Virus Life Cycles

nomenclature. The Internationals Committee on Taxonomy of Viruses will keep track of these things in viruses. All recommendations are disseminated to the scientific community at large.

Viruses can be classified in different ways, including their genome types, the viral capsid structure, whether there is an envelope of not, the host range, the pathogenicity of the virus, the similarity in gene sequences, and the range of host. There are certain orders, families, genuses, and species that get clustered together. For example, all viruses of the Picornavirales order have no envelopes, have icosahedral structures, and have a genome with positive-sense RNA. There are many families and genuses under these categories. Once you get to the genus level, the viruses in the same genus are the most similar to one another.

VIRUS LIFE CYCLES

Viruses in general have roughly the same life cycle although, with bacteriophages, there are both lytic and lysogenic cycles to consider. The basic driving force behind all viral infections and their ability to succeed as pathogens is to have attachment to the right host, the proper metabolic pathways to make new virus particles, and the ability to release those particles in ways that affect another host cell. They do not go through division but instead undergo replication. Figure 6 shows the typical viral life cycle:

Figure 6.

Each virus has its own unique life cycle but they generally follow the same basic pattern of replication. It all starts with attachment, which depends on specific interactions between the capsid proteins and host cell receptors on the cell surface. The specificity determines the range of hosts. Each host cell will have a surface protein that interacts with the viral capsid protein. The success of the virus depends on being able to infect cells that will allow it to best replicate. If there is a viral envelope, it will often fuse in order to allow the viral genome to enter the cell.

The next step is penetration of the virus itself or the viral genome into the host cell. This can happen in several ways. The bacteriophage will inject its genome only through the relatively thin cell wall of the organism. Plants are able to be accessed only through cell trauma because they have a thick and rigid cellulose wall. Once the virus gains entry, it

can travel from cell to cell through the plasmodesmata between the different cells. In animal cells, there is endocytosis, which is receptor-mediated, and membrane fusion.

If any part of the capsid is taken up into the host cell itself, the capsid must be removed through the uncoating process. It can involve self-degradation using viral enzymes or using host enzymes or through simple dissociation processes. The viral nucleic acid is then a naked piece of RNA or DNA inside the cell. Once this has been accomplished, replication of the genome can begin.

The replication of viruses means the copying of its genome in whatever way the virus need to do this. Viral messenger RNA is synthesized, viral proteins are made, and further viral genome replication is regulated. Sometimes, there are several rounds of messenger RNA synthesis before the nucleic acid is made to make new viral particles.

The next step is viral particle assembly. It sometimes requires protein modification or maturation of virus particles before the particles get assembled in the cell cytoplasm. Finally, there is release of virus particles, usually by lysis of the host cell. It generally means that the cell dies. As mentioned, enveloped viruses like HIV leave through budding, which does not involve death of the infected cell.

Lysogeny is what happens with some bacteriophage infections, creating a lysogenic cycle rather than a lytic cycle. During this type of cycle, the viral genome will incorporate its genome into the host cell genome or will form a circular plasmid inside the cell. The phage virus’s genetic material is called a prophage that gets transmitted genetically to the daughter cells. Certain events, such as chemicals or exposure to UV radiation, will revert the cycle back to a lytic one that will kill the cell and release particles of new viruses. This process does happen in eukaryotic infections but to a lesser degree.

Bacteriophages that only allow for lytic cell replication are called virulent phages, while those that replicate during both types of virus life cycles are called temperate phages. Those daughter cells that contain the viral prophage are called lysogens. They do not die but will switch to the lytic cycle if induction occurs. When there is induction, the prophage is excised and new virus particles are made. Some lysogens have an evolutionary disadvantage, while others are advantageous because they have some gene inside them, for example, that confers resistance to their own host.

Bacteriophage infections might not seem important to humans but, in fact, they can be very important. Bacteria can go from being non-virulent to highly virulent, just because of lysogenic conversion. This is because the virus that infects these bacteria contain what are called morons, which are genetic elements that confer some type of advantage to the bacterial cell. The examples of this include Corynebacterium diphtheriae infections, which only make diphtheria toxin when the bacterium is itself infected by its own phage. Vibrio cholerae, Streptococcus pyogenes, and Shigella species have the same issue where the organism makes a toxin that is only coded for by viral nucleic acids.