Concept 22 DNA words are three letters long.

1966 Cold Spring Harbor Symposium on protein synthesis. (L-R) Marshall Nirenberg, B. P. Doctor, C. T. Caskey.

Marshall Nirenberg at the White House explaining the genetic code to President Lyndon Johnson.

Marshall Nirenberg in his office at the NIH, 1960.

Marshall Nirenberg at the NIH, 1999. He is holding one of the original charts with 'code-cracking' data.

1966 Cold Spring Harbor Symposium on protein synthesis. (L-R) Har Gobind Khorana, Francis Crick, Marianne Grunberg-Manago.

1966 Cold Spring Harbor Symposium on protein synthesis. (L-R) John Cairns, Phil Leder and Robert Thach.

1966 Cold Spring Harbor Symposium on protein synthesis. Phil Leder in the midst of a discussion.

Har Khorana in his laboratory at the University of Wisconsin, mid-1960's.

Har Khorana in his office at the University of Wisconsin, mid-1960's.

Sydney Brenner coined the term "codon" — the trinucleotide unit that specifies one amino acid.

3 nucleotides/codon means 64 possible codons — enough for 20 amino acids and "extras" for stops and redundancy. Why not 4 nucleotides/codon (256 possibilities) for even more choice and variety?

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1966 Cold Spring Harbor Symposium on protein synthesis. (L-R) Marshall Nirenberg, B. P. Doctor, C. T. Caskey.