Start Codon Definition
The start codon is the initiation signal for translation that is found on a messenger RNA (mRNA) strand. Remember that translation is the process that leads to the formation of strings of amino acids when anticodons present on the transfer RNA (tRNA) molecule attach to their complementary codons on the mRNA. This is the way our bodies create proteins that are crucial for various functions in the body. After transcription leads to the production of RNA, translation follows when a ribosome latches itself to an mRNA strand and attaches to the 5’ end of it. The ribosome moves across the mRNA strand until it recognizes a start codon, and that’s where translation begins.
Every codon is a sequence made up of three nucleotides, each represented by a letter, and together coding for a specific amino acid. Several codons put together, therefore, result in the creation of a string of amino acids, which eventually becomes a protein. The start codon, then, tells the ribosome where to begin the process of translation on the mRNA molecule. It is typically AUG, which codes for the amino acid methionine. Once it has been identified and the methionine has been brought over by tRNA, the codons following it are read with no pauses or interruptions, until the end of translation. Note that when AUG is found in the middle of a strand already being translated, it no longer codes for a start codon, but simply methionine.
AUG in Prokaryotes
In prokaryotic cells, such as bacterial cells, AUG codes for N-formyl methionine, rather than methionine, and on its own, it is not enough to initiate translation. Therefore, we find that it is followed by a Shine-Dalgarno sequence, which helps align mRNA with ribosomes to orient the molecules correctly for translation. The Shine-Dalgarno sequence is found about 10 nucleotides upstream from the start codon. The way it is identified is through a specific sequence on the ribosomal RNA (rRNA) molecule that is complementary to the Shine-Dalgarno sequence. Once the rRNA attaches itself to the sequence, it stimulates tRNA to bring over the amino acid N-formyl methionine on to the AUG. Lastly, in addition to AUG, prokaryotes posses other start codons; GUG, which codes for valine, and UUG, which codes for leucine.
AUG in Eukaryotes
Codons other than AUG have also been found to serve as start codons in eukaryotes. In rare cases, some eukaryotes can have CUG, which codes for leucine, as their starting codon. In addition, eukaryotic mitochondrial genome can also have AUA and AUU, which code for isoleucine.
Interestingly, in both eukaryotes and prokaryotes, even in cases where alternative codons that don’t code for methionine serve as start codons, methionine is always the first amino acid produced.