The nucleotide that occurs in DNA molecules but not RNA molecules is thymine (T).

Here's why:

* DNA uses the four nucleotides: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).

* RNA uses the four nucleotides: Adenine (A), Guanine (G), Cytosine (C), and Uracil (U).

Thymine is replaced by Uracil in RNA.