Transcript Viruses - leavingcertbiology.net

Chapter 23: Viruses
Leaving Certificate Biology
Higher Level
Viruses
• Viruses are obligate parasites – they can
only reproduce inside a living host
• Viruses are non-cellular: made of a piece of
DNA or 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 nonliving
– Viruses do not show all the characteristics of
living things: organisation; nutrition; excretion;
response; reproduction
Virus Shapes
• Round/spherical-shaped
• Rod-shaped
• Complex-shaped
Virus Structure
• Viruses are composed of a single piece of
either DNA or RNA surrounded by a
protein coat
Piece of
DNA or
RNA
Protein coat
Viral Replication
1.
2.
3.
4.
5.
Attachment: virus attaches to proteins on the target cell’s
surface
Entry: either the whole virus enters the cell by pushing into
the cell or the virus injects its DNA/RNA into the host cell
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
Assembly of new viruses: the newly synthesised
DNA/RNA and viral proteins assemble themselves into new
viruses
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
Importance 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