Which method uses poly(A) tail as a handle to capture mRNA?
3 prime RACE aims to capture unknown downstream sequence extending to messenger RNA terminus. Most eukaryotic messenger RNAs possess post-transcriptional poly-adenine tail of 100-250 residues at 3 prime end added by poly-A polymerase, important for stability and translation. This homopolymeric feature serves as universal handle for selective priming. An oligo-dT primer containing adapter sequence anneals to poly-A tail during reverse transcription, enabling synthesis of complementary DNA spanning entire 3 prime untranslated region. Subsequent PCR amplification using gene-specific forward primer and adapter reverse primer amplifies unknown region between known sequence and poly-A junction.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.