Describe initiation step of translation in prokaryotes.

Explanation

Initiation Step of Translation in Prokaryotes In prokaryotes, two types of tRNA are present for methionine
(i) `tRNA^("met")` for initiation carrying formyl methionine and (ii) `tRNA^("met")` for carrying normal methionine to growing polypeptide.
The initiation of polypeptide synthesis requires the following components mRNA, 305 subunit of ribosome, formylmethionyl-tRNA `(f^("net")-tRNA^("met"))`, initiation factors IF-1, 1-2 and IF-3, GTP, 50S ribosomal subunit and `Mg^(2+)`
The sequence of events occurring during initiation process is
The smaller 305 subunit of ribosome binds to the transcription factor F-3. It prevents the premature association of two ribosomal subunits.
Interaction of SD region of mRNA and ASD region of ribosome helps the mRNA to bind to 30S subunit. It also helps AUG to correctly positioned at the P-site of the ribosome.
The fMet-`tRNA^("met")` (the specific tRNA aminoacylated to formyl methionine) binds to the AUG codon at the P-site. The `tRNAf^("met")` is the only tRNA that binds to its codon present on the P-site. All other tRNA along with their respective amino acids bind to their codon present at the A-site. Therefore, AUG codon present as initiation codon codes for formylmethionine. When, it is present at other position it codes for normal methionine.
The initiation factor F-1, binds to the A-site. It prevents the binding of any other aminoacyl RNA to the codon at the A-site during initiation.
The GTP bound IF-2 (GTP-IF-2) and the initiating `"fmet-tRNAf"^("met")` attaches to the complex of 305 subunit-IF3-F1-mRNA.
50s subunit then attaches the complex formed in the previous step. The GTP bound to IF-2 is hydrolysed to GDP and Pi. After this step, all the three initiation factors leave ribosome. This complex of 70S ribosome, mRNA and `"fmet-tRNAf"^("met")` bound to initiation codon at P site is known as initiation complex