We earlier imagined DNA as an
instruction book. Let's even make it a reference
book. When you need information about something you
make a copy of the pages (genes) you're interested
in, returning the book to the library. This way you
don't have to risk losing or destroying the book.
In all eucaryotic cells DNA
never leaves the nucleus, instead the genetic code
(the genes) is copied into RNA which then in turn is
decoded (translated) into proteins in the cytoplasm.
Why? Wouldn't it be smarter if DNA itself was
translated into proteins in the cytoplasm instead of
using a RNA intermediate?
The answer, for many reasons, is no. One important
reason is security. The cytoplasm is a dangerous
environment for the DNA and the daily transcription
of genes to proteins would be very harmful to the
DNA, which has to stay intact in order to maintain
life. Therefore, RNA works as a sort of throw-away
version of DNA (like the copies from the reference
book) - good for limited work but not for long-term
storage. Another reason is to regulate the rate of
protein synthesis. This will be further discussed in
the section about protein-translation.