Practice question

Eukaryotic messenger RNAs possess a post-transcriptional poly(A) tail of 100-250 adenine residues at the 3' end, essential for stability, nuclear export, and efficient translation. Oligo(dT) primers consisting of 12-18 thymidylates anneal specifically to this poly(A) stretch through A-T base pairing, providing a free 3' hydroxyl for reverse transcriptase to initiate complementary DNA synthesis. This ensures selective reverse transcription of mRNA rather than abundant rRNA or tRNA contaminants. Random hexamers prime throughout transcripts, while oligo(dT) yields full-length, 3'-biased cDNA libraries enriched for eukaryotic coding sequences.