Stop Codon

A codon is a very specific type of genetic code. Codons carry a certain set of rules with the help of which information is processed and encoded to form genetic material. This material is in DNA or mRNA sequences, from where it is translated into protein molecules. Protein molecules are made up of amino acids, which are laid down in a specific sequence. Whenever there is a change in the codon structure, then the information that is passed on is faulty, leading to formation of faulty protein molecules. Hence, the structure of each codon is very specific so as to help it perform a specific function. Given below are details regarding the stop codon, also known as nonsense codon.

What is a Stop Codon?
Before we go on to knowing about what are stop or nonsense codons, we need to understand what are codons. Codons are nothing but genetic code molecules that are made up of three bases. In DNA molecules, there can be different combinations of three out of four bases. These four bases are adenine, cytosine, guanine and thymine. A codon consists of three bases and codes for a single specific amino acid, which is encoded in the tRNA of the organism.

A stop codon or nonsense codon is a nucleotide triplet within messenger RNA which is responsible for the termination of translation process. Most codons in messenger RNA correspond to the addition of a specific amino acid to a growing protein chain in a specific order. The nonsense codon actually terminates the process of protein formation, by stopping the inclusion of any more amino acids in the protein chain. Thus, the function of the start codon is the opposite as that of the stop codon. Amino acid chain formation stops at this point, thus, the chances of making mistakes during protein formation goes down considerably.

Stop Codon Structure
In the standard genetic code, there are three different types of stop codons, which are, TAA (thymine - adenine - adenine), TAG (thymine - adenine - guanine) and TGA (thymine - guanine - adenine). There is a lot of importance attached to each stop codon. When these codons undergo DNA transcription, they change to UAA (uracil - adenine - adenine), UAG (uracil - adenine - guanine) and UGA (uracil - guanine - adenine). Thus, these are the stop codons that are found in RNA molecules.

The position of a stop codon is not fixed, as the length of the protein molecule and correspondingly, the length of the chain and the number of amino acids laid down varies. Thus, the stop codons in DNA and RNA both may be found at varying intervals in the length of the chain. Stop codons can easily be identified when DNA molecule is sequenced and thus, can be used to identify the specific locations in the genetic code which specifically corresponds to a particular type of protein.

Stop codon mutation is not a new phenomenon. Point mutations can occur at any location along the length of the DNA, and hence, they can affect the nonsense codon as well. The codon may be transcribed incorrectly or the nucleic acids in that codon may change. This may lead to problems when the ribosome the mRNA to build amino acid chains. In a cell, this may lead to a random mutation which may result in malfunctioning or even death of the cell.

This was all about stop codons or nonsense codons. These are an important factor in cutting short the process and length of the protein chain. Thus, they help prevent the protein synthesis from going on endlessly and from going wrong, which could otherwise lead to dire consequences.