A nucleotide analog is a chemical compound that resembles a natural nucleotide, which is the building block of DNA and RNA. These analogs can be incorporated into nucleic acids during replication or transcription, potentially disrupting normal cellular processes. They are often used in research and medicine to study genetic functions or to develop antiviral and anticancer therapies. Nucleotide analogs can mimic the structure of natural nucleotides, such as adenine, cytosine, guanine, and thymine. When incorporated into DNA or RNA, they can lead to mutations or inhibit the synthesis of nucleic acids, making them valuable tools in molecular biology and pharmacology.