Overview
This lecture covers the mechanisms of DNA damage, including the causes and outcomes of point and frameshift mutations, the effects of chemical and radiation-induced damage, and introduces the Ames test for detecting mutagens.
DNA Damage and Point Mutations
- DNA damage commonly results in point mutations (single base changes) or frameshift mutations (insertions/deletions).
- DNA polymerase errors or chemical changes can lead to incorrect base pairing during DNA replication.
- Normal base pairing involves specific hydrogen bonds, but tautomeric shifts or resonance can alter bonding and mispair bases.
- Tautomeric shifts (e.g., keto to enol forms) cause bases like thymine and cytosine to pair abnormally, leading to point mutations.
- Mutations such as transitions (purine-to-purine or pyrimidine-to-pyrimidine substitutions) occur due to these base mispairings.
Chemical Damage to DNA
- Deamination removes amino groups (NH2) from bases, often converting cytosine to uracil, which mispairs during replication.
- Oxidative damage results from reactive oxygen species (free radicals), altering bases and causing mispairing.
- Alkylating agents add alkyl groups (e.g., CH3 or C2H5) to bases, changing their pairing properties and causing transitions.
- Base analogs resemble normal bases and can be incorporated during replication, leading to increased tautomeric shifts and mispairing.
Other DNA Damage Mechanisms
- Intercalating agents (e.g., dyes, aflatoxin) insert between DNA bases, distorting the helix and often causing frameshift mutations.
- Transposons (“jumping genes”) can insert into genes and disrupt their function, sometimes resulting in null mutations.
Radiation-Induced DNA Damage
- Nonionizing radiation (UV light) induces pyrimidine dimers (especially thymine dimers), blocking replication and transcription.
- Ionizing radiation (e.g., gamma rays, X-rays) causes single- and double-strand breaks in the DNA backbone.
- Radiation exposure sources include natural radioisotopes, radon gas, medical imaging, and nuclear accidents.
Ames Test for Mutagen Detection
- The Ames test uses his- (histidine auxotroph) Salmonella bacteria to detect chemical mutagens that cause reversion mutations.
- Bacteria are exposed to chemicals (sometimes pre-treated with liver extract to simulate metabolism) and plated on minimal media.
- Significant bacterial growth indicates the chemical induced mutations that revert his- to his+.
Key Terms & Definitions
- Point mutation — Change of a single base in DNA.
- Frameshift mutation — Addition or deletion of bases that disrupts the reading frame.
- Tautomeric shift — Chemical change in a DNA base, altering its hydrogen bonding.
- Transition mutation — Substitution of a purine with another purine or pyrimidine with another pyrimidine.
- Deamination — Removal of an amino group from a DNA base.
- Oxidative damage — DNA changes caused by reactive oxygen species.
- Alkylating agent — Chemical that adds alkyl groups to DNA bases.
- Base analog — Compound similar to a DNA base, incorporated into DNA and prone to mispairing.
- Intercalating agent — Chemical inserting between DNA bases, causing structural distortion.
- Transposon — Mobile DNA sequence that can move to new genome locations.
- Ames test — Assay using bacteria to test chemicals for mutagenicity.
- Auxotroph — Organism unable to synthesize a specific nutrient, used in genetic testing.
Action Items / Next Steps
- Review chemical structures and effects of base modifications (tautomeric shifts, deamination, alkylation).
- Prepare for next lecture on DNA repair mechanisms, including base excision repair.