A DNA double helix is made up of two strands that wind around each other in a twisted ladder-like structure. Each strand consists of a sugar-phosphate backbone with nitrogenous bases paired in the interior, forming the rungs of the ladder. These two strands are complementary and run in opposite directions (antiparallel), held together by hydrogen bonds between specific base pairs: adenine pairs with thymine, and cytosine pairs with guanine
. Thus, the DNA double helix consists of exactly two strands.
