Cell Cycle Regulation, DNA Damage, and Cancer: Foundations of Cellular Biology

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The Cell Cycle

Overview of Cell Cycle Phases

The cell cycle is a series of events that cells undergo to grow and divide. It consists of interphase (G1, S, G2) and the mitotic phase (M), each with distinct functions and regulatory mechanisms.

G1 Phase: Cell growth and normal metabolic processes.
S Phase: DNA replication occurs, preparing the cell for division.
G2 Phase: Further growth and preparation for mitosis.
M Phase: Mitosis and cytokinesis, resulting in two daughter cells.

Interphase encompasses G1, S, and G2 phases, where the cell prepares for division. The mitotic phase includes mitosis and cytokinesis.

Cell cycle diagram showing phases and their functions

Cell Cycle Checkpoints

Checkpoints are control mechanisms that ensure the cell is ready to proceed to the next phase. They prevent errors and maintain genomic integrity.

G1 Checkpoint: Checks cell size, nutrients, social signals, and DNA integrity.
G2 Checkpoint: Ensures DNA replication is complete and undamaged, and MPF is activated.
Metaphase Checkpoint: Verifies spindle attachment and chromosome segregation.

Cell cycle diagram with checkpoints indicated

Regulation of Cell Cycle Progression

Importance of Regulation

Cell division is tightly regulated to ensure proper growth, resource allocation, and prevention of mutations that could lead to cancer. Regulation involves checkpoints, signaling pathways, and protein complexes.

Ensures sufficient resources for new cells.
Prevents propagation of mutations by halting division if DNA is damaged.

Cell Signaling and Growth Factors

Growth factors are extracellular signals that promote cell division. They activate intracellular pathways leading to the production of cyclins, which are essential for cell cycle progression.

Cyclins: Regulatory proteins whose levels fluctuate during the cell cycle.
CDKs (Cyclin-dependent kinases): Enzymes that are activated by cyclins to drive cell cycle transitions.

Diagram of cell signaling and cyclin production

Cyclin-CDK Complexes

CDKs are always present in the cell but require cyclins to become active. Different cyclin-CDK complexes regulate different phases of the cell cycle.

Cyclin D/CDK4,6: G1 phase
Cyclin E/CDK2: G1 to S transition
Cyclin A/CDK2: S phase
Cyclin B/Cdc2: G2 to M transition

Cyclin-CDK complexes and their roles in cell cycle phases

DNA Damage and Cell Cycle Checkpoints

DNA Damage Response

Cells have mechanisms to detect and repair DNA damage. If damage is detected at checkpoints, cell division is halted, and repair pathways are activated. If repair fails, the cell may undergo senescence or apoptosis.

p53 protein: Key regulator that signals cell cycle arrest or cell death in response to DNA damage.
Nucleotide Excision Repair: Removes damaged DNA segments and replaces them with correct nucleotides.

Diagram of p53-mediated DNA damage checkpoint Nucleotide excision repair process

Mutagens and DNA Damage

Mutagens such as chemicals, UV radiation, and X-rays can cause DNA mutations. These mutations may disrupt normal cell cycle regulation and lead to cancer if not repaired.

UV light: Causes thymine dimers, which distort DNA structure.
Repair proteins: uvrA and recA are essential for DNA repair in bacteria.

UV-induced DNA damage and thymine dimer formation E. coli survival after UV exposure based on repair genes

Telomeres and Cell Cycle Regulation

Telomere Function and Replication

Telomeres are repetitive DNA sequences at chromosome ends that protect protein-coding regions from degradation during replication. Each cell division shortens telomeres, eventually leading to senescence.

Telomere sequence: TTAGGGn
Hayflick limit: Maximum number of divisions before a cell becomes senescent.
Telomerase: Enzyme that restores telomere length in certain cells.

Telomere structure and function

Problems with DNA Replication at Chromosome Ends

DNA polymerase cannot replicate the very ends of linear chromosomes due to the lack of a free 3'-OH group, resulting in telomere shortening.

~70-200 nucleotides are lost from each end per replication cycle.
Once telomeres are depleted, essential genes may be lost, halting cell division.

Problems with copying chromosome ends

Cancer: A Disease of the Cell Cycle

Characteristics of Cancer Cells

Cancer arises when cell cycle regulation fails, allowing cells to proliferate uncontrollably. Cancer cells often re-express telomerase, accumulate mutations, evade cell death, and induce angiogenesis.

Genome instability: Mutations accumulate, increasing cancer risk.
BRCA1 gene: Repairs DNA damage; mutations increase cancer susceptibility.
Multiple mutations: Required for a cell to become cancerous.

Cell Cycle Checkpoints and Cancer Prevention

Checkpoints prevent the propagation of damaged DNA. Loss of checkpoint function, such as p53 mutations, increases cancer risk.

p53: Tumor suppressor; loss leads to unchecked cell division.
Inherited mutations: Increase predisposition to cancer (e.g., Li-Fraumeni syndrome).

Cancer Treatments Targeting Cell Cycle

Some cancer therapies disrupt DNA synthesis or cell division to halt tumor growth.

Quinolones: Inhibit topoisomerase, preventing DNA replication.
Taxol: Destabilizes microtubules, blocking mitosis.

Summary Table: Cell Cycle Checkpoints

Checkpoint	Main Criteria	Key Proteins
G1	Cell size, nutrients, social signals, DNA integrity	Cyclin D/E, CDK, p53
G2	DNA replication complete, DNA undamaged, MPF activated	Cyclin A/B, CDK, MPF
Metaphase	Spindle attachment, chromosome segregation, MPF absent	Spindle proteins, MPF

Additional info: Academic context was added to clarify checkpoint mechanisms, DNA repair, telomere function, and cancer biology, ensuring completeness and self-contained study notes.