<P> The stability of the dsDNA form depends not only on the GC - content (% G, C basepairs) but also on sequence (since stacking is sequence specific) and also length (longer molecules are more stable). The stability can be measured in various ways; a common way is the "melting temperature", which is the temperature at which 50% of the ds molecules are converted to ss molecules; melting temperature is dependent on ionic strength and the concentration of DNA . As a result, it is both the percentage of GC base pairs and the overall length of a DNA double helix that determines the strength of the association between the two strands of DNA . Long DNA helices with a high GC - content have stronger - interacting strands, while short helices with high AT content have weaker - interacting strands . In biology, parts of the DNA double helix that need to separate easily, such as the TATAAT Pribnow box in some promoters, tend to have a high AT content, making the strands easier to pull apart . </P> <P> In the laboratory, the strength of this interaction can be measured by finding the temperature necessary to break the hydrogen bonds, their melting temperature (also called T value). When all the base pairs in a DNA double helix melt, the strands separate and exist in solution as two entirely independent molecules . These single - stranded DNA molecules have no single common shape, but some conformations are more stable than others . </P> <P> A DNA sequence is called "sense" if its sequence is the same as that of a messenger RNA copy that is translated into protein . The sequence on the opposite strand is called the "antisense" sequence . Both sense and antisense sequences can exist on different parts of the same strand of DNA (i.e. both strands can contain both sense and antisense sequences). In both prokaryotes and eukaryotes, antisense RNA sequences are produced, but the functions of these RNAs are not entirely clear . One proposal is that antisense RNAs are involved in regulating gene expression through RNA - RNA base pairing . </P> <P> A few DNA sequences in prokaryotes and eukaryotes, and more in plasmids and viruses, blur the distinction between sense and antisense strands by having overlapping genes . In these cases, some DNA sequences do double duty, encoding one protein when read along one strand, and a second protein when read in the opposite direction along the other strand . In bacteria, this overlap may be involved in the regulation of gene transcription, while in viruses, overlapping genes increase the amount of information that can be encoded within the small viral genome . </P>

Who proposed that in any dna molecule a =t and g = c