DNA Translation:DNA To Protein

DNA Translation:DNA To Protein

Translation refers to the process of polymerisation of amino acids to form a polypeptide.The order of the amino acids is defined by the sequence of bases in the m-RNA.Formation of a peptide bond requires energy, therefore in  the first step amino acids are activated in the presence of ATP.

Step1: Activation of amino acids

• Amino acids react with ATP  to form amino acid –AMP complex and pyrophosphate.

• The reaction is catalysed by a specific amino acid -activating enzyme called aminoacyl-tRNA synthetase.

Step 2: Charging of tRNA

• The amino acid -AMP -enzyme complex joins the amino acid binding site to its specific tRNA where the COOH group of the amino acid binds to the -OH group of the tRNA.

• tRNA +amino acid get separated from enzyme+AMP.

• tRNA +amino acid complexis called charged tRNA.

Step3:Activation of Ribosome

• The small and the large subunits of ribosomes must be joined together for protein synthesis , this is brought by mRNA  chain.

• The mRNA chain joins the small ribosomal subunit by first codon through base pairing with appropriate sequence on rRNA.This is called the initiation complex.

• The large subunit later joins the small subunit forming the active ribosome.

Step 4: Polypeptide Formation

Initiation of polypeptide chain

• Inititation stage brings together an mRNA , a tRNA bearing the first amino acid of the polypeptide ,and the two subunits of the ribosome.Union of all these components forms translation initiation complex.

• On the mRNA lies the initiator codon ,AUG, which signals the start of translation.

• The initiator tRNA joins the initiator codon by its anticodon through hydrogen bonds.

• It carries methionine in eukaryotes.At this stage initiator tRNA lies at the P site of the ribosome, and at the A site is left vacant to let another charged tRNA to enter.

Elongation of the polypeptide chain

• In the  elongationstage of translation,amino acids are added one by one to the first amino acid(methionine, present at P site).

• A charged tRNA enters the ribosome at the A  site(say proline). GTP is hydrolyzed in this step.

• The amino acid(methionine) on tRNA at the P site and the newly  arrived amino acid on tRNA at the A site join by a peptide bond.The reaction is catalyzed by peptidal transferase.

•  In the peptide bond formation ,the linkage between the first amino acid (methionine)and its tRNA is broken,and the  -COOH group(of methionine) now forms a peptide bond with the NH2 group of the second amino acid.(proline).Which means that the second tRNA carries a dipeptide (proline and methionine) and first tRNA is without amino acid(carried methionine earlier).

Translocation

• The tRNA at the A site (carring a dipeptide)moves to the P site .This process is translocation.It requires energy.

• As the tRNA translocates to the P site ,its anti codon keeps bonded to the mRNA codon,so that mRNA moves along  with it and brings its next codon to the A site. In the meantime .the uncharged tRNA  shifts from the P site to  mixed pool of tRNAs in the cytoplasm.

• The next cycle starts, till the whole mRNA is translated.

Step 5: The termination of the polypeptide Chain

• At the end of the mRNA chain there is a stop codon(UAA,UAG,UGA)

• The stop codon reaches the A site but is not read.

• A protein called the release factor joins the stop codon in theA site .It adds water molecule to thepolypeptide  chain.This hydrolyses (cleaves)the completed polypeptide from the t RNA that is at the  P site.