Identifying Genetic Mutations and mRNA Changes

These substitutions are further classified by the chemical nature of the bases involved:

• Transition mutation: A change from a purine to a purine (A $\leftrightarrow$ G) or a pyrimidine to a pyrimidine (C $\leftrightarrow$ T).
• Transversion mutation: A change from a purine to a pyrimidine or vice versa (e.g., A $\leftrightarrow$ C).

Nonsense Mutations

A nonsense mutation is particularly impactful. It occurs when a substitution converts an amino acid-coding codon into a stop codon (such as UAG, UAA, or UGA). This causes the ribosome to stop translation prematurely, resulting in a truncated, often non-functional protein.

Exam Tip: When looking at a sequence, if the protein suddenly stops much earlier than the wild-type, look for a nonsense mutation. If the protein is the same length but one "link" in the chain is different, look for a missense mutation.

Frameshift Mutations

Unlike substitutions, which only affect a single codon, frameshift mutations change the entire "reading frame" of the mRNA. This is caused by the insertion or deletion of nucleotides in a number that is not a multiple of three.

Because the genetic code is read in triplets (codons), adding or removing one or two bases shifts every subsequent codon downstream of the mutation. This typically results in a completely different amino acid sequence from the point of mutation onward, often eventually hitting a premature stop codon.

Interestingly, a "second-site" mutation can sometimes occur. If a second insertion or deletion happens downstream of the first, it can restore the original reading frame, returning the amino acid sequence to the wild-type pattern after the site of the errors.

Summary Table of Mutation Effects

Worked Example: Deducing the Mutation

Scenario:

• Wild-type sequence: Met - Ala - Gly - Trp - Ser
• Mutant sequence: Met - Ala - Gly - STOP

Step 1: Identify the change. The mutation occurs at the 4th amino acid. The wild-type has Tryptophan (Trp), but the mutant has a stop codon.

Step 2: Analyze the mRNA. The wild-type codon for Trp is UGG. If the mutant is a stop codon (e.g., UGA), we look at the change:

• Wild-type: UGG
• Mutant: UGA

Step 3: Classify. The change is a single nucleotide substitution (G $\rightarrow$ A) that created a stop codon. Therefore, this is a nonsense mutation.

Note on Chromosomal Mutations: While the mutations discussed above occur at the single-nucleotide level, larger-scale mutations can involve entire segments of chromosomes, such as translocations, inversions, or deletions. These are often visualized using techniques like FISH (Fluorescent In Situ Hybridization) or karyotyping.