Cell Cycle Made Easy

Stages of Cell Cycle


Mnemonic: Go Sally Go! Make Children
1. G1 phase
2. S phase
3. G2 phase
4. Mitosis
5. Cytokinesis

1. G1 (Gap 1) phase:

  • Early and late phase divided by Restriction (R) point
  • Functions:
    • Preparation for DNA replication (synthesis of replication proteins cyclin D)
    • Thymidine dimer repair
  • Variable duration (8 hours to several days, weeks or months – most cells are in G1 phase)
  • Early G1 phase: requires mitogens to proceed forward
  • G0 (Quiescence): cells can exit G1 phase and enter G0 phase if mitogens are absent
  • R (Restriction point): beyond the R point, cell cycle will progress even in the absence of mitogens
  • Late G1 phase: mitogen is not required to proceed further

2. S (Synthesis) phase:

  • DNA replication occurs
  • Constant duration (6-8 hours)
  • Production of Cyclin B

3. G2 (Gap 2) phase:

  • Cellular growth to prepare for cell division
  • 2-5 hours

Mitosis (~1 hour)

Mnemonic: PMAT
1. Prophase: Prepare for division
2. Metaphase: Meet in midline
3. Anaphase: Alone and Apart
4. Telophase: Torn and towed

Prophase: Chromatin condense into chromosomes

Metaphase: Chromosomal arrangement and attachment to spindles (alignment)

Anaphase: Pulling apart of sister chromatids (Chromosomes split)

Telophase: Chromosomal decondensation, reformation of nuclear membrane and breakdown of spindles

Cytokinesis (Cell division)

Cyclins – Cell Cycle Controllers

Cyclins (Synthesized in specific phase of cell cycle and activates CDKs) + Cyclin Dependent Kinases/CDKs (Constitutive but inactive) = Promotes cell cycle progression by phosphorylation of key proteins

  • Cyclin D + CDK 4 = promotes progression of cell cycle past “R” point (G1-CDK complex)
  • Cyclin E + CDK 2 = promotes progression of cell cycle past G1/S checkpoint (G1/S-CDK complex)
  • Cyclin A + CDK 2 = promotes progression of cell cycle past  S checkpoint (S-CDK complex)
  • Cyclin B + CDK 1 = promotes progression of cell cycle past M checkpoint (M-CDK complex)

Mnemonic: To remember the match for Cyclins and CDKs – DEAB 42 21.

The phosphorylation of RB is a molecular ON-OFF switch for the cell cycle.

Cyclin D is the first cyclin to increase in the cell cycle.

The initiation of DNA replication involve the formation of an active complex between cyclin E and CDK2.

The main mediator that propels the cell beyond prophase is the cyclin B-CDK1 complex.

Proteins of the RAD and ataxia telangiectasia mutated (ATM) families act as sensors. Proteins of the CHK kinase families act as transducers.

Cell cycle regulation

Cell Cycle Inhibitors

CDK-cyclin activation requires phosphorylation of CDK by CDK activating complexes (CAK). 2 ways to inhibit CDK-cyclin complex are:

  1. Phosphorylation of CDK (at a different site than CAK) to inhibit it: Wee1
    • cdc25 removes inhibitor phosphates to activate CDK-cyclin complex.
  2. Binding of inhibitory proteins to CDK-cyclin complex: CDK inhibitors
    • Cip/Kip family components (p21,p27,p57): non-specific
    • INK4a/ARF locus (inhibitor of kinase 4/alternative reading frame): p16INK4a (competitively blocks CDK4 and causes cell cycle arrest at late G1); p14ARF (prevents feedback inhibition of p53).

Cyclical degeneration of Cyclins is mediated through Ubiquitin-Proteasome pathway.


Passage through a checkpoint from one cell cycle phase to the next requires a coordinated set of proteins that monitor cell growth and DNA integrity.

a. “R” (Restriction) point:

  • During G1 : RB gene (Tumor suppressor) + EF2 (Transcription factor) = RB binds and blocks EF2
  • Cyclin D + CDK 4,6 = Phosphorylation of RB gene = Inhibition of RB gene = Transcription of S-phase promoting genes by EF2 (e.g. Cyclin E)

b. G1/S checkpoint:

  • Allows checking of DNA integrity before replication in S phase
  • Cyclin E + CDK 2 allows progression through G1/S check point
  • Controlled by: p53 which induces cell cycle inhibitor p21

c. G2/M checkpoint:

  • Checks for damaged/incompletely replicated DNA and adequacy of cell size.
  • Cells damaged by ionizing radiation activate G2/M checkpoint.
  • Arrest by:
    • p53-dependent mechanism: via cyclinA/CDK2 inhibition
    • p53-independent mechanism: via cdc25 inactivation

d. M checkpoint:

  • Checks if Chromosomes all properly attached to spindles
  • Chromosomes without attachment to spindle sends signal that blocks activation of APC (Anaphase Promoting Complex) which blocks progression from metaphase to anaphase.

Cell types


  • enter G0 and cannot leave
  • e.g. neurons, skeletal, cardiac muscle, RBCs

Stable (quiescent)

  • enter G0 and can leave when given appropriate stimulus
  • e.g. hepatocytes, lymphocytes


  • never go to G0
  • constant division with a condensed G1
  • e.g. bone marrow, skin, gut epithelium

Cell cycle and Cancer Chemotherapy

cell cycle and chemotherapy

Read the principles of cancer chemotherapy.

Disorders of Cell Cycle Regulatory Proteins

G1 checkpoint Inhibiting Proteins

RB – Prevents entry into S phase in the absence of Growth signals by inhibiting E2F transcription factor.

p53 – Slows cell cycle and entry into S phase in response to DNA damage by inducing p21 which inhibits CDK; if DNA repair is not possible, it upregulates BAX which disrupts BCL2 and cytochrome C is leaked out from mitochondria into cytosol leading to apoptosis.

Knudson’s 2 hit theory:

Both copies of tumor suppressor – p53 or RB gene must be knocked out for tumor formation:

  1. p53: Germline mutation (1st hit) + Somatic mutation (2nd hit) = Li Fraumeni Syndrome
  2. RB:
    • Germline mutation (1st hit) + Somatic mutation (2nd hit) = Familial Retinoblastoma (Bilateral)
    • Somatic mutation (1st hit) + Somatic mutation (2nd hit) = Sporadic Retinoblastoma (Unilateral)

S checkpoint Inhibiting Proteins

When DNA break is detected – tumor suppressor genes encode:

ATM (Ataxia Telangiectasia Mutated) protein: halts cell cycle and activates other proteins involved in repairing the break including BRCA 1

  • Inherited mutation of ATM protein = high risk of leukemia and lymphomas

BRCA 1 (Breast Cancer 1) protein: mediate DNA repair or apoptosis

  • Mutation is associated with breast and ovarian carcinoma

G2 checkpoint Inhibiting Proteins

If DNA damage is detected – p53 prevents entry into M phase

M checkpoint Inhibiting Proteins

When chromosomes are not properly attached to the mitotic spindles – MAD (Mitotic Arrest Deficient) proteins inhibit APC (Anaphase Promoting Complex and prevents entry into anaphase.

2 Viewpoints 💬 on “Cell Cycle Made Easy”

    1. The G2-phase checkpoint, also known as G2/M-phase checkpoint, has the function of preventing cells with damaged DNA, lasting from the G1 and S phases or generated in G2, from undergoing mitosis.

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