<Table> <Tr> <Td> </Td> </Tr> </Table> <P> Hydrogen bonding is the chemical interaction that underlies the base - pairing rules described above . Appropriate geometrical correspondence of hydrogen bond donors and acceptors allows only the "right" pairs to form stably . DNA with high GC - content is more stable than DNA with low GC - content, but, contrary to popular belief, the hydrogen bonds do not stabilize the DNA significantly, and stabilization is mainly due to stacking interactions . </P> <P> The larger nucleobases, adenine and guanine, are members of a class of double - ringed chemical structures called purines; the smaller nucleobases, cytosine and thymine (and uracil), are members of a class of single - ringed chemical structures called pyrimidines . Purines are complementary only with pyrimidines: pyrimidine - pyrimidine pairings are energetically unfavorable because the molecules are too far apart for hydrogen bonding to be established; purine - purine pairings are energetically unfavorable because the molecules are too close, leading to overlap repulsion . Purine - pyrimidine base pairing of AT or GC or UA (in RNA) results in proper duplex structure . The only other purine - pyrimidine pairings would be AC and GT and UG (in RNA); these pairings are mismatches because the patterns of hydrogen donors and acceptors do not correspond . The GU pairing, with two hydrogen bonds, does occur fairly often in RNA (see wobble base pair). </P> <P> Paired DNA and RNA molecules are comparatively stable at room temperature but the two nucleotide strands will separate above a melting point that is determined by the length of the molecules, the extent of mispairing (if any), and the GC content . Higher GC content results in higher melting temperatures; it is, therefore, unsurprising that the genomes of extremophile organisms such as Thermus thermophilus are particularly GC - rich . On the converse, regions of a genome that need to separate frequently--for example, the promoter regions for often - transcribed genes--are comparatively GC - poor (for example, see TATA box). GC content and melting temperature must also be taken into account when designing primers for PCR reactions . </P>

What are the four dna bases and what is their bonding pattern
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