The genetic code is based on the sequence of nitrogen bases in DNA. The four bases make up the “letters” of the code. Groups of three bases each make up code “words” called codons. Each codon stands for one amino acid or for a start or stop signal.     There are 20 amino acids that make up proteins. With three bases per codon, there are 64 possible codons. This is more than enough to code for the 20 amino acids plus start and stop signals.     The codon AUG is the start signal. It also codes for the amino acid methionine. After a start signal, all the following codons are read in sequence... Show more

The genetic code is based on the sequence of nitrogen bases in DNA. The four bases make up the “letters” of the code. Groups of three bases each make up code “words” called codons. Each codon stands for one amino acid or for a start or stop signal.    
There are 20 amino acids that make up proteins. With three bases per codon, there are 64 possible codons. This is more than enough to code for the 20 amino acids plus start and stop signals.    
The codon AUG is the start signal. It also codes for the amino acid methionine. After a start signal, all the following codons are read in sequence until a stop codon is reached. The codons UAG, UGA, and UAA are all stop codons.    
They don't code for any amino acids.    
 

The genetic code has three important characteristics.    
- The genetic code is the same in all living things. This shows that all organisms are related by descent from a common ancestor.    
- Each codon codes for just one amino acid. This is necessary so the correct amino acid is always selected.    
- Most amino acids are encoded by more than one codon. This is helpful. It increases the chances that the correct amino acid will still be selected even if there is a mistake in the code.   

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