A reading frame is a way to divide a sequence of nucleotides in DNA or RNA into consecutive, non-overlapping triplets, known as codons. The reading frame determines how the sequence is translated into amino acids during protein synthesis. Because the genetic code is read in sets of three nucleotides, a shift in the reading frame can lead to completely different translations of the same nucleotide sequence.
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A reading frame specifies how consecutive nucleotides are grouped into codons. Starting at position 1 of AUGGCUUAA gives AUG / GCU / UAA; starting at position 2 gives UGG / CUU with AA left over. The letters are unchanged but the groups differ. A possible grouping alone does not establish an actual biological translation initiation site.
For the twelve-nucleotide example, number positions from the end. The three possible forward reading frames start at positions 1, 2 and 3:Parentheses enclose incomplete trailing groups. The second and third rows do not assert initiation of biological translation in a biological cell. A chosen start fixes which positions belong together; the ribosome then advances through consecutive codons rather than searching each three-letter window independently.
