Recording-2026-04-22T16:58:40.060Z

S Phase (Synthesis Phase)
- Replication of DNA occurs during S phase.
- DNA must be opened sufficiently for enzymes to act on it, allowing access for replication factors.
Mitosis
  - The only phase where long chromosomes condense into short, stubby chromosomes.
  - Historically, only visible during mitosis through light microscopy, leading to early assumptions that mitosis was the only action in the cell cycle.
  - Mitosis is considered a fraction of the total time spent in the cell cycle and primarily focuses on dividing replicated chromosomes.
Interphase
- Includes three stages: G1 (Gap 1), S, and G2 (Gap 2).
- Represented as everything that occurs before mitosis (visualized as M phase).

Similarities Among Eukaryotes
- Cell cycle control is highly conserved across eukaryotic organisms; prokaryotes do not exhibit a cell cycle.
- Major discoveries of control mechanisms were initially made in yeast, then applied to more complex organisms.
Mechanism of Control
- Cell cycle control involves a dynamic system of accelerators and brakes.
- It represents a series of biochemical signals that coordinate cell movement through different phases.
Accelerators
  - Controlled largely by cyclin-dependent kinases (CDKs).
  - CDKs are activated through binding with regulatory proteins called cyclins and are responsible for moving the cell from one phase to the next.
  - Activation of CDKs is cyclic, meaning they are sequentially activated and inactivated as the cell progresses through the cell cycle.
Brakes
  - Regulate the cell cycle in response to environmental signals (e.g., nutrient availability, DNA integrity).
  - Key factors inducing brakes include DNA damage or replication issues, low nutrient levels, or unfavorable environmental conditions.
  - During cell cycle arrest, the cell halts its progression to ensure everything is adequate before moving forward.

Definition and Function
  - CDKs are enzymes that phosphorylate various substrates, pushing the cell cycle forward.
  - They require binding to cyclins for activation, hence the term cyclin-dependent kinases.
  - CDKs can act on multiple substrates resulting in significant cellular changes.
Cyclins
  - Cyclins are produced and degraded in a cyclic manner during the cell cycle.
  - Different cyclins play roles in activating specific CDKs at various points in the cell cycle.
  - Cyclin levels dictate the activation of their corresponding CDKs, reflecting their presence at different stages.

Activation Mechanism
  - Initial activation requires binding to a cyclin.
  - Additional phosphorylation may also be needed to achieve full activation.
  - Inhibitory proteins can bind to CDK-cyclin complexes, preventing activation until certain conditions are met.
**Control through Ph