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The original question was to predict the basic requirements for information storage. Then the discussion moved to why is it necessary to include mRNA in the protein translation process. Why can't there be a similar chemical molecule like tRNA which can directly read from DNA and exclude the mRNA intermediate. Also is the current translation process optimal in the sense that can there be a system which does not need mRNA or a similar intermediate altogether. Or is the presence of an intermediate a necessary requirement.

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  • $\begingroup$ Although this does not directly answer the question, I think you could take the inverse point of view and rather ask why/when is DNA needed for the replication of an organism. $\endgroup$ – The Quark Aug 30 '15 at 11:26
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One reason is that an intermediate like mRNA allows for higher amounts of protein expression. You can have multiple mRNA molecules that are translated simultaneously. If you read directly from DNA you can have at most two translations in parallel.

I'm not sure about this, but I would imagine that having to unwind the DNA double strand every time for translation could be a drawback to directly reading from DNA as well. The mRNA is also subject to various forms of regulation, which might be an advantage as well for the added intermediary.

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  • $\begingroup$ Beat me by 15 seconds because I thought I could find more references. $\endgroup$ – user137 Dec 29 '14 at 18:29
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    $\begingroup$ "If you read directly from DNA you can have at most two translations in parallel." - upvoted, but this is not true, you can have many parallel translation in series right after each other in one or many codon distance - depending on the space necessary between them. I think a more accurate explanation, that accessibility would limit the translation speed. $\endgroup$ – inf3rno Dec 29 '14 at 18:38
  • $\begingroup$ Is there data on how fast translation and transcription happen? If translation is slower than transcription (which it might be due to the larger tRNAs diffusing more slowly, having 20 amino acids vs 4 nucleotides, etc), then there would probably be an advantage to quickly transcribe the mRNA from DNA, then pass the mRNA on to ribosomes for the slower translation. Might still be able to do multiple transcriptions at once like inf3rno mentions. $\endgroup$ – user137 Dec 29 '14 at 18:52
  • $\begingroup$ @inf3rno I was thinking about including the fact that multiple ribosomes can read the same mRNA at the same time, but decided to skip that to avoid making it more complicated than necessary. And one codon distance is certainly not possible, the ribosome is quite large. $\endgroup$ – Mad Scientist Dec 29 '14 at 19:21
  • $\begingroup$ @user137 you can perform many translations from a single mRNA. So you don't need to transcribe the DNA into mRNA as often as you translate the mRNA into protein. $\endgroup$ – Mad Scientist Dec 29 '14 at 19:23
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I don't have a lot of references for this, but it's too long for a comment.

Separating the roles of RNA and DNA helps to better control protein production and gene replication. If ribosomes worked directly on DNA, it would probably be very hard to replicate that DNA, as the DNA polymerases would collide with the ribosomes. You'd have to stop protein production and move the ribosomes out of the way before you could copy the DNA. Keeping mRNA separate allows the ribosomes to keep making protein while the DNA is being replicated.

However, transcription of DNA would be similar, where the DNA replication could collide with the transcription machinery. I can't find anything about that situation so I don't know how its regulated or how often it happens.

Additionally, having mRNA allows amplification and further regulation. Every mRNA copy can make several protein copies, so the amount of protein produced from a single round of transcription can be higher than if the DNA were read directly by a ribosome. mRNA also has many binding sites for miRNA and other regulatory RNAs. If DNA were read directly, that wouldn't be possible.

There are also some cases where mRNA is stored and used later, such as immediately after fertilization, or in platelets and erythrocytes where translation can happen even after transcription stops due to loss of the nucleus.

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In lay man terms you need your DNA to remain intact and protected. so instead of shipping it to the cytoplasm where a lot can happen to it, the cell makes an intermediate mRNA. if anything does wrong with mRNA, the cell can discard it and make another copy and after translation the copy of mRNA is also discarded.You also have the amplification; instead of making one protein from one gene on a DNA strand, and then one more and the another, we can just make a bunch of copies of mRNA and send them to the nucleus where we can make as many proteins as we need. by doing this there is minimal manipulation of the original DNA strand and the cell get a lot of product over a short period.

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  • $\begingroup$ "instead of shipping it to the cytoplasm where a lot can happen to it"..... Doesn't apply in case of prokaryotes $\endgroup$ – WYSIWYG Dec 30 '14 at 8:01
  • $\begingroup$ yes thanks for that i was mainly focusing on eukaryotes $\endgroup$ – Carl Cj Nuza Dec 31 '14 at 7:20

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