2. Lesson Aims
• To describe the structure of a virus
• The examine the nature of viruses and their
invasion of cells
• To discuss the alteration of cell instructions
to produce more viruses ( the process of
viral replication).
3. Viral diseases an introduction
• MUMPS, MEASLES, GERMAN MEASLES, and
CHICKEN POX are common childhood illnesses
caused by viruses.
• The common cold is usually due to the rhinovirus.
The "flu" is caused by INFLUENZA viruses.
• HEPATITIS, an inflammation of the liver, may be the
result of one of several viruses. Rabies and yellow
fever are highly lethal viral diseases.
• HIV -
Human Immunodeficiency Virus
6. Size
• Viruses are a lot smaller
than cells.
• Smaller than bacteria
• 20-300 nanometres (nm)
• Viruses can only be seen
using an electron
microscope.
• 200 nm 0.2 µm 0.0002 mm
0.0000002 m
8. • They are not cellular – no nucleus,
membrane, other organelles to sustain it’s
own life.
• However, they can reproduce – but they
rely on living cells to do so.
• Therefore they possess the blue print for life
– Nucleic acid (DNA or RNA)
It has long been debated whether
viruses are living organisms or not.
9. Structure of a Virus
• A virus contains
nucleic acid (DNA or
RNA)
• Surrounded by a
protective coat
(capsid)
10. Spread of the virus - also known as
obligate parasites
• Viruses can be spread by
– touching,
– body fluids, HIV
– aerosols (sneezing, coughing
etc.), e.g. Pneumonia
– Bites, e.g. Rabies, Malaria
Viruses cause disease therefore they are called __________.
Remember Viruses also attack plants, seeds and bulbs!
11. SPECIFICITY
• Viruses are often
specific with respect
to their chosen type
of host cell.
• The poliomyelitis
virus attacks nerve
cells
• The Hepatitis virus
attacks liver cells.
12. INVASION: Entering the Host Cell
• A virus will attach itself to a cell
• Different viruses employ different methods
of entering the host cell (antigen dependant
– HIV-wbc’s, flu-throat cells etc.)
• Process depends on antigenic sites on the
virus binding with certain complementary
molecules in the membrane or the specific
host cell
• Following binding the nucleic acid is
introduced into the host cell
14. Then the virus “injects” its
DNA or RNA into the
cytoplasm
15. Viral DNA
replication and
protein synthesis
• Once in the host cell the virus takes over the
cell’s biochemical machinery (cell metabolism).
• The nucleus then follows the viral DNA
(blueprint) .
• Host cell supplies energy (ATP), nucleotides,
enzymes and amino acids
• Host then makes many identical copies of viral
nucleic acid (Viral DNA) and protein coats.
16. ASSEMBLY
• Many viral protein
coats are then made
using the cell’s free
amino acids and protein
synthesis factory
(ribosomes, enzymes
codons and anti-
codons).
EACH VIRAL DNA BECOMES ENCLOSED
IN A PROTEIN COAT
17. Release
• The protein coats house the
NA and the new viruses is
released out of the cell
either by
– LYSIS (bursting of the host
cell membrane)
– Or BUDDING (the cell
membrane pinches off
containing the new viral
particle)
•
The new viruses are
ready to repeat the
process on more cells.
When the virus bursts
out the cell, the cell
dies.
20. TASK: Testing your knowledge
• Complete Torrance TYK questions on page 63.
21. History of viral diseases
• Humans have been known
to be plagued by viruses
since the beginning of
recorded time.
• An ancient Egyptian relic
from about 1500BC shows
a priest with a shrivelled
leg thought to be due to
poliomyelitis.
22. History of viral diseases
• The conquest of the
Mexican Aztec nation in
the 1500s by a small force
of Spaniards is thought to
be the reason that the
smallpox virus transfer
from Europe to Mexico
causing a lethal epidemic .
24. Smallpox – led to the 1st vaccine
• Up until the end of the 18c smallpox was wide spread
in Britain.
• It causes severe fever and was fatal in 1:5 cases.
• Survivors were left badly scarred
• In 1796 a British doctor Edward Jenner injected a
small boy with a milder strain called cowpox. Jenner
then injected the boy with the deadly strain.
Fortunately he survived.
THE SCIENCE OF ARTIFICIALLY ACQUIRED
ACTIVE IMMUNITY BEGAN!!
25. Watch this - http://www.youtube.com/watch?v=jJwGNPRmyTI
26. Safe vaccines – Polio prevention
• The first vaccine used to cause
immunity against poliomyelitis
virus contained DEAD VIRUS.
• After this vaccines became
ATTENUATED – this means the
NA has been rendered harmless
but the antigenic properties of its
protein coat remain unaltered.
• Use of this vaccine almost caused
the complete eradication of polio.
27. Retrovirus
• A retrovirus which
contains RNA.
• It also contains reverse
transcriptase an
enzyme which
produces viral DNA
from viral RNA.
• The virus can thus
replicate itself.
28. Acquired Immune Deficiency
Syndrome (AIDS)
• AIDS is a retrovirus
• It attacks helper T-
lymphocytes.
• The AIDS virus
attaches itself by
glycoprotein on its
surface to receptors on
the helper T cell
surface.
29. The mechanism of HIV
• The envelope surrounding the HIV particle fuses
with the membrane of the helper T-cell and the
virus enters the host cell.
• Viral DNA becomes incorporated into the host
cell’s DNA where it can remain dormant for many
years.
• Viral mRNA is transcribed and it directs synthesis
of new viral particles inside the host cells
• These escape from the infected helper T cell by
budding.
• The original T cell’s membrane is left perforated
which causes destruction of the cell.
30. Electron micrograph (TEM) showing the cell
membrane of a T-lymphocyte white blood cell (T-
cell). The "bubbles" are budding Human
Immunodeficiency Viruses (HIV).
31.
32.
33. • Video
• And http://www.xvivo.net/zirus-
antivirotics-condensed/
34. Random but interesting…
• In the future:
– Viruses might cure cancer
– They can deliver gene therapy
– They could help us fight antibiotic-resistant
bacteria
35. TASK: Testing your knowledge
• Complete Torrance TYK questions on page 65.
