Cell Cycle Phases

      • Cell Cycle Phases
      • Cell 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 G0are of variable duration.

        • Regulation of cell cycle
        • CDKs
        • Constitutive and inactive.
        • Cyclins
        • Regulatory proteins that control cell cycle events
        • phasespecific
        • activateCDKs
        • 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 hypophosphorylatedRb normally inhibit G1-to-S progression
          • mutations in thes genes result in unrestrained cell division (e.g., Li-Fraumeni syndrome).



        • CELL TYPES

        • 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 classified 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

2. In which phase of the cell cycle DNA is synthesized:

A.            M Phase

B.            Gt Phase

C.            S Phase

D.            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 cyclins?

A.            They are inhibitors in replication

B.            They control cell cycle

C.            They are circular single stranded DNA

D.            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 cyclinsactivate 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


Ans D


Exam MCQ

5.Which of the following cellcycle proteins is involved in DNA damagemediated cell-cycle arrest?


B. cyclin A

C. cyclin D

D. E2F

E. 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

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 progression




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






Cell Cycle (Overview, Interphase)
Cell Cycle
Cell Cycle, Mitosis and Meiosis
Paul Andersen explains how the cell cycle is used to create new cells. The creation of identical diploid daughter cells, through mitosis, is described. The creation of unique haploid daughter cells, through meiosis is also described. The importance of cyclin and cyclin dependent kinases to regulate the cell cycle is included.
NEET BIO – Cell: Phases of cell cycle
This video describes about phases of cell cycle, the cell cycle is divided into two basic phases are interphase and M phase. The M phase represents the phase when the actual cell division or mitosis occurs and the interphase represents the phase between two successive M phases.
Cell Cycle, Interphase, Labile Cell Stable Permanent Tissue Paclitaxel G1 G0 G2 phase
The cell cycle is the process of cell replication where two daughter cells are created from one parent cell. The cell cycle is made up of interphase & mitosis. Mitosis is the phase of actual cell splitting (duplication), while Interphase is made up of the G0, G1, S, and G2 phases which prepare the cell for mitosis. The S phase involves “Synthesizing” new DNA and doubling the amount of DNA present so there is enough for both daughter cells. G stands for “Gap” as these are the gaps in time between mitosis and the S Phase. Cells in G0 are no longer in the cell cycle. Cells in G0 are “resting” & no longer actively going through the phases of the cell cycle. Some tissue can easily transition between G0 & G1 with the right signals while other cell types are “stuck” in G0.
Anti-metabolites chemotherapy drugs like 5-flourouracil & sulfa drugs (antibiotic) inhibit DNA synthesis and stall the S phase of the cell cycle. Chemotherapy Paclitaxel inhibits microtubule action involved in Mitosis. Therefore, cells being treated with this medication are stuck in M phase.
Permanent tissues are “stuck” in the G0 phase and cannot undergo further mitosis. Permanent tissues include muscle (cardiac & skeletal) and the CNS. These tissues can only undergo hypertrophy (not hyperplasia) to deal with increased stress and form fibrous nonfunctioning scars during the healing process. Stable tissues are those that primarily reside in the G0 phase, but given the right signals can reenter the cell cycle. For example, lymphocytes are most often in a resting period, but during infection they are triggered to divide. Liver cells are another example of a stable tissue that can undergo replication following injury to the organ to facilitate healing. Labile Tissues are cells which are constantly replicating with no time in the G0 phase. These cells act as the stem cells for tissues such as the bone marrow and skin.
Paul Andersen describes the process of mitosis. He begins by discussing the importance of the cell cycle in development, regeneration, asexual reproduction and wound healing. He differentiates between haploid and diploid cells and describes the structure of the chromosome. He then moves through all the phases of mitosis; interphase, prophase, metaphase, anaphase, telophase and cytokinesis.