Viruses

Classifying Viruses How to tell them apart

Viral Targets Not every virus is a killer Viruses are selective Specific viruses enter only specific cells – this is called the Host range Host Range: the number of host species, tissues or cells that can be infected by a virus or parasite – Some viruses can have a broad or narrow range – Broad Range: Swine Flu – Can infect humans and hogs Rabies – Can infect many mammals including humans – Narrow Range: Human cold virus – usually infects only the respiratory cells HIV – infects the immune system

Viral Replication From One to Hundreds

Viral Replication Replication occurs in a variety of ways, but there are generally four basic steps. 1. Attachment and Entrance The virus chemically recognizes a host cell, attaches to it, and enters it. Depending on the virus, either the whole virus or only its genetic material enters the cell 2. Synthesis of Protein and Nucleic Acids Molecular information in the viral DNA or RNA directs the host cell in replicating viral components – capsid proteins, nucleic acids, enzymes, etc 3. Assembly of the Units The viral nucleic acids, enzymes, and proteins are brought together and assembled into new virus particles 4. Release of New Virus Particles The newly formed virus particles are released from the infected cell and the host cell dies (lysis)

Lytic and Lysogenic Cycel This is known as the LYTIC CYCLE The entire process can be completed in as little as min and produce hundreds of new virus particles Certain types of viruses, such as those that cause cancer or those that infect bacteria, have a LYSOGENIC CYCLE

Lysogenic Cycle The virus does not kill the cell It coexists with the cell and can be carried through many generations without harm to the host Like the lytic cycle, the virus enters the host cell Instead of taking control, its nucleic acid becomes integrated into the host cell’s DNA This extra nucleic acid act as additional genes in the host cells When the cell replicates, the virus genome also replicates and is passed onto daughter cells The virus is said to be in a dormant state or lysogeny The virus can be activated by a stimulus, which would then trigger the virus to enter the lytic cycle

Lytic and Lysogenic Cycles