The translation is a process by which ribosomes present in the cytoplasm (on endoplasmic reticulum) synthesize the proteins from an mRNA. The RNA Translation occurs by 3 steps like
1.Initiation
2. Elongation
3. Termination.
A protein is a polypeptide that has a sequence of amino acids coded by the mRNA.
There are different types of proteins based on their structures and functions and the aminoacid sequences in their polypeptides.
The components involved in RNA translation are
- mRNA
- ribosomes,
- tRNA and
- aminoacyl-tRNA synthetases.
The mRNA is a messenger RNA, and tRNA is a transfer RNA.
Francis Crick theorized the presence of an adapter molecule that would, on the one hand, read the code and, on the other hand, would bind to specific amino acids.
A codon is a set of three bases on the mRNA coding for a single amino acid. Each codon has only one corresponding amino acid. A single amino acid may be coded for by more than one codon. Thus, the genetic code is degenerate. There are 61 different codons for amino acids.
A particular codon (AUG), called initiator codon, signals the initiation of translation. The three codons (UAG, UAA, and UGA), called non-sense codons, signal the termination of the translation process.
Every tRNA molecule has an anticodon on one end that is complementary to the mRNA codon. Thus, it can bind to the mRNA.
The other end attaches to its specific amino acid, which can be determined by following the genetic code.
The process can be studied in the following steps.
Activation
The amino acids first need to be attached to their respective tRNA molecules before they can be incorporated into the proteins.
This reaction is catalyzed by aminoacyl tRNA synthetase that binds the amino acid to its specific tRNA, forming aminoacyl-tRNA.
The resulting complex is charged due to the presence of a high-energy bond.
The tRNA is clover leaf-shaped, and one side has a sequence of three bases while another end is attached to the corresponding amino acid.
Each codon has its own tRNA, and each tRNA has its own amino acid attached to it.
Amino acid + ATP=Aminoacyl-AMP+2Pi
Aminoacyl-AMP + tRNA= Aminoacyl-tRNA + AMP
A translational unit in mRNA is the sequence of RNA that is contained between the start codon (AUG) and the stop codon. It codes for a single protein.
An mRNA also has extra sequences that remain untranslated and are referred to as untranslated regions (UTR). The UTRs are present at both 5′ -end (before start codon) and at 3′ -end (after stop codon). They are essential for translation.
Initiation
To start the process of translation, the start codon AUG must first be recognized. This codon codes for the amino acid methionine. A eukaryotic ribosome has two parts: the smaller 40s subunit and the larger 60s subunit. First, the tRNA carrying methionine attaches to the small ribosomal subunit. Together, they then link to the 5′ end of the mRNA by recognizing the 5′ GTP cap, which is added during the processing of the mRNA in the nucleus. Thus, the 5’ end of mRNA binds to the 40s subunit of the ribosome (with methionyl-tRNA). The codons of an mRNA are read in order from the 5′ end to the 3′ end. The ribosome forms the initiation complex, initiator tRNA carrying the first amino acid in the protein, and the mRNA, which will code for the protein.
Elongation
The first tRNA is bonded out to the mRNA in the middle slot of the ribosome called the P site. Adjacent to this bonded codon, there exists another codon set that is free. This is called the A site and is the landing site for the next tRNA, which has an anticodon that is complementary to this codon. The next tRNA attaches to the codon on the mRNA with its corresponding amino acid bonded to its other end. The two amino acids of the corresponding tRNAs are in such close proximity that they form a peptide bond between them. This link is energetically favorable. The amino acid bonded to the first tRNA breaks its bond with the tRNA, and the amino acid attaches to the 2nd. The ribosome acts as a catalyst and contains the enzyme ribozyme, which catalyzes this peptide bond formation.
Once the first peptide bond is formed, the mRNA is pulled forward through the ribosome by the distance of exactly one codon. This shift allows the first empty tRNA to leave the tRNA through the exit site. It also exposes a new codon in the A site, where another tRNA can bind, and the first two amino acids get connected with the amino acid on this tRNA by a peptide linkage. Thus, the cycle repeats.
Termination
Termination is the process by which translation ends. Termination occurs when the ribosome encounters a stop codon (UAA, UAG, or UGA) on the mRNA. Stop codons are recognized by special proteins called release factors that fit into the codon even though they aren’t tRNA. These factors make the enzyme add a water molecule at the end of the last amino acid instead of forming a peptide bond. This reaction separates the newly formed amino acid chain from the tRNA, and the newly made protein is released.
Thus, a new polypeptide chain that constitutes a protein is formed by RNA translation.
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