The base normally used in the synthesis of RNA but not DNA is uracil (U).

Here's why:

* RNA uses uracil: RNA uses uracil to pair with adenine (A).

* DNA uses thymine: DNA uses thymine (T) instead of uracil to pair with adenine.

Uracil is structurally similar to thymine, but it lacks a methyl group on its ring. This difference is significant for the stability of the genetic material, as uracil is more susceptible to degradation than thymine.