Some suggest that virocells must have predated the current modern cells because they can infect both prokaryotes and eukaryotes. The current data indicate that viruses originated back when cells were so primitive that they lacked ribosomes as well. Without ribosomes, the ability to reproduce independently was impossible as we know it. Capsid proteins were probably acquired after the genetic material was created, indicating that the first genomes were possibly intracellular. The fact that viruses are parasitic may have also been a late finding in these organisms. For this to be true, the ancient viruses must have had more genes in their genome and must have gradually lost some of these genes that necessitated their parasitic nature. In that sense, the reduction of genomes might have been a driving force in evolution rather than genome expansion. Genome reduction probably started in viruses but also occurred in other organisms later on. The most primordial viruses may have once been able to have their own metabolism.
VIRAL STRUCTURES As mentioned, all viruses have a genome and a protein capsid, while some have a lipid envelope. In their totality, each viral particle is called a virion. Those virions without a lipid envelope are called naked viruses. The protein capsid has the capability of recognizing and surrounding just viral DNA. It is suggested that there is a packaging sequence or signal on the viral genome that indicates the necessity of capsids to surround them. Capsids are made of repeating capsomeres that, in most non-phage viruses, are either helical or icosahedral. Each capsomere is usually but not always a single polypeptide chain, also called a structural subunit or protomer. The first helical capsid to be studied is that of the tobacco mosaic virus, which is shown in figure 1:
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