A nucleotide is the monomer unit of DNA. Nucleotides are composed of a sugar, a phosphate and a nitrogenous base. DNA is a polymer consisting of many of these monomers linked together.
The sugar and phosphate of one nucleotide links with the sugar and phosphate of another nucleotide in an alternating pattern to form the backbone of a strand of DNA. The nitrogenous base portion of the nucleotide links with a complementary nitrogenous base of another nucleotide to form the middle of a strand of DNA.
DNA consists of four types of nitrogenous bases. Adenine and guanine are larger bases and are classified as purines. The other two bases, cytosine and thymine, are pyramidines. Adenine links to thymine, and cytosine links to guanine to form a connection between two strands of nucleotides. These bases link together through weak chemical bonds between their hydrogen atoms.
Once the strands of nucleotides are connected by the hydrogen bonds between nitrogenous bases in the middle, the DNA structure forms a shape that is known as a double helix. A double helix is a ladder-shaped structure that twists in a spiral. The ladder twists due to the weak chemical bonds between each of the nitrogenous bases.