Final answer:
In the DNA double helix, the two strands run antiparallel, with adenine pairing with thymine and cytosine pairing with guanine, creating a stable structure with a uniform width due to complementary base pairing.
Step-by-step explanation:
In a DNA double helix, the two DNA strands run antiparallel. This means that the orientation of the two strands is opposite to each other; specifically, the 5' end of one strand aligns with the 3' end of the opposite strand. The base pairing rules state that adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). This is facilitated by hydrogen bonding, where adenine and thymine form two hydrogen bonds, and cytosine and guanine form three hydrogen bonds. DNA's structure is also stabilized by its sugar-phosphate backbone, where the phosphate of one nucleotide bonds covalently with the sugar of the next, resulting in a continuous chain.