Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): DNA has evolved from RNA with chemical modifications that make it more stable.
Reason (R): RNA, being a catalyst, was reactive and hence unstable.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
NCERT holds that RNA was the first genetic material. Essential life processes such as metabolism, translation and splicing evolved around RNA, and RNA acted both as genetic material and as a catalyst.
Assertion (A): DNA has evolved from RNA with chemical modifications that make it more stable. — True.
Reason (R): RNA, being a catalyst, was reactive and hence unstable. — True.
A is about where DNA came from and why it was changed chemically. R is about the weakness of RNA as a store of genetic information.
Why R explains A? Because RNA worked as a catalyst, it was reactive and so unstable, which is a drawback for a molecule that must keep genetic information unchanged. DNA evolved from RNA with chemical modifications that made it more stable, which overcomes exactly this drawback. NCERT joins the two with “Therefore”, so R is the correct explanation of A.
NCERT Extract:
RNA World – “… RNA was the first genetic material. There is now enough evidence to suggest that essential life processes (such as metabolism, translation, splicing, etc.), evolved around RNA. RNA used to act as a genetic material as well as a catalyst (there are some important biochemical reactions in living systems that are catalysed by RNA catalysts and not by protein enzymes). But, RNA being a catalyst was reactive and hence unstable. Therefore, DNA has evolved from RNA with chemical modifications that make it more stable.”