Three sequences of nucleotides called a codon on the mRNA code for amino acids. There are 64 codons. Sixty-one of them code for amino acids including AUG which is a start codon corresponding to methionine. Three codons are referred to as STOP codons. Stop codons terminate translation of mRNA.
The stop codons are UGA, UAA, and UAG. Remember,
- UGA – “You go away”
- UAA – “You are away”
- UAG – “You are gone”
The tRNA is used in translation to transfer amino acids corresponding to codons on the mRNA. As you can see from the figure below, the tRNA forms the shape somewhat of a “t”. Amino acids attach to the tRNA at the 3′ end where there is the nucleotide sequence, AAC. Two arms called the D and T-loop on the tRNA play an important role in the function of the tRNA and its interaction with the ribosome. The sequence of the tRNA that reads the codon on the mRNA is called the anticodon.
In order to bind the tRNA, amino acid first reacts with ATP to produce amino acyl-AMP with the help of amino acyltransferase. The amino acid is then transferred to tRNA with the help of aminoacyl tRNA synthetase. AMP is released in the process.
Along with mRNA and tRNA, ribosomes complete the machinery needed for translation. Ribosomes are a combination of rRNA and proteins. There are two subunits of the ribosome, a large and a small. In eukaryotes, the large unit is called the 60s subunit, and the small is called the 40s subunit.
Steps in Translation
The small subunit positions itself onto the mRNA with the help of initiation factors, and the tRNA is brought into position (also with the help of initiation factors).
The large subunit is engaged with the help of initiation factors to complete the initiation complex. The tRNA is now ready to begin translating, starting at the peptidyl site (or p-site) where the start codon AUG codes for methionine.
After the initial installation of the first amino acid (methionine), another tRNA reads the codon on the aminoacyl site (or A site). The movement of tRNA to this site is facilitated by elongation factors.
The first amino acid that was translated is transferred to the second using peptidyl transferase.
Now tRNA in the p-site is empty (has no amino acid) while the second has two amino acids. This is the beginning of a chain of amino acids that will make the new protein.
The “empty” tRNA moves over to the exit (or e-site) while the tRNA that was previously in the a-site moves over to the p-site.
The empty tRNA in the e-site then exits the complex, leaving behind the tRNA in the p-site.
The process continues with a string of amino acids being joined together each time a new tRNA enters the A site. Eventually, a STOP codon is reached. No tRNA binds to these codons.
Instead, a release factor comes in and binds the STOP codon resulting in the release of the protein a dissociation of the complex.