How many strands are in Watson and Crick’s standard DNA model? There are two strands.
The classic DNA molecule is a double helix made from two long polynucleotide chains coiled around the same axis. Each chain has a sugar-phosphate backbone, while the nitrogenous bases point inward and pair with bases on the opposite strand.
The two strands are complementary: adenine pairs with thymine, and guanine pairs with cytosine. They are also antiparallel, meaning one strand runs from its 5′ end to its 3′ end while the other runs in the opposite direction. This arrangement is essential to the geometry and copying of DNA.
“Double helix” refers to the two intertwined molecular strands, not to two turns or two molecules of DNA. DNA can form other structures, and some viruses have single-stranded genomes, but the standard Watson–Crick model describes ordinary double-stranded DNA. When the strands separate during replication, each serves as a template for a new complementary strand.