Question:
Assertion (A) : The two chains of DNA have antipolarity.
Reason (R) : In one chain of DNA, ribose sugar at \(5^{\prime}\) end consists of a free phosphate moiety, while at the other end, the ribose sugar has free \(3^{\prime}\) - OH group.
Assertion (A) : The two chains of DNA have antipolarity.
Reason (R) : In one chain of DNA, ribose sugar at \(5^{\prime}\) end consists of a free phosphate moiety, while at the other end, the ribose sugar has free \(3^{\prime}\) - OH group.
Show Hint
DNA has deoxyribose sugar, so the reason that names ribose is false while the assertion about antipolarity is true.
Updated On: Oct 23, 2026
- Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
- Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
- Assertion (A) is true, but Reason (R) is false.
- Assertion (A) is false, but Reason (R) is true.
Show Solution
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The Correct Option is C
Solution and Explanation
Step 1: Check the Assertion
The two strands of DNA run in opposite directions. One runs \( 5^{\prime} \rightarrow 3^{\prime} \) and the other runs \( 3^{\prime} \rightarrow 5^{\prime} \). This is called antiparallel orientation or antipolarity. So (A) is true.
Step 2: Check the Reason
The sugar of DNA is deoxyribose and not ribose. Ribose is the sugar of RNA. The statement describes the free phosphate at the \( 5^{\prime} \) end and the free -OH at the \( 3^{\prime} \) end correctly, but it names the wrong sugar. So (R) is false as printed.
Step 3: Choose the option
A is true and R is false. This is option (C), which is also the official marking scheme key.
Final Answer:
Option (C): Assertion (A) is true, but Reason (R) is false.
The two strands of DNA run in opposite directions. One runs \( 5^{\prime} \rightarrow 3^{\prime} \) and the other runs \( 3^{\prime} \rightarrow 5^{\prime} \). This is called antiparallel orientation or antipolarity. So (A) is true.
Step 2: Check the Reason
The sugar of DNA is deoxyribose and not ribose. Ribose is the sugar of RNA. The statement describes the free phosphate at the \( 5^{\prime} \) end and the free -OH at the \( 3^{\prime} \) end correctly, but it names the wrong sugar. So (R) is false as printed.
Step 3: Choose the option
A is true and R is false. This is option (C), which is also the official marking scheme key.
Final Answer:
Option (C): Assertion (A) is true, but Reason (R) is false.
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