<P> Kissing hairpins are formed when a single strand of nucleic acid complements with itself creating loops of RNA in the form of a hairpin . When two hairpins come into contact with each other in vivo, the complementary bases of the two strands form up and begin to unwind the hairpins until a double - stranded RNA (dsRNA) complex is formed or the complex unwinds back to two separate strands due to mismatches in the hairpins . The secondary structure of the hairpin prior to kissing allows for a stable structure with a relatively fixed change in energy . The purpose of these structures is a balancing of stability of the hairpin loop vs binding strength with a complementary strand . Too strong of an initial binding to a bad location and the strands will not unwind quickly enough . Too weak of an initial binding and the strands won't ever fully form the desired complex . These hairpin structures allow for the exposure of enough bases to provide a strong enough check on the initial binding and a weak enough internal binding to allow the unfolding once a favorable match has been found . </P> <P> Complementarity allows information found in DNA or RNA to be stored in a single strand . The complementing strand can be determined from the template and vice versa as in cDNA libraries . This also allows for analysis, like comparing the sequences of two different species . Shorthands have been developed for writing down sequences when there are mismatches (ambiguity codes) or to speed up how to read the opposite sequence in the complement (ambigrams). </P> <P> A cDNA library is a collection of expressed DNA genes that are seen as a useful reference tool in gene identification and cloning processes . cDNA libraries are constructed from mRNA using RNA - dependent DNA polymerase reverse transcriptase (RT), which transcribes an mRNA template into DNA . Therefore, a cDNA library can only contain inserts that are meant to be transcribed into mRNA . This process relies on the principle of DNA / RNA complementarity . The end product of the libraries is double stranded DNA, which may be inserted into plasmids . Hence, cDNA libraries are a powerful tool in modern research . </P> <P> When writing sequences for systematic biology it may be necessary to have IUPAC codes that mean "any of the two" or "any of the three". The IUPAC code R (any purine) is complementary to Y (any pyrimidine) and M (amino) to K (keto). W (weak) and S (strong) are usually not swapped but have been swapped in the past by some tools . W and S denote "weak" and "strong", respectively, and indicate a number of the hydrogen bonds that a nucleotide uses to pair with its complementing partner . A partner uses the same number of the bonds to make a complementing pair . </P>

Which sequence of bases would be found on the complementary strand of mrna