The smaller bases, built on a single-ring structure, are pyrimidines. Cytosine and thymine are pyrimidines. The larger double-ringed bases are purines. Adenine and guanine are purine bases. Each base is typically symbolized by its initial.
Nucleotides can join to each other by phosphodiester bonds.
Each phosphodiester bond connects the 3' carbon of one nucleotide to the phosphate group of the 5' end of the next nucleotide. The sugar and phosphate groups form the backbone of a chain of nucleotides. The bases, extend from the backbone. Together, this chain of nucleotides makes up one strand of the DNA molecule. Each complete DNA molecule has two strands of nucleotides.