Cell Cycle Made Easy

Stages of Cell Cycle

Interphase

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 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.

Checkpoints

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

Permanent

• 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

Labile

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

Cell cycle and Cancer 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.