What is reading frame?

• A. The order of nucleotides in a genome.
• B. The process of synthesizing RNA from a DNA template.
• C. The way in which mRNA-translating machinery groups nucleotides into triplets called codons.
• D. The mechanism by which proteins are degraded.

Answer: (C)

Reading frame is how the ribosome interprets the mRNA sequence by grouping nucleotides into consecutive, nonoverlapping codons, each codon being three nucleotides long. The frame starts with the first nucleotide of the start codon (AUG) and then proceeds in steps of three, so every codon specifies a particular amino acid. If the reading frame shifts—for example, by insertions or deletions that aren’t in multiples of three—the entire set of codons changes, leading to a different amino acid sequence and often a nonfunctional protein. This concept is distinct from the overall nucleotide order of the genome, transcription to RNA, or protein degradation.