The "steps" or "rungs" of a DNA molecule are made up of four nucleobases, including adenine (A), cytosine (C), guanine (G) and thymine (T). These bases are arranged in particular pairs. A only bonds with T and vice versa, while C only pairs with G and vice versa.
Deoxyribonucleic acid, or DNA, is a molecule that contains the genetic material of an organism. Its fundamental unit is called a nucleotide. DNA is a type of nucleic acid that forms a double-helix structure due to the two strands that spiral around each other, often likened to the shape of a twisted ladder.
DNA is composed of a phosphate group, a deoxyribose sugar and four nitrogenous bases. Alternating sugar and phosphate molecules form the backbone or "rails" of DNA. The sugar is joined to the phosphate group via the 3'-5' phosphodiester linkage. The four nucleobases that form the rungs pair up through weak hydrogen bonding. The A-T and T-A base pairs contain two hydrogen bonds, while the C-G and G-C base pairs form three hydrogen bonds.
The nucleobases A and G, referred to as purines, are larger compared to T and C, which are called pyrimidines. A purine always forms a pair with a pyrimidine. The hereditary code that determines the traits of an organism is stored in these base pairs.