signals
CDKs are a type of: A) Tyrosine kinase B) Serine/threonine kinase C) Lipid kinase D) Receptor kinase Answer: B) Serine/threonine kinase
How many CDKs have been identified in humans? A) 5 B) 7 C) 9 D) 11 Answer: C) 9
📈 Section 2: Cyclin-CDK Interactions and Functions
What does phosphorylation of RB protein by CDK4/6-cyclin D complex cause? A) DNA damage B) Activation of E2F C) Cell death D) Translation inhibition Answer: B) Activation of E2F
Which transcription factors are released upon RB phosphorylation? A) ATM and ATR B) Cyclins C) E2F and DP1 D) APC and Cdc20 Answer: C) E2F and DP1
What is the primary role of E2F transcription factors? A) Control mitosis B) Activate apoptosis C) Activate S phase genes D) Promote cytokinesis Answer: C) Activate S phase genes
Which cyclin-CDK complex is responsible for centrosome duplication? A) CDK2/Cyclin A B) CDK1/Cyclin B C) CDK2/Cyclin E D) CDK4/Cyclin D Answer: C) CDK2/Cyclin E
During the S phase, which complex phosphorylates replication substrates? A) Cyclin D-CDK4 B) Cyclin B-CDK1 C) Cyclin A-CDK2 D) Cyclin E-CDK2 Answer: C) Cyclin A-CDK2
What triggers G2/M transition? A) CDK2/Cyclin A B) CDK4/Cyclin D C) CDK1/Cyclin B D) CDK2/Cyclin E Answer: C) CDK1/Cyclin B
The CDK1/Cyclin B complex is required for: A) G1/S checkpoint B) G2 to M transition C) S phase entry D) Cytokinesis Answer: B) G2 to M transition
What is required for transition to anaphase and completion of mitosis? A) RB phosphorylation B) DNA duplication C) APC phosphorylation by CDK1/Cyclin B D) ATP synthesis Answer: C) APC phosphorylation by CDK1/Cyclin B
What type of change occurs in CDK’s ATP-binding site upon cyclin binding? A) DNA cleavage B) Protein degradation C) Reorientation of amino acid side chains D) Methylation Answer: C) Reorientation of amino acid side chains
Which residue is phosphorylated to activate CDK? A) Tyr98 B) Thr160 C) Ser45 D) Lys13 Answer: B) Thr160
🧬 Section 3: General Mechanism and Cell Cycle Regulation
What triggers activation of G1 cyclin-CDK complexes? A) Internal DNA damage B) Lack of nutrients C) Pro-mitotic extracellular signals D) Protein misfolding Answer: C) Pro-mitotic extracellular signals
G1 cyclin-CDK complexes help in degrading: A) mRNA B) S phase inhibitors C) Microtubules D) CDKs Answer: B) S phase inhibitors
What pathway marks S phase inhibitors for degradation? A) DNA repair B) Ubiquitination C) Glycosylation D) Acetylation Answer: B) Ubiquitination
Ubiquitinated proteins are degraded by the: A) Golgi apparatus B) Mitochondria C) Proteasome D) Endosome Answer: C) Proteasome
Phosphorylation of pre-replication complexes serves to: A) Halt DNA replication B) Activate and prevent new complex formation C) Remove CDKs D) Promote transcription Answer: B) Activate and prevent new complex formation
What happens if DNA replication occurs more than once per cycle? A) Faster mitosis B) Gene loss C) Cell aging D) Gene dosage imbalances Answer: D) Gene dosage imbalances
Mitotic cyclin-CDK complexes promote: A) DNA repair B) Transcription inhibition C) Chromosome condensation and spindle formation D) Protein synthesis Answer: C) Chromosome condensation and spindle formation
APC promotes degradation of: A) CDKs B) Cyclin D C) Chromosomal kinetochore proteins and mitotic cyclins D) Growth factors Answer: C) Chromosomal kinetochore proteins and mitotic cyclins
