Question

Which RNA molecule with the following feature used for cDNA synthesis?

• A. RNA with highly methylated region near to 3' end.
• B. RNA with leader sequence at 5' end.
• C. RNA with -CCA sequence at 3' end.
• D. RNA with poly(A) tail at 3' end.

Hint:

Oligo(dT) primers specifically target the 3' poly(A) tail of eukaryotic mRNA, ensuring that only protein-coding messenger RNAs are transcribed into cDNA during library construction.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Complementary DNA (cDNA) is synthesized from a single-stranded RNA template (typically mature mRNA) in a reaction catalyzed by the enzyme reverse transcriptase.
To initiate this synthesis, a primer is required to hybridize to the RNA template.
Detailed Explanation:
- Eukaryotic messenger RNAs (mRNAs) uniquely undergo post-transcriptional polyadenylation, resulting in a poly(A) tail (a stretch of 100 to 250 adenine residues) at their 3' end.
- In recombinant DNA technology, this poly(A) tail is exploited as a universal priming site.
- A synthetic oligo(dT) primer (a short sequence of deoxythymidine residues) is designed to hybridize specifically to the poly(A) tail of the mRNA.
- Once hybridized, the reverse transcriptase utilizes the 3'-OH of the oligo(dT) primer to initiate the synthesis of the first cDNA strand in the 5'-to-3' direction.
- This method allows the selective synthesis of cDNA from protein-coding mRNAs, excluding highly abundant structural RNAs like rRNAs and tRNAs which lack poly(A) tails.

Step 2: Final Answer:
RNA containing a poly(A) tail at the 3' end is used for cDNA synthesis, corresponding to option (D).