Viruses Syllabus links 3 5 4 Viruses identify

Viruses

Syllabus links 3. 5. 4 Viruses: identify the problem of definition. Variety of shapes. Basic structure. Viral reproduction. CIT: Economic and medical importance of viruses: two harmful examples, one beneficial example.

Viruses • Can only be viewed under Viral an electron microscope Protein coat • Consist of an outer protein coat and a strand of nucleic acid (DNA or RNA) • Are not cells • No organelles • No normal metabolic Nucleic acid reactions • Obligate parasites • Living and non – living features structure

Shapes of viruses Polyhedral Rod Round Complex

Alive or not? • Not in any of the 5 kingdoms of life

The living and non – living features of viruses Living Non - living Have nucleic acid (DNA or RNA) Non - cellular Have a protein coat Cannot reproduce by themselves Do not have cell organelles Can replicate and pass on genetic information Have only one type of nucleic acid (DNA or RNA) Do not feed or respire

Replication (Bacteriophage) Cycle of a typical T - phage • • • Stages in viral replication Attachment Entry Synthesis Assembly Release

Method of action of some viruses

Retroviruses • RNA and Reverse transcriptase • Diptheria, botulism and scarlet fever release toxins

Medical and economic importance of viruses Disadvantages of viruses Human diseases Measles Mumps Rubella Warts Cold

Plant diseases • Gain entry via a vector (carrier) such as insects • Cause mosaic disease (striped patterns) in major crop plants Tobacco

Animal diseases Foot and mouth SARS* Rabies *Sudden Acute Respiratory Syndrome

Medical and economic importance of viruses Benefits of viruses • Genetic engineering • Restriction enzymes • Control some bacterial infections

Control of virus infections • Viruses are controlled by the body’s general defence system (i. e. skin, stomach acid, phagocytes etc. ) and by the specific defence system (antibodies) • Vaccinations are available for some viral diseases (MMR) • Antibiotics do not affect viruses • Drugs such as interferon and acyclovir have been developed to interfere with viral replication