Biological Data Science: Saving Lives with Software - Hadoop: The Definitive Guide - page 794

The Genetic Code: Turning DNA Letters into Proteins

Without proteins, there is no life. DNA serves as a recipe for creating proteins. A protein is
a chain of amino acids that folds into a specific 3D shape [ 153 ] to serve a particular structure
or function. As there are a total of 20 amino acids [ 154 ] and only four letters in the DNA al-
phabet (A, C, T, G), nature groups these letters in words, called codons . Each codon is
three bases long (since two bases would only support 4 2 =16 amino acids).
In 1968, Har Gobind Khorana, Robert W. Holley, and Marshall Nirenberg received the No-
bel Prize in Physiology or Medicine for successfully mapping amino acids associated with
each of the 64 codons. Each codon encodes a single amino acid, or designates the start and
stop positions (see Table 23-1 ). Since there are 64 possible codons and only 20 amino
acids, multiple codons correspond to some of the amino acids.

Table 23-1. Codon table

Amino acid Codon(s) Amino acid Codon(s)

Alanine GC{U,C,A,G} Leucine UU{A,G} or CU{U,C,A,G}

Arginine CG{U,C,A,G} or AG{A,G} Lysine AA{A,G}

Asparagine AA{U,C} Methionine AUG

Aspartic acid GA{U,C} Phenylalanine UU{U,C}

Cysteine UG{U,C} Proline CC{U,C,A,G}

Glutamic acid GA{A,G} Threonine AC{U,C,A,G}

Glutamine CA{A,G} Serine UC{U,C,A,G} or AG{U,C}

Glycine GG{U,C,A,G} Tryptophan UGG

Histidine CA{U,C} Tyrosine UA{U,C}

Isoleucine AU{U,C,A} Valine GU{U,C,A,G}

START! AUG STOP! UAA or UGA or UAG

Because every organism on Earth evolved from the same common ancestor, every organ-
ism on Earth uses the same genetic code, with few variations . Whether the organism is a
tree, worm, fungus, or cheetah, the codon UGG encodes tryptophan. Mother Nature has
been the ultimate practitioner of code reuse over the last few billion years.

DNA is not directly used to synthesize amino acids. Instead, a process called transcription
copies the DNA sequence that codes for a protein into messenger RNA (mRNA). These