6.2.8—Effects on polypeptide produced

The effect of a gene mutation on a polypeptide must be traced through gene expression. A DNA change may alter the mRNA message, the amino-acid sequence or the amount of product, but it may also have little or no effect.

1. DNA information: Start with the changed base sequence and ask whether it lies in the part of the gene that is read and expressed.
2. RNA message: Transcription copies the relevant information into RNA; processing can produce the mature mRNA message used for expression.
3. Codon reading: A ribosome reads the mRNA codons while tRNA anticodons bring amino acids. A changed codon or a shifted reading frame can change the amino-acid instructions, but the degenerate code means some base changes do not change the amino acid.
4. Polypeptide outcome: The resulting chain may be unchanged, may contain a limited amino-acid change, or may be substantially altered/shortened when many downstream codons are affected.
5. Function boundary: A changed amino-acid sequence can alter folding, shape or function, but a changed DNA base does not guarantee a changed polypeptide or phenotype. The final claim depends on the mutation’s position, reading context and the protein’s role.

The complete reasoning path is DNA change → RNA message/processing → codon and tRNA matching at the ribosome → amino-acid sequence → polypeptide shape and function. This chain explains both possibilities: a mutation can be buffered by the code or location, or it can propagate through the reading frame and produce a markedly different polypeptide.

Do not jump directly from “mutation” to “disease” or “non-functional protein”. Explain each link that is supported: DNA sequence, mRNA/codon, amino-acid chain, then shape/function. Cards 4581 and 4582 define and compare mutation edits; this card is the high-level completion and consequence check, not a repeat of their classifications.