7.9 The Genetic Code

There is a direct correspondence from gene sequence to protein sequence. The genetic code uses groups of three bases, called codons, to represent individual amino acids. Here it is using DNA bases; it's not important to memorize it, though that can be done.

First letter	Second letter
	T	C	A	G
T	TTT phenylalanine F TTC phenylalanine F TTA leucine L TTG leucine L	TCT serine S TCC serine S TCA serine S TCG serine S	TAT tyrosine Y TAC tyrosine Y TAA stop (Ochre) TAG stop (Amber)	TGT cysteine C TGC cysteine C TGA stop (Opal) TGG tryptophan W
C	CTT leucine L CTC leucine L CTA leucine L CTG leucine L	CCT proline P CCC proline P CCA proline P CCG proline P	CAT histidine H CAC histidine H CAA glutamine Q CAG glutamine Q	CGT arginine R CGC arginine R CGA arginine R CGG arginine R
A	ATT isoleucine I ATC isoleucine I ATA isoleucine I ATG methionine M	ACT threonine T ACC threonine T ACA threonine T ACG threonine T	AAT asparagine N AAC asparagine N AAA lysine K AAG lysine K	AGT serine S AGC serine S AGA arginine R AGG arginine R
G	GTT valine V GTC valine V GTA valine V GTG valine V	GCT alanine A GCC alanine A GCA alanine A GCG alanine A	GAT aspartic acid D GAC aspartic acid D GAA glutamic acid E GAG glutamic acid E	GGT glycine G GGC glycine G GGA glycine G GGG glycine G

(RNA codons are the same except replacing all instances of T with U.)

Coincidentally, all guanines (GGG) is a codon for glycine (G), and that's the only letter this is true for. But that is purely a quirk of the symbols we use and doesn't mean anything in terms of the molecules themselves.

The genes in DNA are demarcated by patterns of DNA sequence called a promoter and a terminator. In order to translate a gene into a protein, the cellular machinery must first make an RNA copy of the gene. The promoter is a region of DNA that signals the enzymes to "start here"; the terminator tells it "okay, all done." Once the section of DNA is copied, the new RNA will have the same sequence as the coding side of the DNA; it will have been copied from the opposite DNA strand, so it will be a complement of a complement and therefore identical to the original sequence, except of course with U instead of T.

Protein synthesis always begins with a start codon, which is AUG, which happens to be the codon for methionine (M). (In fact, it is the only codon for methionine.) The protein ends when the sequence reaches a stop codon. There are three stop codons: UAA, UAG, and UGA, code named ochre, amber, and opal, however some organisms have co-opted one or another of the stop codons to code for nonstandard amino acids.

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