1. Viruses

2. Viruses
no cellular structure, no cytoplasm, organelles or cell membranes
no metabolism on their own, cannot grow or respire
Therefore are not classified as living things

3. Viruses continued
Viruses can reproduce, a basic characteristic of life
They can take over control of a host cell
Can direct cell to make new virus particles
Mobile genes that attack cells

4. Parts of a virus Most viruses consist of 4 parts
Core - located in the center of the virus and contains the virus’ DNA or RNA wrapped together with some proteins
2) Capsid - made of protein and forms a shell around the core, protects DNA from enzymes of host cell
3) Matrix - forms a layer between the capsid and the envelope
4) Envelope - consists of lipids stolen from the cell membrane
of the host cell that the virus is growing in
Note: some viruses do not have an envelope and others do not have
a capsid

6. Type of disease caused (poliovirus)
Classifying Viruses
Often viruses are named for:
Type of disease caused (poliovirus)
Those who discovered them (Epstein-barr virus)
Site of replication (rhinovirus or tobacco mosaic virus)
Where they were isolated (sendai-virus)
Classification is now based on
type of nucleic acid, shape,
size etc.

7. Shape and Size. --> determined by the type and
Shape and Size --> determined by the type and arrangement of proteins in the capsid
Cylindrical
Polyhedral
Spherical

8. Site of Replication Specific organism and cell
Wide variety of organisms
e.g. Tobacco mosaic virus

9. Type of disease they cause
21 different groups of viruses infecting humans differ in set of genes (genome) and method of replication
e.g. influenza virus

10. Type of nucleic acid DNA viruses RNA viruses Less virulent
follow the lytic cyle
Eg. Warts, shingles, chicken pox
RNA viruses
Virulent
Follow lysogenic cycle
AIDS, rubella, rabies, measles

11. How big are viruses?
Video

12. Viral Replication
It depends on the metabolism of a prokaryotic or eukaryotic cell to replicate its DNA or RNA and to make protein coats foe each newly formed virus particle
Attaches to specific receptors on the plasma membrane of the host cell and can only enter particular cells with specific receptor sites
Enter host in one of two ways
a) the virus injects its nucleic acid into host cell (lytic cycle)
b) membrane of host engulfs virus creating a vacuole inside host
Host metabolism replicates the viral DNA or RNA and protein coat and new virus particles are assembled

13. New particles are released from the host cell in one of two ways:
The host cell breaks open (Lysis) and releases the new viruses which then infect neighbouring cells; Host cell is destroyed in process
The host cell releases new viruses without being destroyed

14. Bacteriophage Virus that infects a bacteria Video
Same video different site

15. T4 Bacteriophage Specific to E.coli Head  capsid containing DNA
Sheath  support and pump to
move DNA
Base plate  cut or bite a hole in
cell
Tail fibres  find site for
attachment
Core  pipeline to conduct DNA
from head to cytoplasm of
Collar attach tail to head

16. Viral Replication: Lytic and Lysogenic Cycles
Video

17. Viroids Very small infectious pieces of RNA
responsible for some serious plant
diseases
Differ from viruses as RNA does not
code for proteins
May interfere with the normal formation and functioning of RNA in host cell
Eg. Viroid outbreak killed more than
10 million coconut plants in
Philippines

18. Prions
Abnormally shaped infectious protein responsible for some brain diseases in mammals, including humans
When tissues are eaten by another animal, prions enter blood stream and go to its brain
Prions interact with normal proteins causing them to become abnormal and infectious
E.g. Bovine spongiform encephalopathy (BSE)
(Mad Cow Disease)
Humans - Creutzfeldt-Jakob Disease
(CJD)