A reading frame refers to one of three possible ways of reading a nucleotide sequence.

Let's say we have a stretch of 15 DNA base pairs:

       acttagccgggacta

• We can start translating, or reading, the DNA from the first letter, 'a,' which would be referred to as the first reading frame.
• Or we can start reading from the second letter, 'c,' which is the second reading frame.
• Or we can start reading from the third letter, 't,' which is the third reading frame.

The reading frame affects which protein is made. In the example below, the upper case letters represent amino acids that are coded by the three letters above and to the left of them.

reading frame:             123 
                           ||| 
                           acttacccgggacta 
first reading frame          T  Y  P  G  L 
second reading frame          L  T  R  D 
third reading frame            L  P  G  T 

The illustration above shows three reading frames. However, there are actually six reading frames: three on the positive strand, and three (which are read in the reverse direction) on the negative strand.