The Cell Cycle 2024 PDF

# The Cell Cycle ## G1 Phase - Cyclin D - CDK 4/6 complexes - During the early G1 phase, cyclin D levels begin to rise in response to extracellular growth signals. - Cyclin D binds to CDK 4 & CDK 6 , forming active complexes that phosphorylate the retinoblastoma protein (Rb). - Phosphor...

# The Cell Cycle ## G1 Phase - Cyclin D - CDK 4/6 complexes - During the early G1 phase, cyclin D levels begin to rise in response to extracellular growth signals. - Cyclin D binds to CDK 4 & CDK 6 , forming active complexes that phosphorylate the retinoblastoma protein (Rb). - Phosphorylated Rb releases E2F transcription factors, which activate the transcription of genes necessary for S phase entry, including those encoding cyclin E. ## G1/S Checkpoint/Transition - Cyclin E - CDK 2 Complex - As the cell approaches the G1/S transition, cyclin E accumulates and binds to CDK 2. - The cyclin E-CDK 2 complex further phosphorylates, enhancing the release of E2F and transcription of S-phase genes. - This complex is crucial for the initiation of DNA replication by activating proteins involved in the formation of the pre-replication complex at the origins of replication. ## S-Phase (Synthesis Phase) - Cyclin A-CDK 2 complex - Cyclin A is synthesized as the cell enters S phase and binds to CDK 2, forming a complex that drives DNA synthesis. - The cyclin A-CDK 2 complex phosphorylates various substrates to ensure that DNA replication proceeds efficiently and that each portion of the genome is replicated only once. - It also inhibits the formation of new pre-replication complexes to prevent re-replication. ## G2-Phase - Cyclin A - CDK 1 complex - Cyclin A pairs with CDK 1 to prepare the cell for mitosis. - This complex initiates processes such as chromatin condensation and the activation of mitotic proteins, setting the stage for transition into M phase. ## G2/M Transition/Checkpoint - Cyclin B – CDK 1 Complex (Maturing Promoting Factor, MPF) - Cyclin B - CDK 1 complex for the entry into Mitosis (M-phase). - This complex, also known as MPF, phosphorylates key substrates that lead to the breakdown of the nuclear envelope, chromosome condensation, spindle formation and alignment of chromosomes at the metaphase plate. - The activation of MPF is tightly regulated by phosphorylation events: CDK 1 must be dephosphorylated at specific inhibitory sites and phosphorylated at an activating site to be fully and active. ## M-Phase - Cyclin B- CDK 1 complex - Cyclin B – CDK 1 remains active through the early stages of mitosis, ensuring that the cell progresses through prophase, metaphase and anaphase. - Cyclin B is degraded by the anaphase-promoting complex/cyclasome (APC/C) during the metaphase-to-anaphase transition, leading to the activation of CDK 1 and allowing the cell to complete mitosis and enter cytokinesis. ## Factors that drive the cell cycle (at least 10) 1. **Cyclins** - Proteins that regulate the progression of the cell cycle by activating cyclin-dependent kinases (CDKs). - Different cyclins are produced and degraded at specific stages of the cell cycle. 2. **Cyclin-Dependent Kinases (CDK)** - Enzymes that when activated by binding to cyclins, phosphorylate target proteins to drive all cell cycle progression. - The activity of CDKs is tightly regulated by the availability of cyclins. 3. **CDKs Inhibitors (CKIs)** - Proteins that inhibit the activity of CDKs. - They provide a checkpoint control mechanism to halt the cell cycle in response to DNA damage or other cellular stress. 4. **Growth Factors** - Extracellular signals that promote cell division and survival. - These are receptors on the cell surface, triggering intracellular signaling pathways to manipulate the cell cycle progression. 5. **Tumor Suppressor Proteins** - Proteins such as p53 and retinoblastoma protein (Rb) act as negative regulators of the cell cycle. - They can induce cell cycle arrest in response to DNA damage or other abnormalities, allowing for repair or apoptosis. 6. **Oncogenes** - Mutated or overexpressed versions of normal genes (proto-oncogenes) that drive uncontrolled cell proliferation. - Examples include genes encoding growth factor receptors or signaling proteins like Ras. 7. **DNA Damage & Repair Mechanism** - The presence of DNA damage triggers checkpoint pathways that can halt the cell cycle to allow for repair. - Key proteins involved include ATM, ATR and the checkpoint kinases Chk 1 & Chk 2. 8. **Cell Cycle Checkpoints** - Control mechanisms that ensure the cell cycle progresses only when conditions are favorable. - Major checkpoints include the G1/S checkpoint, the G2/M checkpoint and the spindle assembly checkpoint. 9. **Mitogens** - Specific types of growth factors that stimulate cell division. - They act through signaling pathways to promote the transition from Go (quiescent state) to G1 phase and subsequent cell cycle progression. 10. **Nutrient Availability** - An adequate supply of nutrients & energy is essential for cell growth & division. - Cells monitor nutrient levels through various signaling pathways, including the mTOR pathway, to ensure they have sufficient resources to complete the cell cycle. ## Drugs designed to interfere with the cell cycle. 