Given a scientific passage and an answer, generate a question for the given answer.

Passage: Two DNA strands link together in an antiparallel direction and are twisted to form a double helix. The nitrogenous bases face the inside of the helix. Guanine is always opposite cytosine, and adenine is always opposite thymine. Answer: double helix
What is formed when two dna strands link together in an antiparallel direction and are twisted?