Features of the Genetic Code
- The sequence of DNA nucleotide bases found within a gene is determined by a triplet (three-letter) code
- Each sequence of three bases (i.e. each triplet of bases) in a gene codes for one amino acid
- These triplets code for different amino acids – there are 20 different amino acids that cells use to make up different proteins
- For example:
- CAG codes for the amino acid valine
- TTC codes for the amino acid lysine
- GAC codes for the amino acid leucine
- CCG codes for the amino acid glycine
- Some of these triplets of bases code for start (TAC – methionine) and stop signals
- These start and stop signals tell the cell where individual genes start and stop
- As a result, the cell reads the DNA correctly and produces the correct sequences of amino acids (and therefore the correct protein molecules) that it requires to function properly
- The genetic code is non-overlapping
- Each base is only read once in which codon it is part of
- There are four bases, so there are 64 different codons (triplets) possible (43 = 64), yet there are only 20 amino acids that commonly occur in biological proteins
- This is why the code is said to be degenerate: multiple codons can code for the same amino acids
- The degenerate nature of the genetic code can limit the effect of mutations
- The genetic code is also universal, meaning that almost every organism uses the same code (there are a few rare and minor exceptions)
- The same triplet codes code for the same amino acids in all living things (meaning that genetic information is transferable between species)
- The universal nature of the genetic code is why genetic engineering (the transfer of genes from one species to another) is possible