The structural characteristic seen in RNA but not DNA is the presence of uracil (U) instead of thymine (T).

Here's a breakdown:

* RNA: Contains adenine (A), guanine (G), cytosine (C), and uracil (U).

* DNA: Contains adenine (A), guanine (G), cytosine (C), and thymine (T).

Uracil and thymine are very similar in structure, but uracil lacks a methyl group present in thymine. This difference is significant in terms of base pairing:

* In RNA, uracil pairs with adenine (A-U).

* In DNA, thymine pairs with adenine (A-T).