The phrase that best describes the structure of DNA is "double helix." The DNA molecule resembles a ladder that has been twisted. This double-helical structure was first proposed by researchers James Watson and Francis Crick.
The model of the DNA molecule proposed by Watson and Crick features two chains hydrogen-bonded to one another. Hydrogen bonds are weak bonds between a slightly negative region of one molecule and the slightly positive region of another. The two chains run antiparallel to one another. Each side of the DNA molecule consists of two ends, a 5' end and a 3' end. In the DNA molecule, the sides are parallel to one another, but the 3' end of one side lines up with the 5' end of the other.
DNA molecules have sides made up of alternating phosphate groups and sugars whereas the rungs comprise two nitrogenous bases bonded to one another. The phosphate groups of DNA are called inorganic phosphates, which consist of one phosphorus atom bound to several oxygen atoms. The phosphate group is then bound to a sugar called deoxyribose. The sugar portion connects to one of four nitrogenous bases: adenine, thymine, guanine or cytosine.
The four bases connect in a very specific way. Adenine and guanine are called purines whereas thymine and cytosine are pyrimidines. The rungs of the DNA ladder are only large enough to accommodate one purine bonded to one pyrimidine. According to the base pairing rule, adenine bonds mainly to thymine whereas guanine bonds to cytosine. The two sides of the DNA molecule are held together by the hydrogen bonding between base pairs.