Chapter 23: VirusesChapter 23: Viruses

Leaving Certificate BiologyLeaving Certificate Biology

Higher LevelHigher Level

VirusesViruses• Viruses are obligate parasites – they can

only reproduce inside a living host• Viruses are non-cellular: made of a piece of

DNA oror RNA surrounded by a protein coat• Syllabus: “Viruses: identify the problem of

definition”– The problem of definition comes from the debate

as to whether or not viruses are living or non-living

– Viruses do not show all the characteristics of living things: organisation; nutrition; excretion; response; reproduction

Virus ShapesVirus Shapes

• Round/spherical-shaped

• Rod-shaped

• Complex-shaped

Virus StructureVirus Structure

• Viruses are composed of a single piece of either DNA or RNA surrounded by a protein coat

Protein coat

Piece of DNA or RNA

Viral ReplicationViral Replication1. Attachment: virus attaches to proteins on the target cell’s

surface2. Entry: either the whole virus enters the cell by pushing into

the cell or the virus injects its DNA/RNA into the host cell3. Synthesis of viral DNA and viral proteins: the virus

causes the cell’s own DNA to become inactive and uses the cells ribosomes and organelles to make new DNA/RNA and protein

4. Assembly of new viruses: the newly synthesised DNA/RNA and viral proteins assemble themselves into new viruses

5. Release of new viruses: the number of new viruses build up in the host cell until the host cell bursts and releases all the newly-formed viruses which go on to infect other cells

Economic and Medical Economic and Medical Importance of VirusesImportance of Viruses

• Two harmful effects of viruses:– AIDS (acquired immunodeficiency syndrome) caused

by the HIV (human immunodeficiency virus) – which is a retrovirus (contains RNA)

– Human papilloma virus: causes cervical cancer by integrating its own DNA into the host cells DNA and remains dormant

• Beneficial effect of viruses:– Used in genetic engineering to transfer genes

especially between bacteria– It is possible that bacteriophages might be used to kill

bacterial infections in the future