Viral Genomes and their Implications in Disease

VIRAL GENOMES AND THEIR IMPLICATIONS IN DISEASE

There doesn’t seem much rhyme or reason why some viruses have one type of genome and others have yet another type altogether. In some cases, the genome type determines what the virus can do inside an infected cell and how it causes disease.

One commonly studied virus that does not have major implications in human diseases is the T phage that infects E. coli bacteria. It consists of one molecule of double-stranded DNA. These viruses have an icosahedral head and a helical protein tail. The DNA is injected into the cell, where it can make about a hundred new virions every twenty minutes. The cell then lyses to release these phages. Within 20 minutes of infection, the metabolic processes in the cell will be turned over to make phage proteins only.

Temperate phages like the lambda phage in E. coli are also well-studied. These are double-stranded DNA viruses that might become circular to create ordinary progeny that are released through lysing. They might also enter the genome of the bacterium to assume a lysogenic cycle that does not involve immediate lysing. We will talk more about this in a minute. The environment around the cell will determine what type of life cycle happens in the cell.

Small DNA phages generally only code for less than twelve proteins and have also been extensively studied with regard to their life cycle. These are simple viruses that require a great deal of help from the host in order to make the proteins necessary to make the phage progeny. Research on these small DNA phages has helped researchers determine which cellular proteins are necessary to participate in DNA replication.

RNA phages also infect E. coli but have an RNA genome. Most of these have their genomes directly made into proteins as is the case with eukaryotic messenger RNA. The phage RNA can make many but not all of the proteins necessary for the making of phage proteins. Some will just make RNA polymerase to transcribe viral RNA, an enzyme that dissolves cell walls of bacteria, and two capsid proteins used to make the phage capsids.

As you learn about the animal viruses and, in particular, the human viruses, you’ll see that these are largely randomly named or named after the diseases they cause in the human host. It can be confusing because, in the case of respiratory viruses, the

symptoms might be the same while the virus causing it might be very different from another virus causing the same symptoms.

The type of nucleic acid in the cell makes a very big difference in its overall life cycle in the host cell. It also provides some way of classifying the different viral particles. Viral mRNA can have a plus strand or a minus strand. The plus strand is able to be quickly translated into proteins but the minus strand cannot do this without some intermediate step. Any strand of DNA that is complementary to a positive RNA viral strand is also considered a minus strand. All plus strands that are not already part of the genome must have a minus strand of either DNA or RNA as templates. This leads to six possible classes of animal viruses used in virology.

Class I and class II DNA viruses are both based on DNA as the major genome. Class I viruses are all double-stranded DNA viruses. The viral DNA gets into the cell nucleus of the infected cell, gets transcribed into viral messenger RNA to make viral proteins. Some examples are human adenoviruses that cause GI and respiratory system infections, SV40 simian virus found in monkey kidney cells, herpesviruses to include those that cause cold sores, shingles, and chickenpox, and chickenpox, and human papillomaviruses that cause warts, including genital warts. Poxviruses are class I viruses, including smallpox virus or variola, which has been eradicated.

Class II viruses are known as parvoviruses. These are single-stranded DNA viruses, some of which have both plus or minus strands held in different virions, while others just have minus strands. Either way, the DNA must get made into double-stranded DNA before it can become messenger RNA for the cell.

RNA viruses are from classes III through VI. These cause a great variety of human and other animal viral infections. Class III viruses have double-stranded RNA as their genome. There will be a minus strand that acts solely as a template for the making of plus strands or messenger RNA strands. They contain many strands of up to twelve separate double-stranded RNA strands each. One strand will code for just one to two polypeptides. Because of this unique structure, these have segmented genomes. Many of these have enough capacity to make a full complement of enzymes.