Start Codon

What is a Codon?
A codon is nothing but a genetic code. A genetic code is a set of rules by which certain information is encoded in the genetic material, which may be either DNA or mRNA sequences, from where it is translated into proteins. Proteins consist of amino acids that are strung together in a specific sequence. Any change in this sequence signifies a change in the coding, which is what is normally seen in cases of genetic mutations. Each codon is made up of three bases. There are altogether four bases that are present in DNA - adenine, cytosine, guanine and thymine. Each codon consisting of a group of three bases, codes for a single amino acid, a mapping that is encoded in the tRNA of the organism. Given below are details about a specific codon, known as start codon.

 

What is a Start Codon?
A start codon is a codon which initiates the translation of the first amino acid in the polypeptide chain. The first three bases of the coding sequence of mRNA to be translated into protein is where the initiation codon is located. This is an important structure, because the actual protein sequence that is translated is defined by a start codon. The initiation codon is almost always preceded by an untranslated region, 5' UTR. 5' UTR, also known as the leader sequence, is a particular section of mRNA, which starts at the +1 position, which is where transcription begins and ends just before this start codon of the coding region.

This is usually the first AUG codon in the mRNA sequence. Additionally, in cases of DNA molecules, the start codon typically consists of the bases ATG. It is only in very rare cases that higher organisms, that is, eukaryotes, have non AUG initiation codons. However, in addition to AUG there are certain alternative codons seen as well. These include GUG and UUG. These codons seen in lower and less differentiated organisms, that is, in prokaryotes. For example, E. coli uses ATG (AUG) 83% of the time, GTG (GUG) 14% of the time and TTG (UUG) 3% of the time. One or two others, like ATT and CTG are very rarely seen. There may not always be the same start codon even within a species. Bacteria and archaea, have UUG and GUG as their initiation codons on most occasions. However, it has been seen that in certain rare cases, specific proteins may use alternative initiation codons, which may not be used by that species.

All start codons code for methionine, as this is the first amino acid that is coded for during protein synthesis. Even if alternative initiation codons are present, it eventually does get translated as methionine, even if the codon present normally does encode for a different amino acid. This happens because a separate tRNA is used for initiation in such cases. Translation starts with chain initiation codon or start codon. There is one major difference between start codon and stop codon. Unlike stop codons, the initiation codon alone is not sufficient to begin the process of protein synthesis. Nearby sequences and certain initiation factors are also required so as to start translation.

In cases of start codon mutation, the mutated mRNA would be shunted to the ribosomes as usual, but the translation would not take place. This is because an initiation codon is what is responsible for starting translation, not transcription. Hence, it cannot necessarily produce proteins because start codon for transcription lacks a proper nucleotide sequence that can act as a reading frame.