Question

According to Watson and Crick's model of DNA, sequencing the deoxyribonucleotides are joined together by phosphodiester bond between

• A. 3' hydroxyl of deoxyribose sugar of nucleotide and a free 5' phosphate group on sugar residue of other nucleotide
• B. 5' hydroxyl of sugar of a nucleotide of a free 3' phosphate group on sugar residue of other nucleotide
• C. Both A and B
• D. None of the above

Answer 1

The correct option is A 3' hydroxyl of deoxyribose sugar of nucleotide and a free 5' phosphate group on sugar residue of other nucleotide

Two deoxyribonucleotides are joined together by phosphoryl group transfer. The 3’ hydroxyl group of the nucleotide at the 3’ end of the growing strand serve as nucleophile. Nucleophilic attack occurs at the $\alpha$-phosphorus of the incoming deoxynucleotide with release of inorganic pyrophosphate. Nucleotides have phosphate group at their 5’ carbon, the 3’ carbon has hydroxyl group; thus, the phosphoester bond cannot be formed between 5’ hydroxyl and 3’ phosphate group, option B is incorrect. Correct answer is A.