What ensures telophase and cytokinesis? A) RB phosphorylation B) Cyclin degradation by APC C) CDK synthesis D) Centrosome duplication Answer: B) Cyclin degradation by APC
What complex promotes cyclin degradation during mitosis? A) ATM B) ATR C) APC D) CDK4 Answer: C) APC
🔁 Section 4: Specific Cyclin-CDK Complexes and Their Roles
Which is the first cyclin produced during the cell cycle? A) Cyclin A B) Cyclin B C) Cyclin D D) Cyclin E Answer: C) Cyclin D
Cyclin D binds to which CDK to form the first active complex? A) CDK2 B) CDK6 C) CDK4 D) CDK1 Answer: C) CDK4
Cyclin D-CDK4 complex leads to: A) Cell arrest B) p53 activation C) RB phosphorylation D) Centrosome destruction Answer: C) RB phosphorylation
Which genes are activated due to E2F? A) CDK inhibitors B) Histones C) Cyclin E, Cyclin A, DNA polymerase D) tRNA synthetases Answer: C) Cyclin E, Cyclin A, DNA polymerase
Cyclin E binds to CDK2 to form a complex for: A) S to G2 transition B) G2 to M transition C) G1 to S transition D) M phase Answer: C) G1 to S transition
Cyclin A-CDK2 initiates: A) G1 arrest B) DNA repair C) G2/M transition D) Cytokinesis Answer: C) G2/M transition
Cyclin B-CDK1 activation causes: A) DNA synthesis B) Nuclear envelope breakdown C) Apoptosis D) Endocytosis Answer: B) Nuclear envelope breakdown
What causes the cell to exit mitosis? A) Activation of RB B) Deactivation of Cyclin B-CDK1 C) Upregulation of p21 D) Centrosome duplication Answer: B) Deactivation of Cyclin B-CDK1
Which complex triggers prophase initiation? A) Cyclin D-CDK4 B) Cyclin E-CDK2 C) Cyclin A-CDK2 D) Cyclin B-CDK1 Answer: D) Cyclin B-CDK1
Cyclin A also binds to which CDK around S/G2 transition? A) CDK4 B) CDK1 C) CDK3 D) CDK9 Answer: B) CDK1
📚 Section 5: Additional Concepts and Molecular Details
Which cyclin-CDK complex is linked to the phosphorylation of APC? A) CDK4/Cyclin D B) CDK2/Cyclin A C) CDK1/Cyclin B D) CDK7/Cyclin H Answer: C) CDK1/Cyclin B
What ensures that DNA is replicated only once per cycle? A) Cyclin degradation B) Ubiquitination of CDKs C) Prevention of new pre-replication complex formation D) Proteolysis of chromatin Answer: C) Prevention of new pre-replication complex formation
What protein complex helps initiate S-phase by degrading inhibitors? A) ATM B) APC C) G1 cyclin-CDK complex D) p53 Answer: C) G1 cyclin-CDK complex
What is CDK7’s role? A) DNA methylation B) Phosphorylates CDKs on Thr160 C) Inhibits Cyclin E D) Degrades Cyclin D Answer: B) Phosphorylates CDKs on Thr160
Which complex regulates thymidine kinase gene expression? A) CDK1-Cyclin B B) CDK2-Cyclin E C) CDK4-Cyclin D D) CDK9-Cyclin T Answer: B) CDK2-Cyclin E
Which is NOT a function of mitotic cyclin-CDK complexes? A) Chromosome condensation B) Spindle formation C) Cytokine production D) Activation of APC Answer: C) Cytokine production
Cyclin-CDK phosphorylation typically affects: A) RNA polymerase only B) Nuclear lamins, histones, replication factors C) Chloroplasts D) Ribosomes Answer: B) Nuclear lamins, histones, replication factors
Which phase is skipped if cyclin E-CDK2 is non-functional? A) M B) S C) G2 D) G0 Answer: B) S
CDK1 is also referred to as: A) cdc2 B) p21 C) E2F D) Cdc25 Answer: A) cdc2
What is the main outcome of CDK4/Cyclin D pathway activation? A) Apoptosis B) G1 arrest C) Progression into S phase D) Protein translation Answer: C) Progression into S phase