It's important for the cell to distinguish message
(RNA) from gene (DNA) in order to preserve the
genes intact in an environment where the message
is continually broken down and resynthesized.
Substituting U for T provides another criterion of
specificity to ensure that RNAases will not attack
the DNA sequence.
U and T also have
somewhat different H bonding potentials that have
minor impact in a double helix but have a more
significant effect for a single stranded molecule
that will form short regions of base pairing.
This is important in RNA secondary structure, e.g.
the cloverleafs of tRNA.
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