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My very basic understanding is DNA gets transcribed into RNA by effectively replacing thymine with uracil, then RNA is translated to make proteins. Looking at it from an engineering perspective, it would appear that, while the RNA to protein translation is non-trivial, the DNA to RNA transcription process is rather redundant: As it's just a simple replacement, why not start with RNA in the first place? My assumption therefore is that there is something about the RNA molecule (e.g., too reactive, too unstable, etc.) that requires it to be put in "cold storage" as DNA.

I don't have a background in genetics or biology, so could someone ELI5 why DNA is needed?

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closed as off-topic by WYSIWYG, Devashish Das, Bez, Satwik Pasani, Cornelius Aug 7 '14 at 20:32

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Imagine you want to produce a widget. You have thousands of worker, but only one blueprint. Each worker needs the blueprint to build a widget (they're really forgetful and can't build from memory). So only one worker at a time can build your widgets.

What you would do is to create copies of your blueprint and distribute them to your workers. That way multiple workers can produce the widget at the same time.

DNA is the blueprint, and RNA is the copy. Your workers are the ribosomes that translate the RNA into proteins.

This is of course grossly simplified, and there are other reasons for the specific arrangement, but this is certainly one of the reasons why the intermediate RNA is used.

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  • $\begingroup$ To make this analogy work, one has to substitute the "DNA is the blueprint" with "every gene is a blueprint". $\endgroup$ – har-wradim Aug 7 '14 at 12:52
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    $\begingroup$ @har-wradim I intentionally made it only about one widget/gene/protein to avoid making the analogy more complicated than it has to be $\endgroup$ – Mad Scientist Aug 7 '14 at 12:53
  • $\begingroup$ An analogy has to be realistic, otherwise it produces misunderstanding and new questions. $\endgroup$ – har-wradim Aug 7 '14 at 12:58
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RNA was almost certainly the first genetic molecule of inheritance. However, the single-stranded nature of RNA is not particularly stable and thus would not be reliable for the long-term storage of genetic information necessary for reproduction (and ultimately evolution). The necessary stability is provided by DNA.

The question becomes, how did DNA evolve to replace RNA? This is an area of active research but here is a nice summary.

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OK, I found this on Wikipedia. I probably should have checked there first!

DNA has three primary attributes that allow it to be far better than RNA at encoding genetic information. First, it is normally double-stranded, so that there are a minimum of two copies of the information encoding each gene in every cell. Second, DNA has a much greater stability against breakdown than does RNA, an attribute primarily associated with the absence of the 2'-hydroxyl group within every nucleotide of DNA. Third, highly sophisticated DNA surveillance and repair systems are present which monitor damage to the DNA and repair the sequence when necessary. Analogous systems have not evolved for repairing damaged RNA molecules.

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  • $\begingroup$ "Analogous systems have not evolved for repairing damaged RNA molecules" ..because some of these are impossibilities, e.g. the very common C to U deamination couldn't be detected in an RNA genome. This is also the most likely reason why our genomes are made of DNA instead of U-DNA.. $\endgroup$ – 5heikki Aug 7 '14 at 11:20
  • $\begingroup$ No this doesn't explain, why the RNA step is needed. $\endgroup$ – har-wradim Aug 7 '14 at 12:54

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