Viral Life Cycle

An icosahedral capsid is 20-sided, while there are also helical viruses like Ebola and the tobacco mosaic virus. Polyhedral viruses include rhinovirus and poliovirus subtypes. Bacteriophages have tail fibers, a polyhedral head, along with tail pins that attach the virus to the host cell. Poxviruses are shaped like bricks and have unique surface features. Viruses are also classified according to the viral genome they have. There can be positively stranded or negatively stranded RNA viruses, DNA viruses, and further distinction based on whether or not the virus is double-stranded or single-stranded.

VIRAL LIFE CYCLE

As mentioned, viruses do not have an adequate genome to sustain themselves unless they infect a host cell and capture the host cell’s reproductive capacities. DNA viruses in most eukaryotic cells will need to be present in the nucleus for replication, while bacteriophages replicate in the cytoplasm. Large poxviruses are DNA viruses that can replicate in the cytoplasm. RNA viruses replicate primarily in the cytoplasm.

Bacteriophage life cycles have been studied extensively. There are virulent phages that automatically kill the cell and temperate phages that become part of the host genome. These lead to latent infections that ultimately get activated to make progeny viruses or virions that have been newly assembled.

Virulent phages basically take over the cell, reproduce to make new phages, and destroy the cell. There are five stages associated with the bacteriophage lytic cycle. The first stage is attachment, which involves association of the bacterial surface receptors with the virus particle. The second stage is penetration, with injection of the nucleic acids into the host. The virus itself remains outside. Then comes biosynthesis, which is replication of the viral proteins. After this is maturation, in which new virus particles are assembled. Lysis only happens with virulent phages that kill the cell.

In what’s called the lysogenic cycle, the phage genome participates in attachment and penetration, just as with the lytic cycle. The difference is that the phage genome instead is integrated into the host genome and does not immediately kill the cell. In such cases, the integrated genome is referred to as the prophage and the bacterial host along with the prophage is called a lysogen. The entire process is known as lysogeny.

Certain prophages can contribute to the pathogenicity of the bacterial organism. Some toxic viral genes will make a bacterium more pathogenic. When this happens, it is called lysogenic conversion or phage conversion. An increase in pathogenicity does not necessarily have to happen. Some viruses will decrease the pathogenicity of the bacterial organism. In the case of Clostridium botulinum and Vibrio cholerae, the prophage increases virulence. At the time of induction, the viral genome is excised from the host genome and a lytic phase occurs, killing the cell.

Transduction is the process of transferring bacterial DNA from one bacterium to another bacterium because of an infection with a bacteriophage. Sometimes, when the viral genome is being made and replicated, certain mistakes are made in which the host DNA gets spliced off in part and enters the new virion at the time of viral packaging. When the virus particle enters a new cell, the bacterial DNA segment goes into the new host. This can change the metabolic properties or antibiotic resistance of the new host.

There are two kinds of transduction. In generalized transduction, the random DNA piece gets transferred as part of the lytic cycle. With specialized transduction, the transfer occurs at the end of the lysogenic cycle, after the prophage DNA is excised from the bacterial genome. There are certain situations of increased stress on the bacterium that stimulates induction and later transduction. Things like UV light or chemical exposures can all trigger the induction process. This is called specialized transduction because only the DNA next to where the prophage was inserted can get transferred from one cell to another. Transduction is important to the evolution of bacterial organisms.

When viruses infect an animal host, there is attachment, penetration, viral biosynthesis, maturation of viral particles, and particle release. There are differences, though, in penetration, biosynthesis, and release stages. There isn’t injection of the viral genome but endocytosis of the viral particle. The fact that viruses only infect certain types of tissues is called tissue tropism. For example, the influenza virus only infects the respiratory tract.

There are different ways to replicate the viral genome, depending on the type of nucleic acid making up the virus. The different RNA genomes can be translated directly into viral proteins. It cannot happen with a negative single-stranded RNA virus until it gets

turned into a positive-sense RNA molecule. Often, the enzyme to do this comes from the virus itself. Retroviruses are what Positive single-strand RNA viruses are called. HIV is called a retrovirus. The virus must have an enzyme called reverse transcriptase to make a piece of double-stranded DNA that can be incorporated into the host cell. This integrated virus particle is called a provirus.

Persistent viral infections happen when the virus has not been cleared by the host but instead remain infective or silent without killing the host cell. There are two types of persistent infections. One of these is the latent infection, while the other is a chronic infection. Latent infections are asymptomatic and include oral and genital herpes as well as the chickenpox virus, which leads to herpes zoster. Chronic infections usually have some symptoms and include HIV and hepatitis C infections.

Latent infections come from certain latent viruses that initially cause disease before they become dormant. Chickenpox causes acute disease but lives dormant in the host’s nerve cells for many years. When it becomes reactivated, shingles can occur, involving pustules that are only found in the distribution of the nerve ganglion near the spinal cord. Some latent viruses are separate circular genomes, while others become integrated into the genome of the host.

Chronic infections are usually both persistent and recurrent. There will be symptoms. The problem is that the body cannot eliminate the specific virus. The person with HIV has a chronic infection that might not actually be symptomatic throughout the person’s life. If the virus is not held in check with antiviral agents or the person’s immune system, the patient ultimately develops AIDS, which is lethal to the patient.

Plant viral infections are similar to animal viral infections. Plant viruses may be singlestranded or double-stranded and they may consist of RNA or DNA. Most, however, are positive single-stranded RNA viruses that get directly translated into proteins. Some viruses are highly host-specific, while others are not. They can be transmitted via arthropods, nematodes, fungi, or direct contact. Sometimes the vector itself is specific.

Viruses that cause plant diseases are referred to as biotrophic parasites because they rarely kill the host completely. There is penetration and uncoating of the genome, in which the capsid is removed. Then the viral genome gets replicated to make new