explain the steps involved in polypeptide synthesis. how are the amino acids activayed during polypeptide synthesis?

Translation is the process in which mRNA  produced by transcription is decoded by the ribosomes  to produce a specific amino acid chain .

Protein synthesis occur in three steps: 

Initiation - It involves binding of  small  ribosomal subunits to mRNA  and then is joined by the 50S subuint. It includes steps of formation of the peptide bond between the first two amino acids. Protein synthesis is initiated with the amino acid methionine because the AUG translation initiation codon of mRNAs is recognized by the anticodon (UAC) of initiator methionine transfer RNA.

Elongation - It includes all the reactions from synthesis of the first  peptide bond  to the addition of the last amino acid. 

Termination-  completed polypeptide chain is released and the ribosome dissociate from mRNA.

Three codons on mRNA which are not recognized by tRNA are UAG (amber), UGA (opal), UAA (ochre). These codons are collectively known as non sense codons or terminator codons. They are significant since there presence bring the termination of protein synthesis (translation).

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