Preparatory Mindset

Mutations are changes in DNA sequence. Not all are bad — polymorphisms are common variants. The key is distinguishing benign polymorphisms from disease-causing mutations and understanding how different mutation types affect the protein product.

Core Concepts

• Point mutation: Single base change. Transition: Purine↔purine (A↔G) or pyrimidine↔pyrimidine (C↔T). Transversion: Purine↔pyrimidine (A↔C, etc.)


• Transition bias: Among random substitutions, transitions should outnumber transversions 2:1


• Silent mutation: Base change that does NOT alter amino acid (codon redundancy)


• Missense mutation: Base change that ALTERS amino acid (e.g., sickle cell: Glu→Val in β-globin)


• Nonsense mutation: Creates premature STOP codon → truncated (usually nonfunctional) protein


• Frameshift mutation: Insertion/deletion not a multiple of 3 → shifts reading frame → completely altered protein


• Mutation rate: Humans introduce ~0.6 new base pair mutations per billion bp per cell division


• Polymorphism: Variant present at ≥1% frequency in population — generally not disease-causing alone


• DNA repair: Multiple systems (base excision, nucleotide excision, mismatch repair) correct most damage

High-Yield Points

• Transition > Transversion (purine↔purine more common)


• Sickle cell = missense (Glu→Val); Frameshift usually = nonfunctional protein


• Polymorphism = >1% frequency, usually benign


• Mutation rate: ~0.6 new mutations per billion bp per cell division

Topic Summary

Mutations range from silent (no effect) to frameshift (complete loss of function). Transitions are more common than transversions. Polymorphisms are common variants; disease mutations are rare.