The correct answer is Option (2) → 5'
- A nucleoside = Nitrogenous base + Sugar (ribose or deoxyribose)
- A nucleotide = Nucleoside + Phosphoric acid
The phosphoric acid is always attached to the 5′-hydroxyl (–OH) group of the sugar moiety through a phosphoester linkage.
- In DNA/RNA backbone formation, the phosphodiester bond connects the 5′ phosphate of one nucleotide to the 3′ –OH of the next nucleotide.
- Hence, the phosphate group is linked at the 5′ position of the sugar to form a nucleotide.
Why not the others?
- 3′: Used to form the next phosphodiester bond, not the initial attachment of phosphate.
- 2′: Present only in ribose (RNA), but not used for phosphate linkage (in DNA, 2′ has H).
- 1′: Attached to the nitrogenous base.
|
