Reading Frames & ORFs

Reading Frames

the Three-Frame Shift

The same mRNA can be grouped into three forward reading frames, and each frame translates differently.

highlighted = computed this step

Shifting the start regroups every codon

A reading frame is the column where codons begin. Starting at frame 0, frame 1, or frame 2 groups the same mRNA into different codons.

frames={0,1,2}\text{frames} = \{0,1,2\}

Three frames, three different proteins

Frame 0 reads MACK then stop; frame 1 reads WHAN; frame 2 reads GMQI. The bases are unchanged, but the triplet boundaries moved.

frame 0:MACK+stop,frame 1:WHAN,frame 2:GMQI\text{frame } 0: \text{MACK}+\text{stop},\quad \text{frame } 1: \text{WHAN},\quad \text{frame } 2: \text{GMQI}
Reading framesThe same mRNA yields different peptides when translation starts in a different column.mRNAAUGGCAUGCAAAUAAframe 0MACK*frame 1WHANframe 2GMQI

Only one frame is the intended message

The cell does not choose a frame by taste: it picks the frame from the start codon, which is the subject of the next lesson. Honesty note: a reverse-strand reading gives three more possible frames, but this chapter stays on this mRNA strand.

start codon⟶intended frame\text{start codon} \longrightarrow \text{intended frame}