A ribosome binds to the mRNA strand
that codes for the initiation codon (AUG). This ribosome (70S)
consists of two subunits namely, large subunit (50S) and small
subunit (30S), here S signifies the Svedberg unit. On the 70S
ribosome, there are three tRNA-binding sites present, which bridge
the two subunits of the 70S ribosome. They are called the A-site
(aminoacyl), P-site (peptidyl), and E-site (exit). The process of
initiation, elongation and termination takes place as given below
-
- Charging of tRNA - At first, the
amino acid gets acylated and binds to the tRNA, this process is
called the charging of tRNA which prepares amino acid for the
further elongation process. In this process, the uncharged tRNA, an
ATP molecule and a amino acid are involved. Upon binding of the
amino acid with the tRNA, an ATP molecule gets used up and forms an
AMP molecule with the release of a phosphate group.
- Initiation - During initiation the
30S subunit containing the P-site and the A-site bind to the mRNA.
This occurs due to the binding of the IF-3 (Initiation Factor )to
the 30S subunit and IF-1 and the A-site of the 30S subunit.
Further, the charged tRNA (formyl Methionine for the AUG codon)
bound to the IF-2 along with a GTP molecule binds at the P-site. At
the end of the initiation step, the 50S subunit binds to the mRNA
and the IF-1, IF-2 and IF-3 are released. Also, a GDP molecule is
released after usage of a phosphate molecule from the GTP.
- Elongation - The second
aminoacyl-tRNA binds at the A-site of the 70S ribosome upon
recognition of the codon. Further, the peptide bonding between the
-COO- of the first amino acid and the
-NH3+ of the second amino acid takes place.
Right after the peptide bonding happens the tRNA at the P-site gets
deacylated and the tRNA at the A-site becomes dipeptidyl-tRNA. By
the binding of the Elongation Factor - G (EF-G) along with a GTP
molecule onto the ribosome the translocation takes place. After the
translocation, the EF-G along with the GDP molecule is released
upon usage of a phosphate molecule for energy. Also, the
dipeptidyl-tRNA moves to the P-site making space for another
incoming aminoacyl-tRNA. The steps further repeat causing the
elongation of the peptide molecule.??
- Termination - When the ribosome
recognises a stop codon (UAA, UGA, UAG) the release factors (RF)
are recruited and the peptide chain is cleaved from the tRNA. This
occurs as no aminoacyl-tRNA exists for recognising the stop codons.
RF-1 recognises UAA and UAG; RF-2 recognises UAA and UGA.