Question

Select the correct statement regarding protein synthesis

• A. When the small subunit of the ribosome encounters a mRNA the process of translation begins
• B. Peptidase catalyses the formation of peptide bond
• C. UTRs are present between the start codon and stop codon
• D. At the end of translation the release factor binds to the initiation codon
• E. The completed polypeptide is stored in the ribosome and released when required

Answer 1

Answer: (A)

Protein synthesis occurs in ribosomes therefore, they are also called protein factories. Each ribosome has two unequal subunits, small and large. The larger subunit of ribosome has a groove for pushing out the newly formed polypeptide and protecting the same from cellular enzymes. The smaller subunit fits over the larger one like a cap but leaves a tunnel for mRNA. The two subunits come together only at the time of protein formation. The phenomenon is called association. $M{g}^{2+}$ is essential for it. Smaller subunit of ribosome recognise mRNA molecule prior to the start of translation. A sequence of five to nine (typically seven) nucleotides preceding the start codon in mRNA is recognised by the ribosome as the correct site for binding the mRNA molecule prior to the start of translation. These sequence (AGGAGGU) binds a complementary sequence on the smaller ribosomal subunit helping to form a stable complex between the ribosome and mRNA. The role of this sequence was first proposed by John Shine (1946) and Lynn Dalgamo (1935).