Correct answer: B, C, and D

Synthetic oligonucleotides are short DNA or RNA sequences chemically synthesized for molecular biology applications, and standard phosphoramidite synthesis extends chains from the 5′ to 3′ end. Options B, C, and D accurately describe their uses and synthesis direction, while A is incorrect.​

Option Analysis

• (A) Incorrect: Chemical synthesis uses solid-phase phosphoramidite chemistry where the 3′-OH nucleophile attacks the 5′-phosphoramidite, adding nucleotides 5’→3′. Enzymatic synthesis follows this direction too, but chemical methods dominate for oligonucleotides.​

• (B) Correct: Synthetic oligonucleotides serve as primers in site-directed mutagenesis, annealing to target DNA to introduce specific mutations via PCR or extension.​

• (C) Correct: Standard synthesis proceeds 5’→3′, starting from a 3′-attached nucleoside on solid support.​

• (D) Correct: These oligonucleotides can incorporate radiolabels (e.g., 32P, 125I) at ends or internally for use as hybridization probes in blotting or imaging.​

Introduction to Synthetic Oligonucleotides

Synthetic oligonucleotides are custom short nucleic acid chains (15-100 bases) vital in molecular biology, produced via automated chemical synthesis for CSIR NET Life Sciences topics like genetic engineering. Key applications include PCR primers, probes, and mutagenesis tools. Understanding synthesis direction and uses clarifies MCQs on synthetic oligonucleotides chemical synthesis direction.​

Chemical Synthesis Direction

Phosphoramidite method dominates, building DNA from 5’→3′: a 3′-bound nucleoside on beads reacts with protected 5′-phosphoramidite monomers, extending via nucleophilic attack. The 5′-DMT group protects the growing end, removed iteratively; reverse 3’→5′ is rare due to chemistry constraints. This matches option C, refuting A in CSIR NET questions.​

Site-Directed Mutagenesis Applications

Synthetic oligonucleotides hybridize to plasmid DNA with a mismatch, enabling PCR-based mutagenesis (e.g., QuikChange method). They introduce precise point mutations, deletions, or insertions, essential for protein engineering—directly supporting option B.​

Radiolabeled Probes Utility

Oligonucleotides labeled with 32P, 35S, or iodine detect specific sequences via hybridization in Southern/Northern blots or microarrays. End-labeling or internal incorporation makes option D correct for diagnostics and gene expression studies.​

CSIR NET Exam Insights

This MCQ tests synthetic oligonucleotides in molecular biology: focus on phosphoramidite chemistry (5’→3′), mutagenesis protocols, and probe labeling for Part B/C questions. Practice similar PYQs reinforces concepts across genetics and biotech syllabus units.​