The pyrimidine bases of DNA are cytosine and thymine. One pyrimidine combines with one purine to make up a rung of the DNA double helix. Nucleotides, the building blocks of DNA, comprise a phosphate group, a five-carbon sugar and either or a purine or a pyrimidine.
Purines have a double ring, whereas pyrimidines have only a single ring. Each pyrimidine structure is so specific that it only binds with one type of purine. To illustrate, cytosine and guanine pair up, whereas thymine forms a pair with adenine. Purines do not pair with one another, and pyrimidines do not pair with one another. The pairing of a purine only with a pyrimidine is essential because it keeps the width of the DNA molecule consistent.