DNA Damage and Death

Detection of Damage

  • Different types of damage can occur and various mechanisms exist for their repair.

  • Major points for discussion:

    • Types of damage

    • Agents or stimuli causing damage

    • Mechanisms of repair

  • The transition into the topic of programmed cell death (apoptosis) will follow.

Overview of Cell Cycle and DNA Damage

  • Focus on connecting the cell cycle to DNA damage which is critical for understanding repair processes and cell regulation.

  • The cell cycle consists of:

    • Interphase: Divided into G1, Synthesis (S), G2 phases.

    • G1 Phase: Cell growth and preparation for DNA synthesis.

    • S Phase: DNA is replicated.

    • G2 Phase: Preparation for mitosis.

    • Mitotic Phase: Contains mitosis (cell division) and cytokinesis (cytoplasmic division).

  • The examination will cover these cell cycle phases.

  • There are checkpoint systems in the cell cycle that evaluate:

    • Growth status

    • DNA damage

    • Nutrient availability

    • Functional status of the cell; determining whether to enter G0 phase or continue in the cycle.

Regulation of the Cell Cycle

  • Cyclins and Cyclin-Dependent Kinases (CDKs) are pivotal regulators:

    • CDKs: Maintain fairly stable concentrations throughout the cycle.

    • Cyclins: Concentration varies throughout the cycle, modulating CDKs activity.

  • Mechanism of action:

    • Cyclins bind to CDKs, activating them.

    • CDKs phosphorylate target proteins, activating them for specific cycle events.

    • Post-activation, cyclins are degraded, leading to a decrease in their concentration.

  • Cyclins are named appropriately based on the phases they influence (D, E, A, B).

Cancer and Cell Cycle Misregulation

  • Misregulation of cyclins and CDKs can lead to cancer.

  • Analogy of a car:

    • Accelerating too fast (oncogenes): Mutated proto-oncogenes can drive uncontrolled cell growth.

    • Incorrect braking (tumor suppressor genes): Mutations prevent normal regulation, causing unchecked progression through the cycle.

  • Proto-oncogenes vs. Oncogenes:

    • Proto-oncogenes regulate cell growth, differentiation, and death.

    • Mutations can convert them to oncogenes, promoting cancer.

    • Common proto-oncogenes are cyclins D, E, and A. Cyclin B functions differently; involved in mitosis, not interphase regulation.

Tumor Suppressor Genes

  • Control the checkpoints of the cell cycle, providing time for DNA repair.

  • Example: p53 is a key tumor suppressor that activates gene p21, inhibiting cyclins and CDKs, thus slowing the cycle for damage inspection and repair.

  • Analogy of the system:

    • Tumor suppressors are the brakes; proto-oncogenes are the accelerators. Breaking either can result in cancer.

  • DNA damage can affect cyclins (gas) or tumor suppressors (brakes) leading to cancer if unchecked.

DNA Damage and Repair Mechanisms

  • DNA damage needs to be addressed to prevent cell death and maintain integrity.

  • Mechanisms of repair discussed:

    1. Base Excision Repair (BER)

    • Fixes small issues: single damaged or missing bases (AP sites).

    • Recognized by DNA glycosylase; AP endonuclease removes the damage, followed by repair by DNA polymerases and ligase.

    1. Nucleotide Excision Repair (NER)

    • Addresses larger structural distortions in DNA.

    • Recognizes complete loops or bends in DNA, repairing them with larger sections cut out and replaced.

    • Common agents causing damage include intercalating agents which disturb spatial orientation of bases.

    1. Mismatch Repair (MMR)

    • Targets mismatches from replication errors; usually initiated by proteins MSH and MLH.

    • Important in conditions like Lynch Syndrome, which is linked to various cancers.

Types of Damage and Agents

  • Alkylating Agents: Change the structure of DNA bases, leading to mispairing.

  • Deaminating Agents: Modify bases resulting in incorrect pairings (GC to AT or vice versa).

  • Oxidative Damage: Reactive oxygen species modify nucleobases (e.g., formation of oxoG) affecting base pairing after replication.

Advanced Repair Processes

  • Single Strand Break Repair: Recognized by PARP proteins; generally simple repair including polymerase and ligase.

  • Double Strand Break Repair: Two approaches:

    1. Non-Homologous End Joining (NHEJ): Quick, error-prone repair without templates, leading often to deletions.

    2. Homologous Recombination: More precise, using sister chromatids as templates, typically active during S and G2 phases.

    • Important in cancer research, especially understanding BRCA1/BRCA2 roles in breast cancer.

Cell Death Mechanisms

  • Types of cell death:

    1. Apoptosis (Programmed Cell Death)

    • Controlled process where cells shrink and fragment without causing damage to surrounding tissue.

    • Initiated through mitochondrial pathways, involves caspases and the apoptosome complex leading to cellular dismantling.

    • Resilience against excessive DNA damage and cellular stress.

    1. Necrosis (Unprogrammed Cell Death)

    • Resulting from acute injury, leading to cell rupture and inflammation, causing damage to adjacent cells.

  • Importance of apoptosis:

    • Critical for development, tissue homeostasis, and responding to stress in multicellular organisms.

    • Example: Webbing between fingers in fetal development removed by apoptosis; excess sperm eliminated via apoptosis.

Conclusion

  • Understanding the links between cell cycle, DNA damage, and repair mechanisms is fundamental to grasping how disruptions can lead to diseases such as cancer.

  • Grasping these concepts, along with the distinctions between programmed and unprogrammed cell death, underlines the crucial role of cellular regulation in organismal health and development.