Single stranded nucleic acids shouldnt melt as opposed to double strands, but have a melting temperature. How is that so?
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I have never ever heard anyone claim that ssDNA "melts". Two options for what this might mean:
Option 1: "Melting" dsDNA refers to increasing the temperature so the weak hydrogen bonds that bind DNA strands together come apart. This is a colloquial use of the term, since melt literally means "transform from a solid to a liquid". It is possible that someone is referring to the literal "melting" temperature of DNA, the point at which DNA becomes a liquid. Physically, such a thing must be possible, although it would be at such a high temperature (and you'd need so much DNA to see it happen) it's weird to think about.
Option 2: Long segments of ssDNA will sometimes fold back on themselves, make large "loops" (this is what holds tRNA in its traditional cruciform shape). You may be thinking of the temperature at which these loops come apart. It would be lower than dsDNA, because there are fewer hydrogen bonds (the two strands don't match perfectly) and there are likely to be some internal mismatches.
Option 1: "Melting" dsDNA refers to increasing the temperature so the weak hydrogen bonds that bind DNA strands together come apart. This is a colloquial use of the term, since melt literally means "transform from a solid to a liquid". It is possible that someone is referring to the literal "melting" temperature of DNA, the point at which DNA becomes a liquid. Physically, such a thing must be possible, although it would be at such a high temperature (and you'd need so much DNA to see it happen) it's weird to think about.
Option 2: Long segments of ssDNA will sometimes fold back on themselves, make large "loops" (this is what holds tRNA in its traditional cruciform shape). You may be thinking of the temperature at which these loops come apart. It would be lower than dsDNA, because there are fewer hydrogen bonds (the two strands don't match perfectly) and there are likely to be some internal mismatches.