DNA typically consists of two strands that are linked together and wind around each other to form a double helix structure. Each strand is a polynucleotide chain composed of nucleotides, and the two strands are held together by hydrogen bonds between complementary bases (adenine pairs with thymine, and guanine pairs with cytosine)
. While the classic and most common form of DNA is double-stranded, there are rare forms such as quadruple-stranded DNA structures observed in some contexts, including healthy human cells and certain cancer cells, but these are exceptions rather than the norm
. In summary, DNA normally has two strands forming a double helix.
