Viruses• Extremely small (can only be viewed using an electron microscope)

• Non-cellular pathogen

• Core of genetic material – DNA or RNA

• Surrounded by wall of protective protein Capsid

Viruses – Living or Non - Living• Viruses have none of the characteristics of

living things.

• They only respond when they make contact with a suitable host

• All types of cells are attacked by viruses – most viruses are specific to one cell type.

• Do not have ribosomes, mitochondria or other cytoplasmic organelles cannot carry out metabolisms on their own

• Must invade host cell to carry out rxn’s

Living Non-Living

• Possess genetic material DNA/RNA

• Possess protein coat• Can replicate (inside

a living cell)

• Non cellular• Cannot reproduce by

themselves• Do not possess

ribosomes, mitochondria etc

• Only have one type of nucleic acid

A Bacteriophage is a virus that infects bacteria.

- Most complex and best studied viruses

Virus Replication* As viruses are not cells the term

replication is used instead of reproduction

Attachment – virus attaches to a suitable host cell

Entry – the whole virus or its nucleic acid enters the host cell

New virus DNA made using bacterial organelles Synthesis – the host

nucleic acid is inactivated. The viral nucleic acid uses the host’s organelles to produce new viral nucleic acids and protein

Assembly – Viral nucleic acid and proteins come together forming new viruses

Bacterium

bursts

Many viruses

released

Release – host cell bursts to release between 100 and 100,000 new viruses

Bursting of host cell is called lysis

Retroviruses

• Contain RNA instead of DNA

• Contain an enzyme that converts virus RNA to DNA DNA then makes new copies of virus RNA and new viruses inside the host cell.

• Example: HIV is a retrovirus

Some viruses do not destroy host DNA. Instead the viral DNA joins with the host DNA and remains inactive

When the host DNA is copied, the viral DNA is also copied and passed on to the daughter cells.

New host cells may behave normally or may produce new chemicals due to the presence of the virus

Example : Diphtheria, scarlet fever and botulism

Economic importance of Viruses

• Crop damage e.g. mosaic disease of potatoes, tomatoes and tobacco

• Livestock diseases e.g. foot and mouth, rabies

• Work days lost due to viral infections e.g. flu, colds

Medical Importance of Viruses• Causes disease e.g. cold, cold sores, flu,

measels, rubella, mumps, chickenpox, warts, hepatitis, viral meningitis, HIV(AIDS)

• Some cancers are caused by viruses

• Vaccines – immunisation against viral diseases

Beneficial uses of Viruses• Act as vectors in genetic engineering

used to transfer genes from one organism to another.

• Biological control : against pest rabbit populations

Control and Immunity• Controlled by the body’s general defence

system and specific defence system

• Immunity to certain viruses can be induced artificially by vaccination

• Antibiotics kill bacteria but have no effect on viruses

• Some antiviral drugs are available – they interfere with the virus without affecting the host. E.g. Acyclovir helps treat herpes (cold sores)