Question

Please explain how the sequence of events that occurs when a codon that specifies an amino acid enters a ribosome's A site differs from the sequence of events that occurs when a stop codon enters a ribosome's A site.

Answer 1

Answer-

According to the given question-

Translation- mRNA ---------->> Protein is called Translation that occurs in cytoplasm of the cell.

Information present in mRNA is used for synthesis of protein. mRNA contains four types of nucleotides arranged in a sequence and when read as triplet or group of three nucleotides are called codons and each amino acid are specific for at least one amino acid or there is stop codon which doesn’t code for any protein and translation process stop. There is an adaptor molecules transfer RNA or tRNA that is responsible for recognizing the codon on mRNA and at the same time recognizes and binds with the amino acid on their surface. Aminoacyl-tRNA synthetases enzyme is responsible for covalently attaching the amino acid to the tRNA molecule require ATP.  A ribosome have four binding sites out of which mRNA binds with one site while three other site called E-site, P-site and A-site, are for binding of tRNA. Translation requires mRNA whose information is being translated, tRNA which brings the amino acids and ribosome’s which are responsible for performing translation. There is also need of initiation factors, elongation factors, as well as release factors required for completion of translation.

Initiation- starts when small subunit of ribosomes binds with the initiator tRNA which has initiator amino acid called Methionine. After that this complex binds with the 5’ end of an mRNA and find start codon with the help of initiation factor. After that the large subunit of ribosomal binds and releases initiation factors. Large subunit have A-site which is for entry for new tRNA molecule that are charged with amino-acid or also called aminoacyl-tRNA; P-site is for peptide bond formation and also  growing polypeptide chain while E-site serve as exit site for tRNA.

Elongation: after initiation new amino acid by tRNA enters the A-site and a peptide bond formation takes place between two amino-acids after that releasing amino acid at P-site, tRNA becomes empty and the ribosome moved in right direction one triplet on the mRNA and empty tRNA leaves from the E-site. And the A-site is ready for new amino acid carried by tRNA.

Termination: Termination occur when stop codons is present at A-site in this case tRNA molecule cannot occupy the  A-site because there is no tRNA have amino acid that matches with this stop codon sequence and this codon was recognized by a protein called release factor which binds and cleave the bond present between the growing  polypeptide chain and tRNA due to which the newly formed polypeptide is released from the ribosome and same time the ribosome also disassociated into small and large subunits and again perform the next round of protein synthesis.