1. **Antimetabolites** - Mimic natural substances and interfere with DNA and RNA synthesis. - **Methotrexate**: inhibits dihydrofolate reductase, reducing nucleotide synthesis. - **5-fluorouracil (5-FU)**: inhibits thymidylate synthase, blocking DNA synthesis. - **Cytarabine**: inhibits DNA polymerase, interfering with DNA synthesis. 2. **Mitotic inhibitors** - Disrupt microtubule function, inhibiting mitosis (cell division). - **Paclitaxel (Taxol)**: stabilizes microtubules, preventing their depolymerization. - **Vincristine and Vinblastine**: inhibit microtubule assembly. 3. **Topoisomerase inhibitors** - Interfere with the topoisomerase that control the changes in DNA structure necessary for replication. - **Doxorubicin**: inhibits topoisomerase II, preventing DNA replication. - **Etoposide**: also inhibits topoisomerase II. 4. **Alkylating Agents** - Add alkyl groups to DNA, leading to DNA damage and apoptosis. - **Cyclophosphamide**: cross-links DNA strands, interfering with DNA replication and transcription. - **Cisplatin**: forms DNA adducts, causing DNA cross-linking and apoptosis. 5. **Antitumor Antibiotic** - Intercalate into DNA and inhibit RNA synthesis. - **Doxorubicin (Adriamycin)**: intercalates into DNA, disrupting transcription and replication. - **Bleomycin**: causes breaks in DNA strands. 6. **CDK inhibitors** - Inhibit cyclin-dependent kinases, which are crucial for cell cycle progression. - **Palbociclib**: inhibits CDK 4 & CDK 6, preventing progression from G1 to S phase. - **Ribociclib**: another CDK 4/6 inhibitor. 7. **Proteasome Inhibitors** - Inhibit the proteasome, leading to the accumulation of damaged proteins and cell cycle arrest. - **Bortezomib**: inhibits the 26s proteasome, causes apoptosis in cancer cells. ## Most cancers are targeting CDK 4/6 **Why?** Because these proteins play a critical role in driving cell proliferation by controlling the cell cycle. - Targeting CDK 4/6 with inhibitors helps to restore control over cell division and can stop cancer cells from multiplying. ## Inhibitors currently available in S.A - **Palbociclib** - **Ribocylic** - **Abernacyclic** - Used in breast cancer that is hormone driven. ## Side effects: - Suppresses bone marrow - Diarrhea due to gut continuously dividing. ## BRCA 1/2 & Checkpoint mutations - BRCA 1 & BRCA 2 are crucial genes involved in maintaining genomic stability, particularly through their roles in DNA repair. - **Specifically homologous recombination & cell cycle regulation.** - Mutations in these genes can lead to severe disruption in the cell's ability to repair DNA damage. - Which contributes to cancer development, most notably breast and ovarian. ## BRCA 1/2 & cell cycle checkpoints - Both BRCA 1/2 are intimately connected with the cell cycle checkpoints: 1. **G1/S Checkpoint** - BRCA 1 plays a role in the G1/S checkpoint, where it helps to repair double-stranded breaks in DNA. - When BRCA 1 is mutated, the checkpoint may fail, allowing cells with damaged DNA to enter S phase, where DNA replication occurs. 2. **G2/M Checkpoint** - Both BRCA 1/2 are critical for the G2/M checkpoint. - This checkpoint ensures that DNA damage is repaired before the cell enters M phase (Mitosis). - Mutations in BRCA 1/2 can impair the cell's ability to repair DNA during this phase, leading to the accumulation of genetic mutations. ## Impact of BRCA 1/2 mutations on checkpoints - **Loss of Function:** - These proteins can no longer effectively participate in DNA repair or cell cycle regulation. - As a result, cells with BRCA 1/2 mutations may bypass the checkpoints, especially the G2/M checkpoint, and proceed through the cell cycle with unpaired DNA damage. ## Genomic instability: - The failure of these checkpoints due to BRCA 1/2 mutations leads to genomic instability, a hallmark of cancer. - Cells with damaged DNA replication can accumulate further mutations, promoting tumorigenesis. ## Clinical implications - BRCA 1/2 mutations are associated with a higher risk of developing certain cancers, particularly breast and ovarian cancers. ## In clinical settings: - Individuals with BRCA 1/2 mutations may be candidates for targeted therapies such as PARP inhibitors, which exploit the deficient DNA repair mechanisms in their cancer cells, leading to their death. ## PARP Inhibitors - Are a class of drugs that target the enzymes poly (ADP-ribose) polymerase (PARP). - PARP is involved in several cellular processes, most notably in the repair of single-strand break (SSBs) in DNA through the base excision repair (BER) pathway. - PARP inhibitors are particularly effective in treating cancers with specific in DNA repair mechanisms, such as those involving BRCA 1 & BRCA 2 mutations. ## Mechanisms of Action: 1. **PARP function in DNA repair:** - PARP enzymes, particularly PARP 1, detect and bind to DNA sites such as SSBs and initiate repair by recruiting other proteins necessary for the repair processes. - Once the SSBs are repaired, the PARP enzymes detach from the DNA, allowing the cell cycle to proceed. 2. **Inhibition of PARP:** - PARP inhibitors block the catalytic activity of PARP enzymes, preventing them from repairing SSBs. - This leads to the accumulation of SSBs in the DNA. - During DNA replication, these unpaired SSBs can cause the collapse of replication fork, leading to the formation of double-strand breaks (DSBs). - In normal cells, DSBs are repaired through homologous recombination (HR), a process requiring functioning BRCA 1 & BRCA 2 proteins.