Cell cycle regulation Flashcards
What are cyclins, and what is their role in the cell cycle?
Cyclins are a family of regulatory proteins that fluctuate in concentration throughout the cell cycle. They bind to and activate cyclin-dependent kinases (CDKs), regulating cell cycle progression by controlling key transition points between phases.
What are the different types of cyclins, and when are they active?
- Cyclin D: Active in the G1 phase and promotes progression through the G1 checkpoint
- Cyclin E: Active at the G1/S transition, helping the cell enter S phase.
- Cyclin A: Active during S phase and G2 phase, involved in DNA replication and preparation for mitosis.
- Cyclin B: Active during G2 and M phase, driving the cell into mitosis.
What are cyclin-dependent kinases (CDKs)?
CDKs are serine/threonine kinases that require binding to cyclins to become active. They phosphorylate target proteins to initiate and regulate key events in the cell cycle, such as DNA replication and mitosis.
What is the significance of CDK activation?
CDK activation is crucial for cell cycle progression. Without cyclin binding, CDKs are inactive, preventing transitions between cell cycle stages. Activation drives processes like DNA replication, mitosis, and cytokinesis.
How is CDK activity regulated?
CDK activity is regulated by cyclin binding, phosphorylation, and dephosphorylation. Additional regulation occurs through inhibitors like p21 and p27, which prevent CDK activation in response to DNA damage or stress.
What is the function of CDK inhibitors (CKIs)?
CKIs, such as p21, p27, and p57, bind to CDKs and prevent their activation, serving as key regulators that enforce checkpoints during the cell cycle, especially in response to DNA damage or other stress signals.
What is the Maturation Promoting Factor (MPF)?
MPF is a complex formed by Cyclin B and CDK1. It is critical for the G2/M checkpoint, where it drives the cell from G2 phase into mitosis by initiating events like chromosome condensation and nuclear envelope breakdown.
How does MPF promote the G2/M transition?
MPF activates key proteins that control mitosis, including the condensins for chromosome condensation, and the lamins for nuclear envelope disassembly. MPF also phosphorylates proteins involved in spindle assembly.
How is MPF regulated during the cell cycle?
MPF activity is tightly controlled. During interphase, Cyclin B accumulates and binds to CDK1, forming an inactive MPF complex.
CDK1 is then phosphorylated and dephosphorylated at specific sites (T161 for activation and T14/Y15 for
inactivation) to regulate MPF activity.
Cyclin B degradation at the end of mitosis leads to MPF inactivation.
What is the role of phosphorylation and dephosphorylation in regulating MPF activity?
Phosphorylation of CDK1 by Wee1 and dephosphorylation by Cdc25 regulate MPF activation.
Phosphorylation of CDK1 by Wee1 inhibits MPF activity, while Cdc25 dephosphorylates CDK1, activating MPF and driving the cell into mitosis.
How does the degradation of Cyclin B affect the cell cycle?
The degradation of Cyclin B, mediated by the Anaphase Promoting Complex/Cyclosome (APC/C), leads to the inactivation of MPF, which is required for exit from mitosis.
This process is crucial for ensuring that the cell completes mitosis before progressing into the next cell cycle.
What is the role of the APC/C in the regulation of MPF?
The Anaphase Promoting Complex/Cyclosome (APC/C) is an E3 ubiquitin ligase that targets Cyclin B for ubiquitination and degradation.
This degradation of Cyclin B inactivates CDK1 and leads to the termination of mitosis, ensuring proper exit from M phase.
