Final answer:
The monomers that make up the DNA double helix are nucleotides, consisting of a sugar, a phosphate group, and a nitrogenous base, joined together in long chains.
Step-by-step explanation:
The monomers repeatedly joined together to form a DNA double helix are called nucleotides. A nucleotide is composed of three parts: a pentose sugar (deoxyribose), a phosphate group, and a nitrogenous base (adenine, cytosine, guanine, or thymine).
Long chains of nucleotides form polynucleotides, resulting in the structure of the DNA double helix. The phosphate group and sugar of one nucleotide bond with the phosphate group and sugar of the next, creating the structural 'backbone' of DNA. By adhering to Chargaff's rules, complementary bases across the two DNA strands pair with each other through hydrogen bonds, which allows DNA to maintain its characteristic double helix shape.