Question

In DNA molecule at the 5$'$ end there is a free ___________.

• A. oxygen
• B. nitrogen
• C. phosphate
• D. hydroxyl

Hint:

Always remember: {5$'$ end → phosphate} and {3$'$ end → hydroxyl}.

Answer 1

The Correct Option is C

Step 1: Understanding DNA polarity.
DNA strands have directionality due to the sugar-phosphate backbone. Each strand has a 5$'$ end and a 3$'$ end.
Step 2: Chemical groups at DNA ends.
At the 5$'$ end of DNA, a free phosphate group is present, whereas the 3$'$ end has a free hydroxyl (–OH) group.
Step 3: Analysis of options.
(A) oxygen: Incorrect. Oxygen is not present as a free terminal group.
(B) nitrogen: Incorrect. Nitrogen is part of nitrogenous bases, not terminal groups.
(C) phosphate: Correct. The 5$'$ end carries a free phosphate group.
(D) hydroxyl: Incorrect. Hydroxyl group is present at the 3$'$ end.