Oligo thymine

RNA to initiate cDNA synthesis. All cellular mRNA contains multiple repeats of adenine bases (poly-A tails). Therefore the complementary thymine bases (oligo-dT) can be used as a primer that binds to the mRNA template required for the reverse transcriptase to synthesize the cDNA. In the case of pancreatic mRNAs (Figure 4.2), the signihcantly higher mRNA for insulin compared with other proteins allowed success in isolating the insulin-specihc cDNA. Subsequent insertion of cDNA into a bacterial expression vector allowed the production of functional insulin that is now marketed as a successful therapeutic product (Figure 4.2). 

A third alternative starts with an extract of RNA, not DNA. Mature eukaryotic mRNA contains a long run or tail of adenine residues at its 3 end. The poly(rA) tail can be hybridized with an oligomer of thymine residues, and the oligo(dT) can then be used as a primer for a particular kind of DNA polymerase known as reverse transcriptase. This enzyme, a polymerase associated with retroviruses, will use RNA as a template to make a complementary DNA copy of the RNA, creating a DNA-RNA double-stranded hybrid. In another round of synthesis, the enzyme can replace the RNA strand entirely with DNA, so that the RNA-DNA hybrid is completely converted to double-stranded DNA containing an exact copy of the original RNA sequence. This DNA molecule is known as cDNA because it has a strand that is complementary to (or a copy of) the original RNA. 

The MALDI-technique is not limited to proteins/peptides. With a suitable matrix you can also bring DNA and RNA into the gas phase. What for For example, you can sequence oligos by means of oligonucleotide ladders (Limbach et al. 1995). The smallest MW difference between DNA bases—between adenine and thymine—amounts to at least 9 Dalton. For RNA bases, the smallest difference—between cytosine and uracil—lies at 1 Dalton. 

Suda, Y, Kono, M., Inaki, Y, and Takemoto, K., Photochemical reactions on oligo- and polyeth-yleneimine containing pendant thymine bases Effects of spacer, J. Polymer Sci, Polymer Chem. Ed., 22, 2427,1984. 

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