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Viruses
What is a Virus?
• Particles of nucleic acid and protein
• reproduce by infecting living cells
What do all viruses have in
common?
• They enter living cells and, once inside,
use the machinery of the infected cell to
produce more viruses.
Comparing the size of a virus, a bacterium, and an animal cell
Most viruses are so small they can only be seen with
the aid of a powerful electron microscope
A virus is smaller
than a bacterium
Virus
Bacterium
Animal
cell
Animal cell nucleus
0.25 m
Viral Structure
A typical virus is composed of a core of either DNA
or RNA surrounded by a protein coat
Capsid – made
of protein
DNA or RNA
sheath
T4
Bacteriophage
Viral structure
Virus consists of a protein shell called a Capsid
The capsid may have many shapes
Capsomere
of capsid
RNA
Capsomere
DNA
Membranous
envelope
Head
Capsid Tail
sheath
RNA
Tail
fiber
Glycoprotein
Glycoprotein
18  250 mm
70–90 nm (diameter) 80–200 nm (diameter)
20 nm
50 nm
(a) Tobacco mosaic virus (b) Adenoviruses
Respiratory infections
DNA
80  225 nm
50 nm
50 nm
(c) Influenza viruses (d) Bacteriophage T4
How does a virus get inside a cell
• The capsid proteins bind to receptors on
the host cell
• It “tricks” the cell into allowing it inside.
• Once inside the viral genes are expressed.
• The host cell makes more viruses and the host
cell is destroyed.
Viruses are very specific to the cells they
infect
a particular virus can only infect a specific
cell.
Bacteriophage –
infects bacteria
Reproductive Cycles of Phages
• Two types
– Lytic cycle or Virulent
• Host cell is lysed (bursts) and immediately destroyed
– Lysogenic cycle or Temperate
• The virus embeds its DNA into the host’s DNA
• The virus’s DNA is replicated along with the host
cell’s DNA
• A host cell makes copies of the virus indefinitely.
The lytic cycle of phage T4, a virulent phage
1
Bacteriophage attaches to
2
The bacterium’s cell wall
Bacteriophage injects its
DNA into the cell
5 The bursting of the cell
Releases new bacteriophage
particles
Phage assembly
4 New viruses are assembled. The
Head
Tails
Tail fibers
3
Virus wrecks the cell causing it to
The cell makes mRNA from the
burst
Bacteriophage’s genes
Lysogenic Infection
• The host cell is not immediately destroyed
• Viral DNA inserted into the host cell as a
prophage
A comparison of the lytic and lysogenic cycles
of a bacteriophage
Phage
DNA
The phage attaches to a
host cell and injects its DNA.
Many cell divisions
produce a large
population of bacteria
infected with the
prophage.
Phage DNA
circularizes
Phage
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Bacterial
chromosome
Lytic cycle
Lysogenic cycle
Certain factors
determine whether
The cell lyses, releasing phages.
Lytic cycle
is induced
New phage DNA and
proteins are synthesized
and assembled into phages.
or
Lysogenic cycle Prophage
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
The viral DNA is embedded in the
Host’s DNA
Retroviruses
• Contain RNA as their genetic information
• They produce a DNA copy of their RNA
– RNA produces DNA
– (this is the reverse of what normally happens in healthy cells)
• This DNA is inserted into the DNA of the host cell
– Example….AIDS
Is a Virus a living thing?
Some biologists do not consider viruses to be
alive because they can’t reproduce
independently.
Viruses do have some characteristics of a
living organism once it infects a host cell,
so they are at the borderline of living and
non-living things.
Viruses and Cells
Virus
Cell
Structure
Structure
DNA or RNA, core, capsid
Reproduction
only within a host cell
Genetic Code – DNA or RNA
Growth and Development – NO
Obtain and Use Energy – NO
Response to the Environment NO
Change Over Time - YES
cell membrane, cytoplams;eukaryotes
also contain nucleus and organelles
Reproduction
Independent cell division either
asexually or sexually
Genetic Code – DNA
Growth and Development – YES
Obtain and Use Energy - YES
Response to the Environment - YES
Change Over Time - YES
influenza
Hand, foot and mouth
mumps
HPV
Ebola
Cold sore
Chicken pox
rubella
AIDS
Small pox
Viral Disease
• Diseases in Humans
– Polio, measles, AIDS, mumps, influenza,
yellow fever, smallpox, the common cold, and
many others
• Viral Diseases in Humans
• Disrupt the bodies normal equilibrium
– Symptoms caused by
• Toxins produced by infected cells
• Toxic components of the viruses
themselves
• Cells killed or damaged by the virus
• Body defense mechanisms fighting the
infection
Viral Damage
• Depends on the ability of infected tissue to
repair itself
– Epithelium of respiratory tract heals fast so we
get over colds
– Polio –damage to nerve cells is permanent
Cancer causing viruses
• Oncogenic viruses
– Examples:
• Human papilloma virus
• Human herpes virus
• Hepatitis B
Viral Diseases in Animals
• Rabies
• Parvo
• Hoof and mouth disease
Viral Diseases in Plants
• Have a difficult time invading plants due to
the cell wall
• Plant injuries make a plant susceptible to viral
disease
• Insects act as carriers
The simplest infectious agents
• Viroids – small infectious molecules of
single stranded RNA that have no
surrounding capsids.
– Affect plant cells
• Cause stunted plant growth
• Prions – protein infectious particles
– Linked to several degenerative brain diseases
• Ex. Mad cow, Creutzfeldt-Jacob
– Transferred in food
– Prions are nearly indestructible
Ways to protect against viruses
• Vaccines
• Variants or derivatives of pathogens
– Stimulate the immune system to react against
the actual disease
– Few drugs treat or cure viral diseases
• Antiviral drugs that are effective interfere with viral
DNA or RNA synthesis
• Example polio, smallpox, rubella, mumps, Hepatitis
B
Most viral infections cannot be
cured once they occur
– Antibiotics are powerless against viruses
• Viruses do not contain the enzymes which
antibiotics work against.
How the body attacks a virus
http://www.bbc.co.uk/science/0/22028517