RNA Synthesis, Processing and Modification MCQs

Ans. is 'B' i.e., Small nuclear RNA [Ref Harper 29th/e p. 378, 390] Splicesome Splicesome is an assembly made up of small nuclear RNA (snRNA), some proteins and hnRNA. snRNA combines with proteins to form small nuclear ribnonucleoprotein particles (snRNPs or snurps) that mediate splicing. It is snRNA component of snurps that catalyzes splicing°. Snurps are U,,U,,U3,U4, U5 and U6

Option A: Tetracycline prevents the binding of aminoacyl-tRNAs to the bacterial ribosome A site. Option B: Chloramphenicol works by binding to 23S rRNA, which is interesting in view of the newly appreciated role of rRNA in peptide bond formation through its peptidyl transferase activity. Option C: Puromycin is a structural analogue of tyrosyl-tRNA. Puromycin is incorporated via the A site on the ribosome into the carboxyl terminal position of a peptide but causes the premature release of the polypeptide. Puromycin, as a tyrosyl-tRNA analog, effectively inhibits protein synthesis in both prokaryotes and eukaryotes. Option D: Cycloheximide inhibits peptidyl transferase in the 60S ribosomal subunit in eukaryotes, presumably by binding to an rRNA component. Reference: Harper's illustrated biochemistry 32 nd edition CH 37 PN 407

Option A: Menkes syndrome, characterized by kinky hair and growth retardation, results from a dietary deficiency of copper, an essential cofactor for the enzyme lysyl oxidase that functions in formation of the covalent cross-links that strengthen collagen fiber. Option B & Option D: Lysyl and Prolyl hydroxylases are required for the postsynthetic modification of procollagen to collagen. Both this enzyme requires vitamin C. In vitamin C deficiency(scurvy), leading to skin changes, fragility of blood capillaries, gum decay, tooth loss and bone fracture, which can be attributed to impaired collagen synthesis(leading to delayed wound healing). Option C: Prolyl oxidase does not exist.

Ans. is 'c' i.e., Hydroxylysine [Ref Harper's 29th/e p. 540] Hydroxylation of proline and lysine residue takes place during post- translational modification in rough ER. The enzyme catalyzing the reactions are prolyl hydroxylase (for proline) and lysyl hydroxylase (for lysine). Both these enzymes are dioxygenases° using molecular oxygen (02) and cofactor for both these enzymes is vitamin C (ascorbic acid)°. a- Ketoglutatrate is a coreductant, which is oxidized to succinate.

Explanation: Rifampicin inhibits bacterial RNA synthesis by binding to the DNA-dependent RNA polymerase, effectively stopping transcription. This antibiotic is a critical component of tuberculosis (TB) treatment and is often combined with other drugs to prevent resistance. Its unique mechanism and effectiveness against Mycobacterium tuberculosis make it a first-line drug for TB therapy.

Ans. is 'a' i.e., RNA dependent DNA synthesis [Ref Harper 29thle p. 348] Synthesis of RNA from DNA is called transcription. In transcription, RNA is synthesized by RNA polymerase. RNA polymerase is also called DNA dependent RNA polymerase because it is dependent on DNA (non-coding strand) for RNA synthesis. Reverse transcription, as the name suggests, is the reverse of transcription i.e. synthesis of DNA form RNA. In reverse transcription DNA is synthesized by reverse transcriptase. Reverse transcriptase is also called RNA dependent DNA polymerase because it is dependent on RNA for DNA synthesis.

The process of formation of apo B-48 from apo B is by RNA editing. Reference: Harper's illustrated biochemistry 32 nd edition, page 950

Rifampicin inhibits the initiation of transcription by binding to the beta subunit of prokaryotic RNA polymerase, thus interfering with the formation of the first phosphodiester bond So Option A is correct, rest are incorrect options. Reference: Harper’s illustrated biochemistry 32 nd edition CH 36 PN 376 & Lippincott’s Illustrated Reviews: Biochemistry, 7 th edition (SAE) PN 627.