The cell cycle describes the life cycle of a cell.
It includes processes such as mitosis and meiosis, which are types of cell division.
Focus is on the mechanisms controlling the stages of mitosis and meiosis, rather than going through basic definitions of these processes.
Introduction to Cell Division
Cell division begins the cell’s life cycle, with a cell either undergoing mitosis or meiosis.
Each newly formed cell begins its own life cycle after cell division.
- New cells are created, such as after mitosis, which results in two identical cells.
The life cycle of a cell can end in two ways:
1. Through another division (mitosis or meiosis).
2. Through injury or disease, leading to conventional cell death.
Stages of the Cell Cycle
The G1 Phase
G1 Phase: This stage is also referred to as the growth phase (formerly known as the gap phase).
- After division, the new cell is typically small and grows in size during G1.
- The cell performs its designated functions based on its type (e.g., heart cells contract, stomach cells release digestive chemicals).
G1 is usually the longest stage in mammals.
The S Phase
S Phase (Synthesis): In this stage, DNA is synthesized, meaning the cell duplicates its DNA in preparation for division.
- The cell must copy its DNA to ensure that each new cell has a complete set.
The G2 Phase
G2 Phase: This phase serves as another gap phase but focuses on preparing for mitosis.
- The cell checks to confirm it has all it needs to undergo division (mitosis/meiosis).
Transition to Cell Division
Following G2, if the requirements are met, the cell will proceed to undergo mitosis or meiosis, marking the end of its life cycle.
- The new cells (post-mitosis) enter their own life cycles, designated as B and C, respectively.
Mechanisms of Regulation in the Cell Cycle
The transitions between stages of the cell cycle must be carefully regulated to avoid skips (e.g., moving from G1 to G3 directly).
Cyclins and Cyclin-Dependent Kinases (CDKs)
Two key components in regulating cell cycle progression are cyclins and cyclin-dependent kinases (CDKs).
- Cyclin-Dependent Kinases (CDKs): Biochemical enzymes that regulate the cell cycle.
- Cyclins: Proteins that activate CDKs; their concentration varies cyclically throughout the cell cycle.
CDKs are dependent on cyclins for activation, hence the name cyclin-dependent kinases.
Activation of Cyclin-Dependent Kinases
Activation requires:
1. A cyclin to bind to the CDK (resulting in partial activation).
2. An activating phosphate to fully activate the CDK, provided by an additional enzyme called CAK (cyclin-dependent kinase activating kinase).
Activity Levels of Cyclins and CDKs
There are several critical CDKs and their specific functions based on their respective phases:
- G1/S CDK: Activated during the G1 and beginning of S phase.
- S Phase CDK: Active during the S phase.
- Mitotic CDK: Involved in mitosis activities.
Mechanisms to Achieve Coordinated Progression
The cell uses multiple strategies to temporarily inactivate active CDKs to allow for coordinated transitions between phases.
- Inhibitory phosphates added by B1 kinase can temporarily inactivate a cyclin/CDK complex.
- Specific phosphatases (like CDC25) can remove inhibitory phosphates to activate the CDK again.
Specific Functions of G1/S Cyclin-Dependent Kinase
One function adds phosphate groups to the protein CDH1, impacting downstream processes in mitosis.
Another function activates the S phase cyclin-dependent kinase by removing inhibitory proteins, which allows the cell to enter the S phase and begin DNA replication.
S Phase Events
During S phase, the S phase CDK becomes active, leading to DNA synthesis.
- Origin Recognition Complex (ORC) is established during previous cycles to prepare for DNA replication in the upcoming cycle.
- Loading of helicases occurs during low CDK activity, allowing them to bind without initiating unwinding too early.
Key proteins involved include:
- helicases (and associated proteins such as DDK) to facilitate DNA strand separation and replication.
Phosphorylation and Activation Process
During S phase:
- Inhibitory proteins are phosphorylated to remove their functional interference.
- Activators then join helicases, leading to DNA strand separation and replication processes.
- Phosphorylation ensures helicases and other proteins like DNA polymerase are prepared for appropriate actions.
Importance of Timing in Cell Cycle Regulation
The changes in regulated activity levels prevent premature or excessive DNA replication by ensuring helicases do not load during active high CDK states to avoid multiple copying of DNA strands.
This redundancy helps maintain genomic integrity during cell division.
Future Discussion on Mitosis Using Cyclins
Next session, the focus will turn to the events in mitosis and the roles of mitotic cyclin-dependent kinases, emphasizing their multiple functions in coordinating cell division processes.