penetrated. Because plant and fungal cells have thick cell walls, they can often only get in when the plant is traumatized. When they do get in, they pass from cell to cell through pores called plasmodesmata. Bacteria also have strong walls but the viruses have evolved to have injection mechanisms that leave the capsid outside of the bacterium and just inject the nucleic acids. Those that have capsids still inside the infected cell will have degradation of the capsid to release the viral genome. •
Replication—this is multiplication of the genome via viral messenger RNA, viral protein synthesis, assembly of viral proteins, and viral genome replication. There are “early” genes that replicate the viral genome, and “late” genes that usually are responsible for the making of viral structural proteins.
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Assembly—there is the natural self-assembly of the virus particles as well as modification or “maturation” of the viral proteins. This maturation process usually happens after the virus has been released from the host cell.
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Release—the virus particles can be released by lysis or breakdown of the host cell. This happens in many bacterial and in certain animal viral infections. There is also the possibility of incorporation of the viral genome into the host genome. The viral genome is then referred to as a provirus or prophage (in bacterial host cells). These proviruses can become activated later on, which will kill the cell. In HIV disease, the virus is enveloped so the virus buds out of the infected cell.
THE REPLICATION OF THE VIRAL GENOME Unlike the replication of DNA that occurs in cellular, non-viral organisms, which is basically the same between organisms, the way viruses replicate their genome is different, depending on the virus type. There are notable differences between DNA viruses and RNA viruses. DNA virus genome replication happens in the nucleus of the host cell. The genome enters the host cell by several means. Remember, the virus has no metabolic abilities of its own; it needs the metabolism of the host cell. This includes the RNA processing machinery and the ability to make proteins. Large viral genomes may partially encode 38
