The statement that describes DNA but not RNA is:

DNA is a double-stranded helix.

Here's why:

* DNA: Deoxyribonucleic acid is a double helix, meaning it consists of two strands of nucleotides that wind around each other like a twisted ladder.

* RNA: Ribonucleic acid is typically single-stranded, although it can fold into complex three-dimensional structures.

Let me know if you'd like to explore more differences between DNA and RNA!