Indexes to the reassociation and stability of solution DNA hybrids

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Abstract
The computation, assumptions, and properties of DNA-hybrid stability and reassociation indexes were reviewed. Different methods of computing the same index typically yielded similar values. However, because dissociation curves change from asymmetric to symmetric as increasingly divergent DNAs are compared, adequate determination of mode required fitting a complex function. Delta Tm, delta mode, and delta T50H correlated well up to ca. 12, and all were found to be useful indexes of genomic similarity in that range. They also exhibited similar levels of error, even though T50H comprises a percent reassociation component with relatively large variance. At greater distances, the delta Tm scale became markedly compressed because of the boundary imposed by the temperature of hybrid formation (incubation temperature). Though not compressed or technically limited by it, delta mode and delta T50H could not be extrapolated with certainty below the incubation temperature. Among theoretical problems discussed: Tm and mode index an increasingly small percentage of the genome as the extent of reassociation decreases, and they may compare different genomic segments as DNAs become highly diverged. T50H relies upon the assumptions that all sequences evolve at a constant rate and that reassociation behavior is the same among all sequences regardless of their extent of divergence. Tm and T50H may be biased by selfhybridization of repetitive elements or cross-hybridization of paralogous sequences. Delta mode is free of such biases as long as the genomes under comparison are not too diverged. No index was found to be best in all circumstances.