How Universal is the Genetic Code?
The table of codons used by organisms to translate mRNA into proteins is shown on the bottom of the page. As was mentioned earlier in this lesson, the genetic code needed to be cracked one time because all organisms used the same codons to encode amino acids. As scientists began to sequence the coding regions of genes from different organisms, they discovered something called a codon preference. When you look at the codon table, you can see that the genetic code is redundant. This means that more than one codon can encode the same amino acid. This is because there are 61 codons that code for the placement of 20 different amino acids. A codon will only work in coding if a tRNA with a complementary anticodon is also found in the same cell and has the appropriate amino acid to deliver. Therefore there could be 61 different tRNAs, one to complement each codon. Each different tRNA needs to be encoded by a different gene. If that gene is not expressed in the cell, the tRNA will not be found and a codon that needs to be complemented by that tRNA will not be complemented. In this case, the codon will act like a stop codon. The ribosome will halt it's translation and the protein made will be a shorter version of the intended protein. Obviously organisms would not benefit from this situation so there is a tight complementation between what tRNAs genes are present and expressed in an organsism's cells and what codons are used to encode a specific mRNA. In this way the genetic code will have a dialect. The language is universal but certain words are used preferentially.
Scientists are not sure why codon preferences are a part of the gene expression process in organisms. It may provide another level for the organism to control the amounts and kinds of proteins made in its cells. Recent experiences in genetic engineering of plants and animals, however, has made codon preference an important consideration. For example, scientists have put genes from a soil bacteria into corn plant cells in order to give the corn plant the ability to make a protein that is toxic to European corn borer, a common pest to corn producers. They found that the gene would be transcribed but the mRNA would not be translated to make the desired protein. One reason was codon usage. Some of the codons the bacteria uses to encode amino acids are rarely used by corn. The corn plant either lacked the tRNA to complement the codon or make the tRNA at such low levels that there were not enough copies in the cell to accommodate translation of the Bt mRNA. Therefore, the genetic engineers needed to make synthetic coding regions that substituted codons preferred by corn for those preferred by bacteria. The end result was they were able to get higher levels of the Bt protein made once these changes were made in the gene. Codon preference thus makes the genetic engineering process more challenging.