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5.1. Viruses encode enzymes and follow unique pathways:

Almost all viruses encodes unique proteins and enzymes, moreover they follow unique pathways to transfer their genetic information. This phenomenon is more pronounced in case of RNA viruses, they either use RNA dependent RNA polymerase or in case of retrovirus (HIV) RNA dependent DNA polymerase to complete their replication cycle. Both of these processes requires unique enzymes that are encoded by the virus following infection to the host cells and are generally absent elsewhere.

The RNA dependent RNA polymerase and reverse transcriptase have minimal proofreading ability, as a result their error rate is very high (1 in 10,000) as compared to the DNA replication. This means that an RNA virus particle will contain 1 or more mutation from its parental wild type virus. Presence of many different subspecies of virus particle in a population is also called as quasispecies nature of RNA viruses. The error prone activity of RNA virus polymerase restricts the upper size limit of the genome above which they cannot survive. As a result of this phenomenon most of the RNA virus have their genome size in the range of 5-15kb (coronavirus 30kb). The opposite is true in case of DNA viruses where proofreading and error repair activity ensures accurate replication of the viral DNA as big as 800 kb. The fact that DNA is more stable chemically than RNA likely explains us why all thermophilic hosts contain viruses that have dsDNA as their genetic material.