Question

Assertion :Synthesis of daughter or new strand occurs continuously along the parent 3 $\to$ 5 strand. Reason: DNA polymerase can polymerise nucleotides in 3 $\to$ 5 direction on 5 $\to$ 3 strand.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Both Assertion and Reason are incorrect

Answer 1

The correct option is C Assertion is correct but Reason is incorrect

DNA-polymerase can polymerize nucleotides only in 5 → 3 direction on 3 → 5 strand because it adds them at the 3 end. Replication over the two templates proceeds in opposite directions. One strand with polarity 3 → 5 forms its complementary strand continuously because 3 end of latter is always open for elongation. It is called leading strand. Replication is discontinuous on the other template with polarity 5 - 3 because only a short segment of DNA strand can be built in 5 → 3 direction due to exposure of a small stretch of template at one time. Short segments of replicated DNA are called Okazaki fragments. DNA strand built up of Okazaki fragments is called lagging strand. As one strand grows continuously while the other strand is formed discontinuously, DNA replication is semi-discontinuous.

So, the correct answer is 'Assertion is correct but Reason is incorrect'.