Genetic Code


Rules of the Genetic Code

Summary Rules of the Genetic Code

In addition to the principles we have already discussed, there are basic rules that govern the translation of the genetic code into a protein. There are three principle rules we will discuss:

  1. The sequence of bases in a codon must follow the direction of translation.
  2. The code is non-over-lapping.
  3. The code is read in a fixed reading frame.

Rule 1

The first rule is somewhat basic. It says that since mRNA is translated in the 5' to 3' direction, the codon sequences must occur in a similar orientation so that they will be properly translated. This simply means that the first base of a codon must be located at the 5'-most end of the codon. Codons must always be read from 5' to 3'.

Rule 2

The second rule means that any one nucleotide can only be a part of one codon. It cannot be a part of two different codons. Therefore, successive codons are composed of adjacent, not over-lapping, trinucleotides. For example, given the code AACT, AAC could be a codon with T starting a new codon or ACT could be a codon with the first A the last letter of a previous codon. But AAC and ACT cannot both be codons at the same time.

Rule 3

The final rule states that once you begin reading the code from a specific nucleotide, you continue reading it by threes until the end. The beginning of an amino acid sequence is specified by a start codon located somewhere in the mRNA sequence, this is usually an AUG, but can also be a GUG. The end of a sequence is specified by one of three stop codons: UAA, UAG, or UGA. A consequence of this rule is that the genetic code can be read in three different reading frames depending on which base one begins with. For example, the sequence: ACGACGACGACGACG can be read in the three following ways.

1.) ACG ACG ACG ACG ACG (each codon specifies the amino acid threonine)

2.) A CGA CGA CGA CGA CG (each codon specifies the amino acid arginine)

3.) AC GAC GAC GAC GAC G (each codon specifies the amino acid asparagine)