Why is RNA single stranded, why isn it double stranded like DNA?
This is more of a philosophical question than a biochemical question. There are both double stranded RNAs and single-stranded DNAs in biological situations (e.g. certain viral genomes exist transiently as each). Teleologically thinking though, one might guess that DNA is double stranded because it is the genetic material and by having two complementary strands, an error or mutation to one strand will not necessary effect the genetic information (because there is always a backup copy to code for repair of the other strand). In contrast, most RNAs function either as structural molecules (rRNAs, tRNAs) or as transient messangers (mRNAs), contexts in which it is not essential that high fidelity be maintained (if an mRNA gets altered between its synthesis and its translation, you can always make another one). For the structural molecules, it is hard to do much if you are locked into a double helix. By being single stranded, rRNAs and tRNAs are able to fold into complex three dimensional str
