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  Initiation of polypeptide chain

Initiation of polypeptide chain:

Protein synthesis is started through the selection and transfer of the first amino acid in ribosomes. This procedure needs ribosome subunits, amino acyl tRNA complex, mRNA and initiation factors (IF). Beginning of polypeptide chain includes the following steps:

1. The 30s ribosomal subunit connects to the 5’ end of the mRNA to create an mRNA 30s complex. This procedure needs the initiation factor IF-3 and Mg2+ ions. The attachment is ready at the 1st codon of the mRNA.

2. The 1st codon of mRNA will be all the time AUG. This codon identifies the amino acid methionine. Thus the 1st amino acid in the synthesis of whichever polypeptide chain is methionine.

2137_polypeptide chain1.jpg


                                                  Diagram: 30s ribosomal subunit is attached to the 5' end of mRNA

3. The tRNA comprising the anticodon UAC (complementary to AUG) transports methionine to the 30s ribosome and connects itself to the beginning codon on mRNA. The tRNA, mRNA and 30s ribosome subunit create a complex known as 30s - pre initiation complex. This procedure needs initiation factors and GTP.

1898_polypeptie chain2.jpg

4. 30s - Pre initiation complex joins along with 50s ribosomal subunit to create initiation complex. The initiation complex is created by 70s ribosome, mRNA and met -RNA or methionine RNA.

5. The 70s ribosome comprises two slots for the entry of amino acyl tRNA, that are

  • P site (peptidyl site) and
  • A site (amino acid site).

The first tRNA that is met RNA is joint to the P site of 70s ribosome.

985_polypeptide chain3.jpg

                                                                Diagram: Formation of initiation complex

 

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