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I have seen in textbooks referring to ds RNA and ssDNA. How a RNA can be double stranded and likewise how a DNA can single stranded and if they do exist why are there names not interchanged?

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The main differences between RNA (ribonucleic acid) and DNA (deoxyribonucleic acid) is that DNA contains a hydrogen atom where RNA has a hyroxyl (-OH) group and RNA's uracil is substituted for DNA's thymine. The below image shows several deoxyribose molecules bound together, and off to the side, a ribose molecule. It is the OH group on the bottom right of the ribose molecule that is exchanged for a hydrogen in deoxyribose. enter image description here The structures of RNA and DNA are almost identical, so there is no reason why you can't have double stranded RNA. Similarly DNA can be single stranded. In fact, for DNA replication and transcription, the DNA strands must be separated. dsRNA and ssDNA are most commonly found in viruses. Check out these lecture slides for more about them: http://www.columbia.edu/itc/hs/medical/pathophys/id/2009/viruses2Color.pdf

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  • $\begingroup$ "DNA contains one less oxygen atom" — please edit your answer to be more precise. The naive reader would have no idea what you mean, and the difference between ribose and deoxy-ribose is what produces different structures of dsDNA and dsRNA. $\endgroup$ – David Jan 9 at 20:46
  • $\begingroup$ edited for clarity, though the reason I added the image originally was to provide the reader with a fairly self-explanatory comparison. $\endgroup$ – Astrolamb Jan 11 at 0:08
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Complementary base-pairing is possible for RNA

As the poster is no doubt aware, the double-stranded structure of DNA is stabilized by A–T and G–C base pairs. He will also be aware that in transcription of DNA to RNA (in which A is replaced by U) A–U base pairing occurs. Hence, there is no physico-chemical reason why dsRNA cannot occur between suitable complementary strands.

Viruses provide examples of single- and double-stranded DNAs and RNAs

As is mentioned in another answer, the genomes of bacterial and eukaryotic viruses provide examples of dsDNA, ssDNA, dsRNA and ssRNA.

It takes two to tangle

Whether or not a particular DNA or RNA is double-stranded or single-stranded in Nature depends on whether both complementary strands are present. That depends on biology, not chemistry. Thus, a virus that is single stranded has evolved a replicative strategy whereby many copies of one strand are copied from a few copies of the complementary template. In double-stranded viruses (and organisms) the replication method starts with a double-stranded genome and makes a copy of each at the same time, resulting in equal numbers. (They are also synthesised in a way that allows them immediately to hydbridize.)

Nature is full of black swans

The fact that most DNA encountered is double-stranded and most RNA encountered is single-stranded, just reflects the nature of the genome and translation apparatus of cellular organisms. But transfer-RNA and ribosomal-RNA (and mRNAs) have double-stranded helical portions, and there are a variety of less abundant species of RNAs with individual functions that may involve helix formation. And, of course there are also DNA/RNA hybrids, for example the Okazaki fragments that occur during DNA replication.

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I'll try a third pedagogical approach, the above answers are sufficient.

The typical college-entry level textbooks on biology will state that DNA is double-stranded and RNA is single-stranded. This is misleading later on, especially when you encounter an instance of single-stranded DNA and double-stranded RNA.

The posters above have already answered your question well - dsRNA and ssDNA exist all over the tree of life. However, I'd like to emphasize that they also exist transiently all the time in every single cell...

3 critical examples:

  1. DNA must unwind and become single-stranded for DNA replication.
  2. The same is true for transcription.
  3. RNA has a tertiary structure too. Ribozymes rely on this. However, the most abundant RNA species in all cells is ribosomal RNA. It forms these structures:

rRNA

These are 'single-stranded', but are capable of dimerizing with another RNA species if there is sufficient complementarity between local regions. Below you see many RNA hairpins and loops and locally these are double-stranded, even though it's a single molecule.

In situ hybridization is also another common technique in biology, it takes advantage of pretty much all 'exceptions to the rule' you mention. Nowadays, RNA is increasingly probed. Here is a simple diagram of how it works:

FISH

If they do exist why are there names not interchanged?

The names for RNA and DNA are not interchanged because they are different chemically! They just share features, most notably the ability to complement and hybridize to themselves and other nucleic acids, given enough complementarity.

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