Pathways of biological information flow. In almost all living systems, information is stored in DNA, then transcribed into RNA, which is processed and translated into protein. DNA is replicated to prepare for cell division. The transfer and maintenance of genetic information are regulated at each of these stages. Exceptions to this general pattern are found in certain viruses (RNA viruses and retroviruses) that store their genetic information in RNA. Viruses with RNA genomes make use of additional pathways (denoted by the red arrows)—RNA replication and reverse transcription (creation of DNA from RNA, instead of the other way around)—to maintain their genomes. The yellow highlighting represents points of regulation. Processes in the gray shaded box, along with occasional errors in replication, reverse transcription, and RNA replication, give rise to genomic alterations (mutations) that fuel evolution.