Higher Human Biology
Unit1: Cell function and Inheritance
Chapter 8:Viruses
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).
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
Different types of viruses
• Influenza virus
• AIDS virus
COLD SORES CAUSES BY THE
HERPES SIMPLES VIRUS
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
Different Shaped Viruses
It has long been debated whether
viruses are living organisms or not.
• 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)
Structure of a Virus
• A virus contains
nucleic acid (DNA or
RNA)
• Surrounded by a
protective coat
(capsid)
Spread of the virus - also known as
obligate parasites
Viruses cause disease therefore they are called __________.
• Viruses can be spread by
– touching,
– body fluids, HIV
– aerosols (sneezing, coughing
etc.), e.g. Pneumonia
– Bites, e.g. Rabies, Malaria
Remember Viruses also attack plants, seeds and bulbs!
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.
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
Binding With the Host Cell
Then the virus “injects” its
DNA or RNA into the
cytoplasm
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.
ASSEMBLY
• Many viral protein
coats are then made
using the cell’s free
amino acids and protein
synthesis factory
(ribosomes, enzymes
codons and anticodons).
EACH VIRAL DNA BECOMES ENCLOSED
IN A PROTEIN COAT
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.
Before and After Lysis
Budding
TASK: Testing your knowledge
• Complete Torrance TYK questions on page 63.
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.
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 .
Smallpox
Smallpox – led to the
st
1
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!!
Watch this - http://www.youtube.com/watch?v=jJwGNPRmyTI
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.
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.
Acquired Immune Deficiency
Syndrome (AIDS)
• AIDS is a retrovirus
• It attacks helper Tlymphocytes.
• The AIDS virus
attaches itself by
glycoprotein on its
surface to receptors on
the helper T cell
surface.
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.
Electron micrograph (TEM) showing the cell
membrane of a T-lymphocyte white blood cell (Tcell). The "bubbles" are budding Human
Immunodeficiency Viruses (HIV).
• Video
• And http://www.xvivo.net/zirusantivirotics-condensed/
Random but interesting…
• In the future:
– Viruses might cure cancer
– They can deliver gene therapy
– They could help us fight antibiotic-resistant
bacteria
TASK: Testing your knowledge
• Complete Torrance TYK questions on page 65.
Essay question 2009
2B. Describe the nature
and reproduction of
viruses. (10)