Once the structure of DNA was discovered, the next challenge was determining how the sequence of letters coded for the 20 amino acids. In theory, one or two letters can only code for 4 or 16 amino acids, respectively. A scheme using three letters, a triplet code, is the minimum necessary to encode for all the amino acids.
AP Biology (2012–13)
1.B.1.a, 3.A.1.a,b,c, 4.A.1.a,b
IB Biology (2009)
3.2.2, 3.2.5, 3.3.1-3.3.5, 3.4.1-3.4.3, 3.5.1-3.5.5, 7.2.1-7.2.3, 7.3.1-7.3.4, 7.4.1-7.4.4, 7.4.6, 7.4.7
NGSS (April 2013)
HS-LS1-1 (LS1.A), HS-LS4-1 (LS4.A)