Viral Animal (Animal viruses)
Animal viruses vary based on the type of nucleic acid they possess, as well as whether they are naked or enveloped. Each variance results in different types of life cycles. A naked and an enveloped virus will attach to and penetrate a host differently. Animal viruses without an envelope are referred to as naked viruses. They bind to the surface of the host cell and inject their DNA into the host cell in a manner similar to bacteriophages. Some enveloped viruses infect their host by binding to receptors on the host cell. The envelope of the virus then merges with the host membrane and the capsid enters the cell. After entry, the capsid opens and releases the viral genetic material into the cytoplasm of the cell. The type of nucleid acid in an animal virus will determine how the viral nucleic acid and proteins are synthesized. When the single-stranded DNA genome of a parvovirus enters a host cell and invades its nucleus, a complimentary strand is produced for the single-stranded viral DNA.
This DNA is then replicated normally. Messenger RNA is transcribed and transported into the cytoplasm, where viral capsomere proteins are produced. At some point after synthesis has begun, the capsomere proteins enter the nucleus of the host, and virions containing the original single stranded DNA are assembled. In a virus with double-stranded DNA in its genome, the DNA also migrates to the nucleus, where it is replicated by the host enzymes in the same way that the host’s DNA is normally replicated. Messenger RNA is made from the viral DNA in the nucleus, and viral capsomere proteins are made in the cytoplasm from the messenger RNA. The capsomere proteins enter the nucleus, and the virions are assembled in the host nucleus. After assembly, the virions accumulate in the cytoplasm and prepare for release from the host cell.
Some viruses have a single stranded RNA genome that can act directly as messenger RNA. This strand is called the positive strand or sense strand RNA. This messenger RNA is read by host ribosomes to make viral proteins. The virus carries a unique RNA polymerase that makes a complimentary negative strand (also called an antisense strand) from the original positive strand. There is no known animal equivalent of this RNA-dependent polymerase. The negative strand RNA can act as the template for more positive strand RNA. Positive strand single-stranded RNA viruses are assembled in the cytoplasm of the host cell when the RNA and the proteins have all been produced. Viruses with single stranded negative sense RNA are in a special situation: their RNA will not act as messenger RNA until it has been transcribed into the positive (sense) strand compliment of the original negative (antisense) strand RNA. To accomplish this feat, the viruses carry their own RNA-dependent RNA polymerase, which transcribes the RNA into the positive strand. When the positive strand has been produced, it acts as the template for both protein synthesis and more negative strand RNA. When the virion is packaged, it must contain the negative strand RNA in addition to copies of the RNA-dependent RNA polymerase.
When doble-stranded RNA viruses infect a host cell, the positive (sense) strand of the RNA acts as messenger RNA for protein production. One of the proteins coded is an RNA polymerase that uses the negative strand to make more positive strand RNA. The positive strand also acts as a template to make more negative strand RNA. The two strands come together and are packaged, along with the polymerase, in the virion. Retroviruses make up a special class of positive strand RNA viruses. When a retrovirus infects a cell, its positive strand RNA is reverse-transcribes into a DNA negative strand by a viral reverse transcriptase. This negative strand DNA is furthet transcribed to double-stranded DNA, which serves as atemplate for the viral RNA genome. This positive strand RNA also serves as the template for viral protein synthesis. The reverse transcriptase is packaged with the mature virion, as it is essential for the successful reproduction of the virus. Just like the beginning of the life cycle, naked viruses differ from enveloped viruses in how they assemble and release. Naked viruses generally leave their host cell by acumulating to such numbers that the cell lyses, spiling virions into the extracellular medium, free to infect new hosts. This kills the host cell and gennerally causes inflammation and infection in the tissue where the host cell is found.
Enveloped viruses leave the host cell by merging with one of the cell’s membranes-the nuclear membrane, the endoplasmic reticulum, or the cytoplasmic membrane. During synthesis, some viral glycoproteins are embedded in cellular membranes, and these glycoproteins act as a recognition system for the viral capsid. The cell membrane surrounds the capsid and forms a bud on the outer surface of the cell, which pinches off and forms a protective envelope for the virus, which is then free to infect other host cells. An advantage of the envelope is that the viruse does not need to break the cell to be released, so the cell can stay alive longer while it continues to produce and bud off more viruses. The type of nucleic acid, along with the presence or absence of an envelope will determine the type of life cycle a given animal virus will have.