Viruses

1. Viruses

2. Characteristics of Viruses • Non cellular structures – do not have membranes or any cell organelles. • Consist of an outer protein coat (capsid) and a strand of nucleic acid either DNA or RNA :

3. Characteristics of Viruses • Do not carry out metabolic reactions on their own – require the organelles and enzymes of a host to carry out such reactions. • Come in a variety of shapes.

4. Living or Non-living??? • Living • have nucleic acid – DNA or RNA. • Have a protein coat. • Can replicate (inside a living cell) and pass on genetic information. • Non – Living • Non – cellular • Cannot reproduce by themselves • Do not have cell organelles • Have only 1 type of nucleic acid • Do not feed or respire

5. Shapes of Viruses • Viruses may have 3 distinct shapes: • Rod shaped e.g. tobacco mosaic virus • Round e.g. influenza or mumps

6. Shapes of Viruses • Complex e.g. the T phage ( a virus that infects bacteria)

7. Virus Replication Viruses enter the host cell and use the energy and organelles of the host to produce new viruses and for this reason are known as obligate parasites. Bacteriophage - is a virus that infects bacteria and whose replication cycle takes approx. 30 minutes e.g. T-phage (this replication cycle is similar to the way in which most viruses infect humans)

8. Stages in virus replication

9. Stages in virus replication Stage 1: Attachment Virus attaches to the host cell – proteins on the virus match up with receptor sites on the host wall. Stage 2: Entry Virus forms a hole in host cell. The DNA of the virus enters the host while the protein coat stays outside. (In the case of animal cell these protein coats enter and are digested)

10. Stages in virus replication Stage 3: Synthesis Host DNA is made inactive. The viral DNA uses the host’s organelles to produce new viral DNA and proteins. OR Sometimes viral DNA joins with host DNA. When host DNA is copied so is the viral DNA and passed on to the daughter cells. Resulting daughter cells may behave normally or produce new chemicals e.g. scarlet fever.

11. Stages in virus replication Stage 4: Assembly Using these new viral molecules new viruses are formed inside the host cell. Stage 5: Release The host cell bursts (Lysis) to release new virus particles and the process repeats.

12. Stages in virus replication

13. Stages in virus Replication Retroviruses:contain RNA instead of DNA and an enzyme that converts the virus RNA to DNA. This DNA then makes new copies of the virus RNA and new viruses inside the host cell e.g. HIV virus.

14. Medical and Economic Importance of Viruses Must know 2 disadvantages and 1 beneficial example of viruses • Disadvantages of viruses: • Viruses cause human diseases e.g. colds, flu, measles, rabies, AIDS etc. • Human diseases cause medical problems resulting in economic losses such as medical costs and lost time at work. • Viruses cause animal diseases e.g. foot and mouth and rabies.

15. Medical and Economic Importance of Viruses • Disadvantages of viruses: • Viruses cause plant diseases e.g. tobacco, tomato and potato mosaic diseases. • Benefits of viruses: • Genetic Engineering – in genetic engineering viruses are sometimes used to transfer genes from one organism to another. These viruses are known as vectors.

16. Medical and Economic Importance of Viruses • Benefits of Viruses: • Control of Infections – there is hope that bacteriophages may be used to control some bacterial infections.

17. Control of Virus Infections • Virus are controlled by the body’s general defence system (e.g. skin, stomach acid etc.) and the specific defence system (e.g. production of antibodies.) • Vaccinations are available for some viral diseases e.g. MMR • Antibiotics have no affect on viruses. • Some drugs have been produced to interfere with viral replication e.g. interferon, acyclovir.