I know that there are two most important directions of genetic information transfer in living organisms: DNA->DNA and DNA->RNA. The first is replication, and the second is transcription. I wonder if there is a reason for this choice of directions. According to this article, all other directions are possible. Why do we use DNA for example? RNA is capable of self-replication since it happens in viruses. And why do we use RNA, not DNA, as messenger molecules? Is it just an accident or is it possible to explain why this is the right way of doing it?
DNA is more chemically stable than RNA, which makes it ideal for long-term storage. RNA viruses like HIV have a short lifespan and must replicate to survive, which is why they can get by with a less chemically stable genome.
RNA is a useful format to transcribe since it has multiple forms and functions (e.g. rRNA, mRNA, tRNA, siRNA, snRNA, miRNA, etc.). RNA can sometimes function like a protein in which it carries out cellular actions without needing to be translated. It has been hypothesized that RNA were the first molecules as precursors to life since they can function for both storage and action. The theory is that RNA was the first molecule but was then able to be translated into proteins (which were more variable/useful) and able to be stored as DNA (which was more stable as a storage medium).
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If you had a complex life form which used only DNA or RNA, it would have no way to tell transcribed mXNA from genomic gXNA. This would cause problems during cellar replication, as you could also replicate your mXNA along with your gXNA. It would also cause problems repairing breaks in your gXNA, as you would run the risk of including mXNA during the repair process.
Therefore it seems advantageous to have a storage system for information which is not currently being translated into protein (i.e., DNA), as compared to just having RNA.
On the other hand a completely DNA organism would need RNA for functional ribosomes anyway. If RNA is being used for ribosomes, using it for mRNA as well to avoid confusion with genomic DNA seems advantageous.
In order to test this hypothesis, you would need to create a completely RNA/DNA based lifeform and investigate its properties. Short of starting life from scratch as RNA based and monitoring its evolution over a few million years, a conclusive proof as to why things are the way they are as opposed to being down to an evolutionary historical accident is difficult to obtain!