<P> Complementarity can be found between short nucleic acid stretches and a coding region or an transcribed gene, and results in base pairing . These short nucleic acid sequences are commonly found in nature and have regulatory functions such as gene silencing . </P> <P> Antisense transcripts are stretches of non coding mRNA that are complementary to the coding sequence . Genome wide studies have shown that RNA antisense transcripts occur commonly within nature . They are generally believed to increase the coding potential of the genetic code and add an overall layer of complexity to gene regulation . So far, it is known that 40% of the human genome is transcribed in both directions, underlining the potential significance of reverse transcription . It has been suggested that complementary regions between sense and antisense transcripts would allow generation of double stranded RNA hybrids, which may play an important role in gene regulation . For example, hypoxia - induced factor 1α mRNA and β - secretase mRNA are transcribed bidirectionally, and it has been shown that the antisense transcript acts as a stabilizer to the sense script . </P> <P> miRNAs, microRNA, are short RNA sequences that are complementary to regions of a transcribed gene and have regulatory functions . Current research indicates that circulating miRNA may be utilized as novel biomarkers, hence show promising evidence to be utilized in disease diagnostics . MiRNAs are formed from longer sequences of RNA that are cut free by a Dicer enzyme from an RNA sequence that is from a regulator gene . These short strands bind to a RISC complex . They match up with sequences in the upstream region of a transcribed gene due to their complementarity to act as a silencer for the gene in three ways . One is by preventing a ribosome from binding and initiating translation . Two is by degrading the mRNA that the complex has bound to . And three is by providing a new double - stranded RNA (dsRNA) sequence that Dicer can act upon to create more miRNA to find and degrade more copies of the gene . Small interfering RNAs (siRNAs) are similar in function to miRNAs; they come from other sources of RNA, but serve a similar purpose to miRNAs . Given their short length, the rules for complementarity means that they can still be very discriminating in their targets of choice . Given that there are four choices for each base in the strand and a 20bp - 22bp length for a mi / siRNA, that leads to more than 7012100000000000000 ♠ 1 × 10 possible combinations . Given that the human genome is ~ 3.1 billion bases in length, this means that each miRNA should only find a match once in the entire human genome by accident . </P> <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>

When making mrna adenine bonds to which base
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