Disclosed are methods and compositions for inhibiting DNA synthesis in a cell using RNA. Inhibition of DNA synthesis by RNA can be used, for example, in analytical methods, as a research tool to affect cells under study, to synchronize cell cycle in a cell culture, and to inhibit cell growth. For example, inhibition of DNA synthesis in cancer cells can be used to inhibit cancer cells and treat cancer. The RNA can be any RNA, such as whole cell RNA, whole cell mRNA, whole cell ribosomal RNA, whole cell transfer RNA, synthetic RNA, recombinant RNA, modified RNA, or a combination. The composition can comprise RNA and a pharmaceutically acceptable carrier or RNA, a targeting molecule, and a pharmaceutically acceptable carrier. The targeting molecule can be a tumor-targeting peptide.
