How do you go from four letters to 20 amino acids?
The Genetic Code
How is the information in a gene encoded? The answer is the genetic code. The genetic code consists of the sequence of nitrogen bases—A, C, G, U—in an mRNA chain. The four bases make up the “letters” of the genetic code. The letters are combined in groups of three to form code “words,” called codons. Each codon stands for (encodes) one amino acid, unless it codes for a start or stop signal.
There are 20 common amino acids in proteins. There are 64 possible codons, more than enough to code for the 20 amino acids. The genetic code is shown in the figure below.
Reading the Genetic Code
As shown in the figure above, the codon AUG codes for the amino acid methionine. This codon is also the start codon that begins translation. The start codon establishes the reading frame of mRNA. The reading frame is the way the letters are divided into codons. After the AUG start codon, the next three letters are read as the second codon. The next three letters after that are read as the third codon, and so on. This is illustrated in the figure below. The mRNA molecule is read, codon by codon, until a stop codon is reached. UAG, UGA, and UAA are all stop codons. They do not code for any amino acids. Stop codons are also known as termination codons.
Characteristics of the Genetic Code
The genetic code has a number of important characteristics.
- The genetic code is universal. All known living organisms use the same genetic code. This shows that all organisms share a common evolutionary history.
- The genetic code is unambiguous. Each codon codes for just one amino acid (or start or stop). What might happen if codons encoded more than one amino acid?
- The genetic code is redundant. Most amino acids are encoded by more than one codon. In the figure above, how many codons code for the amino acid threonine? What might be an advantage of having more than one codon for the same amino acid?
- The genetic code consists of the sequence of bases in DNA or RNA.
- Groups of three bases form codons, and each codon stands for one amino acid (or start or stop).
- The codons are read in sequence following the start codon until a stop codon is reached.
- The genetic code is universal, unambiguous, and redundant.