Answer:
4. A small ribosomal subunit binds with mRNA;
1. The first tRNA with Met binds to the P site;
3. tRNA comes in to the A site with a new aa;
2. A peptide bond forms between the new amino acid and a polypeptide chain;
5. tRNA in the P site leaves and the tRNA from the A site translocates to the P site.
Explanation:
The first step in translation i.e. protein synthesis is attachment of small ribosomal subunit i.e. 30S subunit with mRNA. Soon after that a tRNA charged with Met binds to the P site of 30 S ribosomal subunit. To this complex further 50S ribosomal subunit binds. Together all these components form '70 S initiation complex". Since this complex already has one amino acid (tRNA with Met) at the P site, in order to create peptide chain another tRNA charged with amino acid must enter the complex. The upcoming tRNA enters the initiation complex at A site. Next, due to the peptidyl transferase activity of large ribosomal subunit i.e. 50S subunit a peptide bond is formed between the amino acid at P site and amino acid at A site. Now since peptide bond has already been formed tRNA at P site is useless so it has to be expelled from the initiation complex via exit site. As soon as this tRNA is expelled, the tRNA which was at A site translocates into P site so that a new tRNA charged with another amino acid could enter the A site for further elongation of peptide chain.
