DNA is a polymer that forms a double helix structure. Describe the structure of DNA and explain how it is replicated inside a cell.

DNA is a polymer of nucleotides, each consisting of a phosphate, a deoxyribose sugar and a nitrogenous base. Two strands of nucleotides are twisted around each other to form a right‑handed double helix. The bases – adenine, thymine, cytosine and guanine – pair in a complementary way: A with T and C with G, held together by hydrogen bonds.

During replication the double helix is unwound by helicase, separating the two strands. Each single strand then acts as a template for synthesis of a new complementary strand by DNA polymerase, using free nucleotides. The result is two identical DNA molecules, each composed of one original strand and one newly synthesised strand.