Viral disease

1. Viral disease Learning objective: • To be able to describe the structural features of a virus.

2. What diseases do viruses cause? • Diseases caused by viruses include chickenpox, smallpox, common cold, influenza, measles, mumps, rabies, polio, yellow fever.

3. Viruses • Viruses are the smallest and simplest of the microbes. • They are acellular (not made of cells) • Viruses are obligate parasites who can only reproduce inside host cells which get damaged in the process, leading to disease. • Viruses are thought to have arisen from lengths of DNA that became separated from their cells.

4. Living or not? • Viruses lack the mitochondria necessary to derive energy and they cannot reproduce on their own. • They are dependent on their host cells and are only classed as living organisms when they infect host cells. • After reproducing, viruses cause their host's destruction. Viruses are described as obligate intracellular parasites.

5. Outside the host cell viruses are inert and called virons. By taking over the host cells metabolic machinery they are able to replicate. Reproduction is the only common characteristic that viruses have with other living organisms

6. 1. Why can viruses be classified as living and non living? 2. Explain why all viruses are considered to be parasites.

7. Size does matter.. • Viruses are smaller than bacteria – about 20 – 400 nm

8. Morphology Viruses consist of: • A core of nucleic acid which can be DNA or RNA • A protein coat or capsid • There is no protoplasm or cytoplasm. • The capsid is a protein coat on the very outside to protect the genetic material. • It is made up of protein units called capsomeres which link together to form a very geometrical shape.

9. Function of capsid: • Protects the nucleic acid when the virus is not in a host cell. • Helps the virus to gain entry into a host cell and introduce the viral nucleic acid. • Viruses may also possess an envelope and that would have come from a previous host; it’s not their own.

10. Function of envelope: • Binds to the host cell membrane. • It helps the viro-particles to fuse with a new cell., (a disguise) e.g. HIV

11. Structure • Viruses have distinct structures and they identify the cells which they attack by recognising specific cell surface receptors. • Viruses will usually only infect one species. • Viruses which infect bacterial cells are called bacteriophages, those that infect animals and plants are called animal and plant viruses.

12. A complete virus particle is called a virion. • A virion is the dormant form of a virus that is transmitted between cells. • Virions are too small to see with a light microscope and were first seen in the 1930s using the electron microscope. • Once inside a host cell virions dismantle into their separate parts, and the virus can be reproduced.

13. All virions contain • Nucleic acid, which can be DNA or RNA, and single or double-stranded. • Viruses are classified according to the type of nucleic acid they contain. • The nucleic acid typically codes for 5-100 proteins (by comparison, the bacterium E. coli has about 4000 genes). • A protein coat called a capsid, made of subunits called capsomeres. • If the capsid proteins are closely bound to the nucleic acid, then the combination is called a nucleocapsid. • Because capsids are composed of many repeating subunits, they tend to have simple geometrical shapes, such as helix or icosahedron (20 triangular faces).

14. Some virions have very simple structures containing nothing else, but many virions have morecomplex structures, including: • Enzymes, required to replicate the viral nucleic acid or incorporate it into a host. • A lipid envelope, not made by the virus itself, but derived from a host cell membrane. • Matrix proteins to attach the capsid to the envelope. • Glycoproteins to allow the virus to attach to host cells.

15. Some examples of virion structures • Discovered in 1852 by Ivanovsky when investigating tobacco mosaic disease in tobacco plants. • He found that sap from a diseased plant could still cause infection even after being put through a bacterial filter so he concluded there was a disease-causing organism smaller even than a bacterium. • Named from the Greek word meaning poisonous fluid.

16. 3 types of capsid • helical e.g. TMV • icosahedral e.g. HIV • complex which has a polyhedral head and a helical tail e.g. T2

17. Tobacco mosaic virus (TMV) • A coil of RNA surrounded by a helical capsid.

18. Adenovirus • A single stranded DNA surrounded by an icosahedral capsid.

19. Bacteriophages are viruses that infect bacteria An example is the T2 virus which has complex structures that combine icosahedral and helical capsids. Bacteriophage

20. HIV The human immunodeficiency virus (HIV) is an enveloped retrovirus. It comprises 2 copies of single-stranded RNA together with some enzymes, surrounded by an icosahedral capsid, which is in turn surrounded by a sphere of matrix proteins attached to a lipid envelope. Protein coat Nucleic acid strand

22. VIRAL DISEASE • What is a virus? • What characteristics of living things do viruses show? • Do you consider viruses to be alive? • Give reasons to justify your answer. • What does obligate intracellular parasite mean? • Roughly what size are viruses?