Regulation of Cell Cycle Progression and Checkpoints

1. Chap. 19 Problem 1 Passage through the cell cycle is unidirectional and irreversible due to the degradation of critical regulators by proteasome complexes at specific points within the cycle (Fig. 19.30). The APC/C ubiquitin ligase/proteasome degrades the protein securin at the beginning of anaphase, leading to separation of the sister chromatids of metaphase chromosomes. The APC/C ubiquitin ligase/proteasome degrades mitotic cyclins at the end of anaphase, and this triggers telophase processes and ultimately entry into G1. Another ubiquitin ligase/proteasome complex (the SCF/proteasome) is responsible for degradation of the S phase cyclin-CDK inhibitor at the start of S phase.

2. Chap. 19 Problem 9 At the START point in the cell cycle, cells become committed to enter S phase regardless of whether growth factors are present or not. In G0 phase, mitogens stimulate synthesis of G1 cyclin-CDK (cyclin D-CDK4/6) that in turn phosphorylates the Rb protein which controls E2F activity (Fig. 19.15b). Due to release from Rb control, the E2F transcription factor induces transcription of genes that promote entry into S phase, including G1/S cyclin-CDKs (cyclins E/A-CDK2), S phase cyclin-CDKs, and DNA synthesis enzymes. Cells would no longer require mitogens for exit of G1 if cyclin D were overexpressed. In the absence of functional Rb, mitogens and cyclin D would not be required for activation of E2F. p16 (INK4A) inhibits G1/S cyclin-CDKs. Without p16 function, G1/S cyclin CDKs would promote entry into S phase. In the presence of hyperactive E2F, a number of gene products (including E2F itself) that promote entry of cells into S phase would be switched on. (Refer to the first paragraph above).

3. Chap. 19 Problem 12 When S phase cyclin-CDKs are activated at the end of G1 due to the degradation of the S phase cyclin-CDK inhibitor, they phosphorylate two initiation factors and MCM helicase, which leads to unwinding of replication origins and bidirectional DNA synthesis (Fig. 19.19). The phosphorylated forms of the initiation factors cannot rebind to origins preventing re-initiation of DNA synthesis during the remainder of the cell cycle. These factors are maintained in their phosphorylated states by S phase and mitotic cyclin-CDKs throughout the remainder of the cell cycle. Only after these cyclins are degraded at the end of mitosis can dephosphorylated initiation factors assemble again at replication origins.

4. Chap. 19 Problem 15 The activation of APC/C ubiquitin ligase by Cdc20 triggers the separation of sister chromatids during anaphase (Fig. 19.27). Separation is achieved after APC/C-mediated polyubiquitination and proteasome degradation of the protein known as securin. Securin normally inhibits a protease (separase), which cleaves cohesin linkages between sister chromatids when the inhibitor is degraded. The protein known as Mad2 operates at this checkpoint. Mad2 binds to kinetochores that have not yet bound to microtubules of the mitotic spindle. Kinetochore binding activates Mad2, and it in turn inhibits the activity of Cdc20 which controls the activity of the APC/C ubiquitin ligase. This delays degradation of securin and anaphase until all chromosomes have attached to the spindle.

5. Chap. 19 Problem 17 Cell cycle checkpoints are points where the status of a cell’s progression through the cycle is monitored, and the cell cycle arrested if a problem is detected. DNA damage and the completion of DNA synthesis are monitored in G1, S and M phases (Fig. 19.34). Potential problems with chromosome segregation and the assembly of the mitotic spindle are screened in M phase. Checkpoint arrests minimize the transfer of mutations to the next generation.