Protein Translation

If you were going to translate your notes from English to Russian, you would need a translation dictionary.

When you move from the language of nucleotides to the language of amino acids, you still need a "dictionary", something that tells you which nucleotides correspond to which amino acids.

More specifically, you need something to tell you which sequence of three nucleotides corresponds to which one amino acid. A sequence of three nucleotides is called a codon, and the order of codons on mRNA specifies the order of amino acids in a protein.

The dictionary for protein translation is called the Genetic Code.

Because there are four possible nucleotide bases, and codons are groups of three bases, there are 64 (4 x 4 x 4) possible codons. Because there are only 20 different amino acids, some of the amino acids are coded for by more than one codon. The Genetic Code is nothing more, really, than a list of the 64 possible codons and the amino acids they correspond to.

Here's a portion of the Genetic Code:

And so on, and so on, and so on.

So, to figure out the order of amino acids in a protein, all you have to do is look at the sequence of codons on mRNA. Suppose you had a piece of mRNA with the following sequence:

The codons are read in non-overlapping sequence, like this:

So, for this piece of mRNA, the amino acid sequence would be:

Now you can see how a nucleotide sequence on DNA can specify a nucleotide sequence on RNA, and how that same nucleotide sequence can specify an amino acid sequence in a protein.

There are two final things to point out: first, the codon AUG (methionine) is known as the "start" codon, because it's the first codon on all mRNAs and methionine is the first amino acid in all proteins. Second, three of the 64 possible codons do not specify an amino acid.

They specify "stop." In other words, "stop translating, the protein is finished." The three stop codons are UAA, UGA, and UAG.