Question

Describe the structure of a virus, and explain how viruses cause disease. (p. 505)

Short answer

Question: Describe the basic structure of a virus and explain how viruses cause diseases. Answer: A virus is a small infectious agent with a genetic material (DNA or RNA) enclosed in a protective protein coat called the capsid, and sometimes an outer lipid envelope. Viruses cause diseases by infecting host cells and using them for replication. They bind to specific receptors on the host cell's surface, enter the cell, and replicate their genome using the host's cellular machinery. New virus particles are then assembled and released from the host cell through budding, exocytosis, or cell lysis. The host's immune system responds to the infection, which can contribute to disease symptoms such as inflammation, fever, and tissue damage. The severity of the disease depends on factors like the virus's ability to evade the immune system, the host's overall health, and the specific tissues targeted by the virus.

Step-by-step solution

Part 1: Describe the structure of a virus.

A virus is a small infectious agent that can only replicate inside the living cells of an organism. Viruses consist of genetic material - either DNA or RNA - surrounded by a protective protein coat called the capsid. Some viruses also have an additional outer envelope made up of lipids, called the viral envelope. These viral particles (virions) may vary in shape, size, and complexity.

Part 2a: Virus entry

Viruses cause disease by infecting the host's cells and using them to replicate and produce more viral particles. To enter a host cell, the viral particle first binds to specific receptors on the cell's surface. The precise structure of the virus and its receptor-binding proteins determine which cells are susceptible to infection. Once it has successfully bound to the receptor, the virus enters the cell through various mechanisms, such as fusion, endocytosis, or translocation.

Part 2b: Replication of viral genome

After entering the host cell, the virus begins replicating its genetic material. If the virus has an RNA genome, it uses the host cell's machinery to transcribe its RNA into mRNA and subsequently translates it into viral proteins. If the virus has a DNA genome, its DNA is first replicated by host cell enzymes or, in some cases, its own viral enzymes. The newly generated viral genomes and proteins come together to produce new virus particles.

Part 2c: Release of new virus particles

Once new virus particles are assembled, they leave the host cell either by budding, exocytosis, or cell lysis. Budding allows the new virions to obtain their lipid envelope, while exocytosis is a process in which the virions are enclosed in a vesicle and released from the cell. In the case of cell lysis, the large number of newly produced viruses cause the host cell to burst and release the virions, often damaging or killing the host cell in the process.

Part 2d: Immune system response and disease symptoms

The host's immune system responds to the viral infection by activating various immune cells and producing signaling molecules called cytokines that help to control and eliminate the virus. However, this immune response can also contribute to the disease symptoms, such as inflammation, fever, and tissue damage. Ultimately, the severity of the disease depends on factors such as the virus's ability to evade the immune system, the host's overall health, and the specific tissues and organs targeted by the virus.