Translation is a complex process. The steps in this process are translation initiation, polypeptide chain elongation and translation termination.
1 | Initiation
(i) mRNA enters the cytoplasm of the cell through nuclear pores. At this time tRNA is attached to the start codon of mRNA with methionine.
(ii) Small units of ribosome, tRNA and mRNA combine to form initiation complex.
(iii) The large unit of the ribosome binds to the initiation compound. Ribosomes have A site, P site, E site etc.
(iv) tRNA carries the amino acid to the A-site of the ribosome and initiates translation.
2. Elongation
(i) First the tRNA carries an amino acid to the A-site of the ribosome. Amino acids and tRNA are released there.
(ii) A code proceeds by scanning the ribosome. The amino acid remains at the A-site of the ribosome and the tRNA moves from the A-site to the P-site of the ribosome. At this point the second tRNA carries the amino acid to the A-site of the ribosome. There the second amino acid and tRNA are released.
(iii) Another code proceeds by scanning the ribosome. The first and second amino acids are added to the A-site of the ribosome and the second tRNA moves from the A-site to the P-site of the ribosome. The first tRNA moves from the P-site to the E-site of the ribosome. Meanwhile, the third tRNA carries the amino acid to the A-site of the ribosome. There the third amino acid and tRNA are released.
(iv) A further code proceeds by scanning the ribosome. At the A-site of the ribosome, the first, second and third amino acids join to form the chain. The third tRNA moves from the A-site to the P-site of the ribosome. The second tRNA moves from the P-site to the E-site of the ribosome and the first tRNA moves from the E-site of the ribosome to the cytoplasm.
(v) Thus one after the other amino acids come to the ribosome and join to the 5-3 carbon side of the mRNA to form a polypeptide chain. The presence of polyribosomes increases the speed of this process.
(vi) The polypeptide chain continues to form until the A-site of the ribosome reaches the stop codon of the mRNA.
(vii) Movement of tRNA from A-site to P-site and from P-site to E-site of ribosome is called translocation.
3. Termination
(i) A water molecule is added to the polypeptide chain when the A-site of the ribosome reaches the stop codon of the mRNA. As a result, the bond between the polypeptide and the ribosome is broken in the process of hydrolysis.
(ii) The newly formed polypeptide chain or protein molecule is released from the ribosome and translation ceases.