12th Class Biology Molecular basis of inheritance

  • question_answer 80)   Discuss the process of translation in detail.  


                      There are three-stages of protein synthesis (i) Initiation Assembly of Ribosomes on mRNAIn prokaryotes, initiation requires the large and small ribosome subunits, the mRNA, initiationtRNA and three Initiation Factors (IFs). Activation of Amino Acid Amino acids become activated by binding with amino acyltRNA synthesize enzyme in the presence of ATP. Amino acid (AA) + ATP AA-AMP-Enzyme complex +. Transfer of Amino Acid to tRNAThe AA-AMP-enzyme complex formed reacts with specific t RNA to form aminoacyltRNA complex, AA-AMP-Enzyme complex + tRNA AA tRNA + AMP + Enzyme. The cap region of mRNA binds to the smaller subunit of ribosome. The ribosome has two sites, A-site and P-site. The smaller subunit first binds the initiator tRNA then and then binds to the larger subunit so, that initiation codon (AUG) lies on the P-site. The initiationtRNA, i.e., methionyltRNA then binds to the P-site, (i) Elongation Another charged aminoacyltRNA complex binds to the A-site of the ribosome. Peptide bond formation and movement along the mRNA called translocation. A peptide bond is formed between carboxyl group (?COOH) of amino acid at P-site and aminogroup (?NH) of amino acid at A-site by the enzyme peptidyltransferase. The ribosome slides over mRNA from codon to codon in the 5'3' direction. According to the sequence of codon, amino acids are attached to one another by peptide bonds and a polypeptide chain is formed. (ii) Termination When the A-site of ribosome reaches a termination codon which does not code for anyamino acid, no charged tRNA binds to the A-site. Dissociation of polypeptide from ribosome takes place which is catalyzed by a 'release factor'. There are three termination codons,i.e., UGA, UAG and UAA.  

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