Which statement best describes a reading frame and why it must be maintained during translation?

• A. A reading frame is the start site where transcription begins
• B. A reading frame is the number of ribosomes translating the mRNA at once
• C. A reading frame is the grouping of nucleotides into codons; shifting frames changes the downstream amino acid sequence
• D. A reading frame is the process by which tRNA is loaded with amino acids

Answer: (D)

The main idea here is that a reading frame is how the nucleotide sequence is divided into codons, groups of three nucleotides, each specifying one amino acid. During translation, the ribosome starts at a start codon and reads the mRNA in steps of three nucleotides, with each codon guiding the addition of the corresponding amino acid via tRNAs. If the frame shifts—due to an insertion or deletion not in multiples of three—the downstream codons change, producing a completely different sequence of amino acids and often a nonfunctional protein or early termination. That’s why maintaining the reading frame is essential for producing the correct protein. The option that describes tRNA loading is about aminoacyl-tRNA synthetase, not about how the reading frame operates, so it doesn’t describe the concept. The other ideas mix up transcription start sites or ribosome loading with frame maintenance. So the best description is that the reading frame is the grouping of nucleotides into codons, and shifting frames changes the downstream amino acid sequence.