mRNA is read in non-overlapping triplets; four bases in three positions give 64 codons.
highlighted = computed this step
Bases are read three at a time
The ribosome reads mRNA in non-overlapping groups of 3 bases. Each group is a codon, shown here bracketed over the strand it comes from.
codon=3 bases
There are 64 possible codons
With 4 possible bases in each of 3 positions, there are 64 distinct codons.
43=64
The reading frame fixes where codons begin
Unless told otherwise we start at the first base — frame 0. Starting one or two bases later would regroup every codon and change the reading; that shift is the subject of a later lesson.