reading frame

Meaning: A reading frame is the grouping of a nucleotide sequence into codons, read three bases at a time with no overlap and no gaps. One strand can be read in three frames (starting at base 1, 2, or 3). The ribosome sets the frame at the start codon AUG, which codes for methionine, and keeps reading in that frame until it reaches a stop codon (UAA, UAG, or UGA). A stretch that begins with a start codon and has no in-frame stop codon is an open reading frame.

On the MCAT: A passage gives a normal sequence and a mutant one, and asks about the protein. Insertion or deletion of 1 or 2 nucleotides (any number not divisible by 3) causes a frameshift mutation. Every codon after the change is read differently, so the amino acid sequence downstream is usually scrambled, and a premature stop codon often appears, giving a truncated, usually nonfunctional protein. Insertion or deletion of exactly 3 nucleotides keeps the downstream frame and adds or removes one amino acid. Expect comparisons with silent, missense, and nonsense point mutations.

Common confusion: Students treat a frameshift like a point mutation that changes one codon. It alters all codons downstream of the change. A base substitution does not shift the frame, because the number of bases stays the same.