Cyclin and cyclin dependent kinases (cdk) | Cell cycle regulation lecture 3

Cell Cycle Control and Checkpoints

Overview of Cell Cycle Checkpoints

• The cell cycle includes checkpoints that determine whether a cell will divide based on internal conditions, such as size and DNA replication status.

• Proper cell division requires multiple proteins to work together, particularly cyclins and cyclin-dependent kinases (CDKs).

Role of Cyclins and CDKs

• Cyclins are proteins produced in small amounts that activate CDKs, which are enzymes responsible for phosphorylating other proteins to initiate cellular functions.

• The activity of CDKs is dependent on the presence of specific cyclins; without them, CDKs cannot function properly.

Stages of the Cell Cycle

• The cell cycle consists of interphase (G1, S, G2 phases) and mitotic phase (M phase), with M phase further divided into prophase, metaphase, anaphase, and telophase.

• During G1 phase, cells check cytoplasmic content and readiness for DNA replication. In S phase, DNA is replicated; in G2 phase, cells verify proper replication before entering M phase.

Checkpoints in the Cell Cycle

• There are critical checkpoints between G1/S phases and G2/M phases to ensure proper conditions for progression through the cycle.

• At the M phase checkpoint, cells assess if microtubules are correctly attached to chromatids and if chromosomes are aligned properly.

Cyclin-CDK Combinations

• Specific combinations of cyclins and CDKs drive progression through each stage:

• G1-S transition involves G1 cyclin,

• S-G2 transition involves S-phase cyclin,

• G2-M transition involves G2-M cyclin.

Understanding Cyclin-CDK Complexes and Their Role in Cell Cycle Regulation

Activation of Cyclin-CDK Complex

• The cyclin-CDK complex activates other proteins through phosphorylation, initiating cellular progression to the next level.

• Phosphorylation is crucial for various processes, including DNA replication and maintenance of cellular structures like actin filaments.

Specificity of Cyclins for Cell Cycle Phases

• Each cyclin is specific to a particular phase of the cell cycle; for instance, G1 cyclins do not function during G2 or S phases.

• If cyclins remain active beyond their designated phase, it could lead to uncontrolled cell division due to constant activation of CDKs.

Sequential Activation and Degradation of Cyclins

• Cells produce specific types of cyclins that only activate CDKs at appropriate times, ensuring orderly progression through the cell cycle.

• As cells transition from one phase to another (e.g., from G1 to S), previous cyclins are degraded while new ones are synthesized.

Steady State of CDKs vs. Fluctuating Levels of Cyclins

• CDKs maintain a steady concentration throughout the cell cycle, while levels of different cyclins fluctuate according to their respective phases.

• This fluctuation ensures that only the necessary cyclins are present at any given time, preventing premature activation of CDKs.

Mechanism Behind Cyclin-CDK Activation

• The activation process involves auto-phosphorylation and dephosphorylation events which regulate whether the complex is active or inactive.

Understanding Cyclin-Dependent Kinases (CDKs) and Their Activation

The Role of Cyclins in CDK Activation

• Cyclin-dependent kinases (CDKs) are present throughout the cell cycle, but they require cyclins to become active. In the M phase, cyclins bind to CDKs, rendering them inactive until further modifications occur.

Phosphorylation Mechanisms

• After cyclin binding, two different kinase molecules phosphorylate the CDK. This process involves both an inhibitory kinase (v1) and an activating kinase (CAC), which modify specific amino acid sequences on the CDK.

Dual Phosphorylation Dynamics

• The phosphorylation by v1 inhibits CDK activity while CAC's phosphorylation activates it. Despite being bound to cyclin, dual phosphorylation is necessary for full activation of the CDK complex.

Activation Process of MPF

• For the CDK-cyclin complex to become fully active as maturation-promoting factor (MPF), a phosphatase enzyme called CDC 25 removes the inhibitory phosphate from v1. This step is crucial for initiating processes like chromatid separation during cell division.

Importance of Checkpoints in Cell Cycle Regulation

• The necessity for dual phosphorylation ensures that activation only occurs when conditions are favorable. This mechanism acts as a regulatory checkpoint, preventing premature activation of MPF before confirming proper chromosome alignment and microtubule attachment.

Preparation for Rapid Cell Division

• High concentrations of cyclin-CDK complexes are produced in advance during metaphase preparation. This strategy allows rapid response once all conditions are verified as suitable for cell division.

Monitoring MPF Concentration During Cell Cycle

Understanding the Cyclin-CDK Complex Dynamics

The Role of Dual Phosphorylation in Cyclin-CDK Activation

• The cyclin-cyclin-dependent kinase (CDK) complex exhibits a sharp peak in concentration, indicating rapid activation upon receiving a positive signal.

• This rapid increase in concentration is attributed to a dual phosphorylation event, which plays a crucial role in the activation process.

• The interplay between cyclins and CDKs is essential for cell cycle regulation, particularly during critical phases such as mitosis.

• Understanding this mechanism provides insights into how cells respond to signals that trigger division and growth.