Module 1: The liver and hepatitis viruses - section 3

In this document:

Structure, types of viruses and reproduction:

Activity: Group discussion and lecture presentation
Section Time: Approximately 20 minutes

This section allows a basic exploration of viruses and their replication before looking at the family of hepatitis viruses in more detail.

Ask the large group to answer to the question ‘what is a virus’ Expect to spend only a few minutes and then refer to Slide 1.7 (What is a virus?) as a summary:

Supporting content:

A virus is a small infectious agent, much smaller than bacteria that can only replicate inside the cells of another organism. The study of viruses is known as virology. Viruses are found in almost every ecosystem on Earth and these minute structures are the most abundant type of biological entity. All living things, plants, animals or bacteria are susceptible to viral infections, which are usually specific to a particular organism. With the possible exception of bacterial viruses, which can kill harmful bacteria, all viruses are considered harmful. 

There is some debate as to whether viruses are living organisms, as outside of living cells they are wholly inert and cannot perform any of the activities typical of life such as respiration, reproduction and breaking down and digesting food. Their structure is very simple. They are made up of genetic material, ribonucleic (RNA) or deoxyribonucleic acid (DNA), which contains coded instructions for making copies of the virus, enclosed in protective shells that are made of proteins.

Using Slide 1.8 (Virus reproduction) explain how viruses replicate:

Supporting content:

The particles of a virus are so small that they cannot contain the complex apparatus required to reproduce themselves but require assistance from an infected host. Viruses have evolved to persuade certain cells in living organisms to let them in, to become infected, and then once inside they take over the operation of the cell to reproduce themselves.

The lytic cycle is typically considered the main method of viral replication, since it results in the destruction of the infected cell. Viruses of the lytic cycle are called virulent viruses. The lytic cycle is a six-stage cycle. In the first stage, called penetration or absorption the virus injects its own genetic material into a host cell causing the infected cell to mistakenly replicate or copy the DNA thousands of times. Then the viral DNA assemble themselves as viruses inside the cell. When the number of new viruses inside becomes too much for the cell to hold, the cell wall ruptures and the viruses are free to infect other cells.

Viral infections in animals provoke an immune response that usually eliminates the infecting virus. These immune responses can also be produced by vaccines, which give immunity to specific viral infections. However, some viruses including HCV and HIV evade these immune responses and cause chronic infections.

By way of differentiation, explain viruses are also more difficult to combat with drugs than bacterial infections because of the risk of damaging the host cell.

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