VirusesViruses are non-cellular obligatory intracellular parasites, i.e. they require living hosts in order to reproduce and are unable to function for long (if at all) outside of a living host cell. A complete viral particle or virion contains a single type of nucleic acid (DNA or RNA) encased in a protein coat or capsid. Once a virus enters a host cell, the viral nucleic acid becomes active and the virus begins its replication, diverting the host’s cellular machinery to the task of building new viruses. This often results in the death of the host cell. Viruses invade all types of organisms. Some viruses are specific to one host species, while others are able to infect several taxa. Viruses are a difficult group to classify as they are non-cellular. A summary of viral classification is provided below.

Structure and Classification of Human Viruses

The basic shape of a virus is determined by the organization of the protein subunits that make up the capsid. Capsids may be helical (rod-like), polyhedral (20-sided), or complex (having polyhedral and helical portions). In viruses with a complex capsid structure (ie. bacteriophages), the contractile tail region has fibres for attachment to the host. In the enveloped viruses, the covering envelope is made of lipids, proteins, and/or carbohydrates that completely surround the capsid. The envelope may be covered with glycoprotein spikes. These are important in recognition of, and attachment to the host cell.

Questions:

1. Use your textbook (pages 487-491) and notes to help you determine the living and non-living characteristics of viruses and complete the table below.

Living Non-Living

2. Viruses are considered non-living. What is the one main reason for this classification?

Use the information on the previous page to answer the following questions.3. Determine four ways in which viruses are classified (organized into groups).

4. Name one virus with the following structures:

a. Double stranded DNA virus with no envelope ___________________________

b. Single-stranded RNA virus between 28-30nm ___________________________

c. Multiple stranded RNA virus causing haemorrhagic fevers ___________________________

d. Double stranded retrovirus ___________________________

e. Single stranded RNA virus causing upper respiratory tract infections and the common cold ___________________________

5. Explain how retroviruses differ from other viruses.

6. How are glycoprotein spikes similar to the tail fibres of a bacteriophage?

Tobacco Mosaic Virus (TMV) A single stranded RNA plant virus, with helical capsid.

Bacteriophage T4 A complex virus that uses its contractile tail region to inject DNA into its host.

Herpesvirus A polyhedral, enveloped, DNA virus responsible for herpes simplex types I and II

Ebola virus A helical shaped, RNA filovirus. Ebola causes severe haemorrhagic disease.