Cell cycle phases Cyclins CDKs | Anatomy2Medicine

Cell cycle phases Cyclins CDKs

action of cyclins and cyclin-dependent kinases

(CDKs) of which there are several. The activity of

the CDKs is dependent on interaction with

cyclins. However, the presence of CDK

inhibitory proteins, CIPs, results in inhibition of

the activity of CDKs. CIPs bind and inhibit

CDK1, 2, 4, and 6 complexes. Thus, an increase

in the activity of p53, which occurs in response to

DNAdamage, results in a block to cell-cycle progreCell cycle phases


      • Checkpoints control transitions between phases of cell cycle.


  • This process is regulated by cyclins, cyclin-dependent kinases (CDKs), and tumor suppressors.


      • Mitosis (shortest phase of cell cycle) includes prophase, metaphase, anaphase, and telophase.


  • G1 and G0 are of variable duration.




  • Regulation of cell cycle
  • CDKs


      • Constitutive and inactive.
    • Cyclins
      • Regulatory proteins that control cell cycle events
      • phase specific
      • activate CDKs


  • Cyclin-CDK complexes –
  • Must be both activated and inactivated for cell cycle to progress.


    • G1/S cyclin: They activate Cdks in late G1 and their level fall in S phase.
    • S cyclin: They stimulate DNA replication and their level remains high until mitosis.


  • M cyclin: Activate Cdks that stimulate entry into mitosis at the G2/M checkpoint.
  • G1 cyclins: Governs the activities of G1/S cyclins.


Don’t Enter After the Bell – How to

    • Cyclin D/E – G1 ➡ S
    • Cyclin A – S ➡ G2


  • Cyclin B – G2 ➡ M




  • Tumor suppressors


    • p53 and hypophosphorylated Rb normally inhibit G1-to-S progression
    • mutations in thes genes result in unrestrained cell division (e.g., Li-Fraumeni syndrome).





  • Permanent




  • Remain in G0, regenerate from stem cells.




  • Neurons, skeletal and cardiac muscle, RBCs.




  • Stable (quiescent)



    • Enter G1 from G0 when stimulated.



  • Hepatocytes, lymphocytes.




  • Labile


      • Never go to G0
      • divide rapidly with a short G1.


  • Most affected by chemotherapy.
  • Bone marrow, gut epithelium, skin, hair follicles, germ cells.



Exam MCQ

  1. A patient has been diagnosed with a melanoma, and molecular analysis has indicated that the tumor has sustained a loss of p16(INK4) activity (inhibitor of cyclin-dependent kinase 4). Such a gene would be best classified as which of the following?

(A) A dominant oncogene

(B) A tumor suppressor

(C) A proapoptotic factor

(D) An antiapoptotic factor

(E) A growth factor

13 The answer is B:

A tumor suppressor. Cyclin-dependent kinase inhibitors (CKI) act to block the action of kinases that are activated by cyclins  .When such an activity is lost (meaning that the gene products from both chromosomes are inactive), uncontrolled cell proliferation can result. Since the activity must be lost, such genes are classifi ed as tumor suppressors, as opposed to the dominant oncogenes, in which an activity is gained via mutation or inappropriate gene regulation. The CKIs are not involved in apoptosis, nor do they act as growth factors.


Exam MCQ

  1. In which phase of the cell cycle DNA is synthesized:
  2. M Phase
  3. Gt Phase
  4. S Phase
  5. G2 Phase

ANS: C.   DNA is synthesized during the S-phase of the cell cycle, this period known as synthetic phase. Mammalian cells contain large quantity of DNA polymerase enzymes. M phase (mitosis) is the cell division stage. Gl phase (gap-1) and G2 phase (gap-2) occurring before and after S phase.

    • Gl phase RNA and protein contents increase.
    • Cytoplasmic enalargment is seen in G2 phase.


3 Which one of the following statement is correct regarding cy clins?

  1. They are inhibitors in replication
  2. They control cell cycle
  3. They are circular single stranded DNA
  4. They are not able to phosphorylate specific proteins

ANS: B.   Cyclins are proteins and their concentrarion increase or decrease through out the cell cycle, that’s why they are named as cyclins. These cyclins activate different cyclin dependent kinases (CDK1, 2, 4, 5 and 6) which phosphorylates specific substrate for the transition of one phase of cell cycle to another.

Exam MCQ

  1. Which of the following cellcycle proteins is involved in DNA damage mediated cell-cycle arrest?
  2. CDK2
  3. cyclin A
  4. cyclin D
  5. E2F
  6. p53

Ans E

The p53 protein is a tumor suppressor whose

function is to ensure that cells with damaged

DNAdo not progress through the cell cycle until

that damage is repaired. The major response to

p53 action is an increase in the expression of the

p21CIP gene. Progression through the cell cycle

requires, among other activities, the concerted



6 .ALL of the following are example of stable cell exept

1- kidney

B- liver

C – smooh muscle

D – GI epethelia

Ans D

GI epethelia are labile cell