Overview
This lecture explains the types of mutations, focusing on small-scale (point) mutations and their effects on gene expression and protein function.
Types of Mutations
- Mutations are changes in DNA and are mostly neutral, contributing to genetic diversity.
- Small-scale mutations include point mutations and frameshift mutations, while large-scale ones affect whole chromosomes.
- Point mutations occur at a single nucleotide during DNA replication.
Point Mutations
- A point mutation (substitution) replaces one nucleotide with another, potentially causing differences between daughter cells.
- If a mutation changes a DNA base, it alters the corresponding mRNA codon during transcription and translation.
- The genetic code is redundant; changes in the third base may still code for the same amino acid (silent mutation).
- Missense mutations change the amino acid, possibly altering protein function depending on chemical properties.
- Nonsense mutations create a stop codon, which can prematurely end protein synthesis and often result in nonfunctional proteins.
Frameshift Mutations
- Frameshift mutations are caused by insertions or deletions of nucleotides, shifting the mRNA reading frame.
- These mutations change all downstream codons, often leading to extensive changes in the amino acid sequence or early stop codons.
- Frameshifts generally have a more drastic effect than point mutations.
Mutation Location and Effects
- Mutations in introns (noncoding regions) usually have no effect because introns are removed during mRNA processing.
- Only mutations in exons (coding regions) can affect protein structure and an organism’s phenotype.
- Most mutations occur in noncoding DNA and are neutral.
- Mutations must occur in gametes (sperm or egg) to be inherited by offspring.
Key Terms & Definitions
- Mutation — a change in the DNA sequence.
- Point Mutation — a single nucleotide change or substitution in DNA.
- Silent Mutation — a base change that does not alter the amino acid sequence.
- Missense Mutation — a base change that results in a different amino acid.
- Nonsense Mutation — a base change that creates a premature stop codon.
- Frameshift Mutation — insertion or deletion of bases altering the reading frame.
- Exon — coding region of a gene that is expressed.
- Intron — noncoding region of a gene that is removed during mRNA processing.
Action Items / Next Steps
- Review genetic code tables and practice identifying silent, missense, and nonsense mutations.
- Complete assigned readings on DNA replication and mutation effects.