ch19

Transition mutation

Purine to purine substitution or pyrimidine to pyrimidine substitution such as G-C changing to A-T

Transversion mutation

Purine replaced with pyrimidine or pyrimidine replaced with purine

How to recognize a transition

G↔A or C↔T changes because the base type stays the same

Frameshift mutation

Insertion or deletion shifts the reading frame changing all downstream codons and amino acids

Why insertions often create completely different proteins

The reading frame changes so every codon after the mutation is altered

Degeneracy of the genetic code

Multiple codons can code for the same amino acid

Why a codon change may not alter protein sequence

Because of degeneracy of the genetic code resulting in a silent mutation

Promoter mutation effect

Usually reduces or alters transcription because RNA polymerase binding is affected

Promoter region

DNA sequence where RNA polymerase binds to initiate transcription

Splice-site mutation effect

Incorrect mRNA processing due to improper intron removal or exon joining

Why splice mutations are dangerous

Incorrect mRNA can produce abnormal or nonfunctional proteins

Somatic mutation during early embryonic development

A large portion of the organism may carry the mutation because many descendant cells arise from the mutated cell

Somatic mutation later in development

Only a small group of cells is affected

Why early embryonic mutations spread widely

The mutation is copied into many daughter cells during development

8-oxoG consequence

Oxidized guanine often mispairs with adenine during replication

Why oxidative damage causes mutations

Altered bases pair incorrectly leading to base substitutions

Thymine dimers

Covalent links between adjacent thymines caused by UV radiation

Trinucleotide repeat expansion in coding region

Often creates repeated amino acids causing protein aggregation and dysfunction

Why coding-region TNREs are harmful

Repeated amino acids can cause misfolding and toxic protein aggregates

Trinucleotide repeat expansion in 5′ UTR

Usually affects gene regulation or translation efficiency rather than amino acid sequence

5′ UTR function

Regulates translation and gene expression without coding for protein sequence

UvrA protein role

Part of nucleotide excision repair that recognizes UV-induced DNA damage

Effect of losing UvrA

Accumulation of unrepaired DNA lesions especially thymine dimers after UV exposure

Translesion DNA polymerase function

Bypasses damaged DNA so replication can continue

Trade-off of translesion synthesis

Increased mutation rate because translesion polymerases are less accurate