Elongating the Protein synthesis Process
The Elongation phase of protein synthesis consists of a cyclic process whereby a new aminoacyl-tRNA is positioned in the ribosome, the amino acid is transferred to the C-terminus of the growing polypeptide chain, and the the whole assembly moves one position along the ribosome.
The next phase in translation is known as the elongation phase. First, the ribosome moves along the mRNA in the 5' –to –3'direction, which requires the elongation factor G, in a process called translocation.
The tRNA that corresponds to the second codon can then bind to the A site, a step that requires elongation factors (in E. coli, these are called EF –Tu and EF –Ts), as well as guanosine tri phosphate (GTP) as an energy source for the process. Upon binding of the tRNA –amino acid complex in the A site, GTP is cleaved to form guanosine di phosphate (GDP), then released along with EF –Tu to be recycled by EF –Ts for the next round.
Peptide bonds between the now –adjacent first and second amino acids are formed through a peptidyl transferase activity. For many years, it was thought that an enzyme catalyzed this step, but recent evidence indicates that the transferase activity is a catalytic function of rRNA.
After the peptide bond is formed, the ribosome shifts, or translocates, again, thus causing the tRNA to occupy the E site. The tRNA is then released to the cytoplasm to pick up another amino acid.
In addition, the A site is now empty and ready to receive the tRNA for the next codon. This process is repeated until all the codons in the mRNA have been read by tRNA molecules, and the amino acids attached to the tRNAs have been linked together in the growing polypeptide chain in the appropriate order. At this point, translation must be terminated, and the nascent protein must be released from the mRNA and ribosome.
Termination of Translation:
There are three termination codons that are employed at the end of a protein –coding sequence in mRNA: UAA, UAG, and UGA. No tRNAs recognize these codons. Thus, in the place of these tRNAs, one of several proteins, called release factors, binds and facilitates release of the mRNA from the ribosome and subsequent dissociation of the ribosome.