Why is mRNA needed in the Protein translation?

Question

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.

Answer 1

There are a few things to consider with your question, one which has to do with the assumption of RNA as just an "intermediate" and the other which has to do with cell compartmentalization.

First, on RNA itself: there is a theory that in the origins of life, RNA was the first major molecule to exist (the "RNA World" theory). The reasons for this theory are that RNA can function as an enzyme (for example, RNA, not protein, is the major catalytic portion of the ribosome -- the protein is just a structural scaffold) and as genetic material (think RNA viruses). When scientists consider about how DNA is copied to make proteins, but proteins are necessary to make DNA, they get stuck in a chicken-egg paradox. RNA being able to perform the functions of both DNA and protein offers a possible solution to this problem -- it could be both the chicken AND the egg! Even though the RNA-world theory has its flaws (see: https://biologydirect.biomedcentral.com/articles/10.1186/1745-6150-7-23) , it is generally accepted that RNA most likely existed before DNA, with DNA taking over as genetic material because it is far more stable. Even the process of splicing is mostly driven by RNA, with some organisms having self-splicing introns (no protein required!).

Second, on compartmentalization: With DNA being confined to the nucleus and the ribosome being confined to the cytoplasm, the RNA intermediate is necessary to maintain this compartmentalization, which increases efficiency of both processes. Even bacteria actually compartmentalize their RNA, which can affect processes such as gene regulation (see: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6264921/).