Translation

Translation is the second step of protein synthesis, where protein is formed from mRNA after transcription.

The process occurs in the ‘protein factory’ of the cell, the ribosome, in both prokaryotes and eukaryotes.

Translation occurs in three stages: initiation, elongation, and termination.

1. Initiation

After the transcribed mature mRNA leaves the nucleus, it travels to a ribosome. The 5′ methylated cap of the mRNA, containing the start codon binds to the small ribosomal subunit of the ribosome consisting rRNA. Next, a tRNA containing anticodons complementary to the start codon on the mRNA attaches to the ribosome.  These mRNA, ribosome, and tRNA together form an initiation complex. The ribosome reads the sequence of codons in mRNA, and tRNA bring amino acids to the ribosome in the proper sequence.

2. Elongation

Once the initiation complex is formed, the large ribosomal subunit of ribosome binds to this complex, releasing initiation factors (IFs). The large subunit of the ribosome has three sites for tRNA binding; A-site, P-site, and E-site. The A- (amino acid) site is the region, where the complementary anticodons of aminoacyl-tRNA (tRNA with amino acid) pairs up with the mRNA codon. This ensures that correct amino acid is added to the growing polypeptide chain at the P- (polypeptide) site. Once this transfer is complete, the tRNA leaves the ribosome at the E- (exit) site and returns to the cytoplasm to bind another amino acid. The whole process gets repeated continuously and the polypeptide chain gets elongated. The rRNA binds the newly formed amino acids via peptide bond, forming the polypeptide chains.

3. Termination

The 3′ poly-A tail of the mRNA holds a stop codon that signals to end the elongation stage. A specialized protein called release factor gets attached to the tail of mRNA, causing the entire initiation complex along with the polypeptide chain to break down. As a result, all the components are released.

The newly formed polypeptide chain undergoes several modifications before becoming a fully functioning protein. Proteins have a variety of functions. Some will be used in the cell membrane, while others will remain in the cytoplasm or be transported out of the cell. Many copies of a protein can be made from one mRNA molecule. This is because several ribosomes can translate the same mRNA molecule at the same time. These clusters of ribosomes that translate a single mRNA sequence are called polyribosomes or polysomes.

The process helps in decoding instructions for making proteins, the building blocks of the body.