Nucleotides can also be biotinylated by incorporation of biotinylated nucleotides. Nucleases are enzymes that break phosphodiester bonds between nucleotides in nucleic acids. Nucleic acids consist of a chain of linked units called nucleotides. Hexamer primers are sequences composed of six random nucleotides. Nucleotide salvage uses pieces of previously made nucleotides and re-synthesizes them for future use. Sets of three nucleotides, known as codons, each correspond to a specific amino acid. The switch regions are loops of alpha-helices with conformations sensitive to guanine nucleotides. It is coded so that every three nucleotides (a codon) corresponds to one amino acid. Ribavirin is a prodrug, which when metabolized resembles purine RNA nucleotides. DNA is a long molecule made up of individual molecules called nucleotides. DNA consists of two complementary antiparallel strands consisting varying patterns of nucleotides. Guanine is one of the four nucleotides that makes up DNA. The two proteins recognize two DNA sequences that are a few nucleotides apart. Both steps involve transesterification reactions that occur between RNA nucleotides. It is from UMP that other pyrimidine nucleotides are derived.