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22.3 Viral Genome The genome of a virus consists of DNA or RNA, whose size and configuration vary. The entire genome can exist as a single nucleic acid molecule or several nucleic acid segments. Also, the DNA or RNA may be single-stranded or double-stranded, and either linear or circular. Not all viruses can reproduce in a host cell by themselves. Since viruses are so small, the size of their genome is limiting. For example, some viruses have coded instructions for only making a few different proteins for the viruses' capsid. On the other hand, the human genome codes for over 30,000 different proteins. Therefore, the lack of coded instructions cause some viruses to need the presence of other viruses to help them reproduce themselves. Such viruses are called replication defective. Lastly, it is worthy to note that 70% of all viruses are RNA viruses. As the process of RNA replication with enzymes and other organelles of the host cell is more prone to errors, RNA viruses have much higher mutation rates than do DNA viruses.