Viruses are important to biologists for several reasons. They are the simplest form of life. Indeed they are so simple that they exist on the borderline between the living and the inanimate non biological world. Viruses also reveal much about more complex biological entities including cells because viral replication is governed by the same principles that govern the lives of cells. Finally viruses are responsible for many human diseases including influenza and AIDS
Some evolutionary biologists argue that all organisms on the planet are simply complex devices whose sole purpose is to make more copies of their own genomes. Viruses take this notion to the extreme. They are mostly DNA that happen to be wrapped in a coating. The capsid affords protection for the viral genes and allows viral genes to gain entrance to appropriate host cells. Viruses exist at the border of the living and non-living because they are unable to replicate on their own. They are obligate parasites in the sense that they can only replicate after they have invaded and parasitized a host cell
Because viral genomes could be isolated from the genomes of the infected cells, viruses were a good source of pure DNA. This explains why viruses were studied so intensively before the advent of gene cloning. For example, the SV40 virus, a double stranded DNA virus carries approximately five genes in its genome and the viral DNA molecules were readily separated from the DNA of the monkey cells infected by this virus
The origins of viruses are even more obscure than the origins of cellular forms of life. Since viruses are obligate cellular parasites we can only assume that they evolved later than cells either as degenerate cells or as renegade cellular genes that learned to manipulate the replication machinery of the cells in which they arose. Viral genomes evolve more rapidly than the genomes of cellular organisms. This rapid genetic change has obscured or erased any relationships that may have existed between various types of viruses and might have been used to illuminate their ancient roots
The viruses that parasitize bacterial cells ( bacteriophages ) and those that parasitize animal cells ( animal viruses ) operate on identical principles, even though the details of their genes and the organization of their genomes give no hint of relatedness. We will focus on animal viruses the mechanisms by which they replicate, and the consequences of their replicative strategies on their disease-causing abilities
Some evolutionary biologists argue that all organisms on the planet are simply complex devices whose sole purpose is to make more copies of their own genomes. Viruses take this notion to the extreme. They are mostly DNA that happen to be wrapped in a coating. The capsid affords protection for the viral genes and allows viral genes to gain entrance to appropriate host cells. Viruses exist at the border of the living and non-living because they are unable to replicate on their own. They are obligate parasites in the sense that they can only replicate after they have invaded and parasitized a host cell
Because viral genomes could be isolated from the genomes of the infected cells, viruses were a good source of pure DNA. This explains why viruses were studied so intensively before the advent of gene cloning. For example, the SV40 virus, a double stranded DNA virus carries approximately five genes in its genome and the viral DNA molecules were readily separated from the DNA of the monkey cells infected by this virus
The origins of viruses are even more obscure than the origins of cellular forms of life. Since viruses are obligate cellular parasites we can only assume that they evolved later than cells either as degenerate cells or as renegade cellular genes that learned to manipulate the replication machinery of the cells in which they arose. Viral genomes evolve more rapidly than the genomes of cellular organisms. This rapid genetic change has obscured or erased any relationships that may have existed between various types of viruses and might have been used to illuminate their ancient roots
The viruses that parasitize bacterial cells ( bacteriophages ) and those that parasitize animal cells ( animal viruses ) operate on identical principles, even though the details of their genes and the organization of their genomes give no hint of relatedness. We will focus on animal viruses the mechanisms by which they replicate, and the consequences of their replicative strategies on their disease-causing abilities