Nucleoside analogues are synthetic compounds that resemble natural nucleosides, the building blocks of DNA and RNA. They are designed to interfere with the replication of viral genomes or cancer cells by mimicking the structure of nucleosides, which can disrupt normal cellular processes. These analogues can be incorporated into nucleic acids, leading to faulty genetic information and ultimately inhibiting the growth of pathogens or tumors. These compounds are commonly used in antiviral therapies, such as those for HIV and hepatitis, as well as in cancer treatments. By targeting the replication mechanisms of harmful cells, nucleoside analogues can effectively reduce the spread of infections or the growth of tumors, making them valuable tools in modern medicine.