Question

A sequence of pictures of polypeptides synthesis shows a ribosome holding two transfer iRNAs. One iRNA has a polypeptide chain attached to it, the other iRNA has a single amino acid attached to it. What does the next picture show?
A. The polypeptide chain moves over and bonds with the single amino acid
B. The iRNA with the amino acid leaves the ribosomes
C. The amino acid moves over and bonds with the polypeptide chain
D. The iRNA with the polypeptide chain leaves the ribosome

Answer 1

Hint:- Translation is the process in which genetic message carried by messenger RNA from the DNA is converted in the form of a polypeptide chain having a specific sequence of amino acids i.e. RNA to protein.

Complete step-by-step solution:- The protein synthesis process through translation requires ribosomes, amino acids, mRNA, tRNAs and aminoacyl tRNA synthetases. The protein synthesis machinery has three important steps- initiation, elongation, and termination.
Initiation-
- It requires factors called initiation factors and GTP.
- mRNA attaches itself to a smaller subunit of ribosome in the region of tunnels.
- Aminoacyl tRNA complex specific for the initiation codon reaches the P-site.
- Anticodon of tRNA establishes temporary hydrogen bonds with initiation codon of mRNA.
Elongation-
- It is the polypeptide chain formation step.
- In this, an aminoacyl tRNA complex reaches the A-site and attaches to mRNA codon next to the initiation codon with the help of anticodon. It requires GTP and elongation factor.
- A peptide bond is established between the carboxyl group of first amino acid to amino group of second amino acid i.e. A-site amino acid.
- After the formation of first peptide linkage and slipping of the freed tRNA of P-site, the ribosomes mRNA rotates slightly. This process is called translocation. It requires a factor called translocase and energy from GTP.
- As a result of translocation, the A-site codon along with peptidyl tRNA complex reaches the P-site. A new codon is then exposed at the A-site. It attracts a new aminoacyl tRNA complex. This process of bond formation and translocation is repeated.
Termination-
- The synthesis of polypeptide terminates when a nonsense codon of mRNA is exposed.
- The release factor binds to the stop codon and terminates the translation.

Correct option: 4.The polypeptide chain moves over and bonds with the single amino acid

Note:- On being released from the ribosome, a polypeptide has only primary structure. It coils and folds further to have secondary and tertiary structure. The polypeptide may get associated with other polypeptides to produce Beta-pleated structure which then forms tertiary structure and quaternary structure.