Learning Part 2 : Protein Synthesis

STAGE 1 : INITIATION

• First, a small ribosomal subunit binds with mRNA

• Then, an initiator tRNA with the anticodon UAC (which carries methionine) base-pairs with the start codon , AUG.

• Later, a large ribosomal subunit binds to the small ribosomal subunit completes the initiation complex

STAGE 2 : ELONGATION

Consists of 3 steps

1. Codon recognition

2. Peptide bond formation

3. Translocation

STEP 1 : CODON RECOGNITION

• The anticodon of an incoming aminoacyl tRNA base-pairs with the codon of mRNA in the A site.

STEP 2 : PEPTIDE BOND FORMATION

• Peptidyl transferase catalyze the formation of peptide bond between amino acid in the A site and the growing polypeptide in the P site.

• This step removes the polypeptide from the tRNA in the P site and attaches it to the amino acid on the tRNA in the A site

STEP 3 : TRANSLOCATION

• The ribosome translocates the tRNA in the A site to the P site.

• At the same time, the empty tRNA in the P site is moved to the E site, where it is released

• The mRNA moves along with its bound tRNAs, bringing the next codon to be translated into the A site.

• Effectively translocation ensures that the mRNA is read 5' end to 3' end codon by codon

STAGE 3 : TERMINATION

• When a ribosome reaches a stop codon on mRNA, the A site of the ribosome accepts a protein called a release factor instead of tRNA.

• The release factor hydrolyzes the bond between the tRNA in the P site and the last amino acid of the polypeptide chain.

• The polypeptide is freed from the ribosome

• The two ribosomal subunits and the other components of the assembly dissociate.