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Cells do not divide arbitrarily; division occurs at specific times and in specific contexts (applicable to both mitosis and meiosis).
The crucial question: How do cells know when to divide?

Cell division is typically initiated by an external signal.
- A signaling molecule binds to a cell surface receptor.
- This action triggers an intracellular signaling pathway, conveying the message that it is time to divide.
Despite the presence of a signal, cells do not immediately initiate division due to internal control mechanisms, known as checkpoints.

Checkpoints assess whether the cell is ready to proceed to the next phase of cell division.
- They determine:
- If DNA replication has been completed.
- If the cell has grown sufficiently large to produce two viable daughter cells.
Only when these requirements are satisfied can the cell progress into division.

Cell division is tightly regulated.
Loss of control can lead to uncontrolled cell proliferation or diseases such as cancer.

The regulation of the cell cycle is mediated by a set of proteins known as cyclins.
- Cyclins appear and disappear in a cyclical fashion.
Cyclins function:
- Bind and activate another set of proteins known as cyclin-dependent kinases (CDKs).
- CDKs are enzymes always present but become active only when bound to cyclins.
Kinases: A large family of enzymes involved in phosphorylating other molecules and catalyzing reactions, leading to phosphorylation.
- Phosphorylation: The addition of a phosphate group onto a molecule which can alter the function of proteins.

The process of cyclin binding to CDKs:
- Cyclin appears and binds to an inactive CDK.
- Binding activates the CDK.
- Activated CDK phosphorylates target proteins, enabling them to become active.
- Phosphorylated target proteins promote cell division and progression through the cell cycle stages.

Multiple cyclin-CDK complexes regulate different stages of the cell cycle.
Key regulatory time points:
1. Transition from G1 to S phase.
2. Throughout the S phase.
3. Transition from G2 to M phase.

Active during the latter part of G1 and during the transition to S phase.
- Promotes:
- Expression of histone proteins for packaging newly synthesized DNA.
- Increase expression and activation of enzymes required for DNA synthesis (e.g., DNA polymerase).

Active during the S phase and the transition to G2.
- Functions include:
- Initiating synthesis of DNA.
- Inhibiting further replication of already replicated DNA.

Active during the latter part of G2 and the transition into M phase.
- Functions involve preparing the cell for mitotic division.
- Breakdown of nuclear envelope during prophase.
- Formation of mitotic spindle necessary for mitosis.

Cells have built-in checkpoints to ensure readiness before progressing to the next stage.
Major checkpoints:
1. DNA Damage Checkpoint: Before G1 to S transition; it checks for DNA damage.
2. DNA Replication Checkpoint: At the end of G2, determines if all DNA has been replicated.
3. Spindle Assembly Checkpoint: Checks if all chromosomes are attached to the spindle before anaphase.

The DNA damage checkpoint is regulated by a small protein called p53.
DNA damage typically manifests as a double-stranded break.
When DNA damage is detected:
- A specific protein kinase activates and phosphorylates p53.
- Phosphorylated p53 accumulates in the nucleus, preventing its export.
- p53 activates transcription of a gene coding for an inhibitor of CDK-cyclin complexes, halting the cell cycle until damage is repaired.

Cancer arises when the regulation of the cell cycle fails.
Through the work of Peyton Rous, the connection between viruses and cancer was established (e.g., Rous Sarcoma Virus).
- This virus contains an oncogene that promotes uncontrolled cell division by encoding an overactive protein kinase.

Oncogenes: Genes that, when mutated, promote uncontrolled cell division.
- Rous Sarcoma Virus and its oncogene cause cancer when introduced to healthy cells.
Proto-Oncogenes: Normal genes that are crucial for cell division but can become cancerous if mutated.
- Encode proteins involved in cell division signaling cascades (e.g., growth factors, cell surface receptors, G proteins, protein kinases).

Encode proteins that inhibit cell division.
- Example: p53 acts as a tumor suppressor by stopping the cell cycle in case of DNA damage.

The cell division is regulated by a balance of proto-oncogenes (promoting division) and tumor suppressor genes (inhibiting division).
A loss of regulatory mechanisms leads to multiple failures that progress from normal cells to cancer.

Cancer typically arises from the accumulation of mutations over time.
- Involves both:
- Over-activation of an oncogene.
- Loss of tumor suppressor activity.
Distinction between benign (localized, slow-growing) and malignant (metastatic) cancers.
Progression to cancer requires multiple mutations in cell cycle regulators.
- The accumulation of these mutations leads to stepwise progression from benign to malignant forms of cancer.

Overview of the discussed topics:
- Phases of the cell cycle: G1, S, G2, and M phase.
- Events of mitosis and meiosis, gametogenesis, and the regulation of the cell cycle.
- Mechanisms of cancer development and the importance of regulatory mechanisms in maintaining controlled cell division.

These concepts should be understood for exams and tests.