A codon is a sort of genetic code that has a set of rules, where specific information is encoded in the genetic material that can either be DNA or mRNA sequences, which is then translated into proteins. The proteins that are synthesized are made up of amino acids that are placed together in a specific order or sequence. If any changes happen in the order these amino acids are placed, it results in changes in the coding, or what is commonly known as mutations.
There are three bases that are the ones that will make up each codon, and they are codes for every single amino acid; these amino acids will form a mapping that is found inside the tRNA of each organism. In total, we have four bases that are in our DNA: adenine, cytosine, guanine and thymine.
What is a Start Codon?
In DNA a starts codon is where the translation of the first amino acid in the polypeptide chain begins. Here is the location of the first three bases of the coding sequence of mRNA that will start to be translated into proteins. It is a very important structure because the start codon actually defines the protein sequence that is to be translated.
This codon is normally preceded by an area that isn’t translated known as 5’UTR, which is also called the leader sequence. It is a specific part of mRNA, that starts at the +1 position. In this particular area transcription begins and finishes, right before the codon start of the coding region.
Usually this will be the first AUG codon you will encounter in the mRNA sequence. When it comes to DNA, the start codon is usually made up of ATG codon bases.
In a few rare cases do higher organisms (eukaryotes) have non AUG start codons. However, other combinations, such as GUG or UUG are seen in lower organisms (prokaryotes). For instance, E.coliuses ATG (AUG) 83% of the time and TTC (UUG) only 3% of the time.
Methionine is the first amino acid that is coded in the protein synthesis, and all start codons code for it. Even in the presence of alternative initiation codons, in the end they all get translated into methionine. The reason behind this is that a separate tRNA is used to start in cases like this.
In order for translation to begin, we need a start codon, and there’s also a stop codon. However, the former isn’t enough to start the whole process on protein synthesis. Sequences and other initiation factors are necessary to start the translation process.
When there’s a start codon mutation, the mutated mRNA would be sent to the ribosomes, but no translation would occur. The reason is that the initiation codon is responsible only for the beginning of the translation, not a transcription start codon. Therefore it doesn’t necessarily mean that it can produce proteins, since this codon doesn’t have the proper nucleotide sequence that serves as a reading frame.