Viruses
Viruses
Which Kingdom?
Viruses
They are considered “lifeless”…why?
They can NOT carry out metabolic and
reproductive functions alone.
They require a host organism
They lack cytoplasm, organelles, and a cell
membrane
5000 influenza viruses can be found on the
head of a pin.
Viral Infections
• Are they all deadly?
• No, viruses range from mild to
deadly
• Ex. Herpes simplex I, West Nile
Virus, cold/flu viruses, Cancer,
HIV/AIDS.
• Each virus is specific to a host
• The host range describes all of
the hosts a virus can infect
• E.g. swine flu virus infects both
pigs and humans
Scientists classify viruses based on their
unique characteristics, including:
size and shape of the capsid (protein coat
surrounding genetic material)
shape and structure of the virus
type(s) of diseases the virus causes
type of genetic material (RNA or DNA)
method of reproduction
Virus Shape and Structure:
There are different shapes for viruses:
1) Helical (Spiral)
2) Polyhedral
Enveloped
Non-enveloped
3) Complex/Phage (many sides with
a tail)
E.g. Bacteriophage
(bacteria eating virus)
An inner genetic
material (nucleic acid DNA or RNA)
Capsid – outer protein
coat
VIRUS TYPES
There are many virus types
including:
Retroviruses
Reoviruses
Adenoviruses
Rhabdoviruses
Orthomyxoviruses, etc
Each has a different inner
nucleic acid, coat and
structure
Methods of Reproduction
Incubation period = the time it takes between
when the virus infects a cell and when
symptoms appear (when the virus ruptures
the cell).
There are two types of reproduction/replication:
 The LYTIC cycle – shorter incubation period
 The LYSOGENIC cycle – longer incubation period
How Do Viruses Replicate?
Viruses replicate through a cycle known as the lytic cycle
Some viruses, such as cancer causing viruses and
bacteriophages, go through a lysogenic cycle
Occurs without the host being aware and the host may not be aware
until years later (e.g. HIV)
Lytic Cycle
The lytic cycle occurs through four steps:
1. Attachment and entrance: the virus recognizes the host
cell and then enters into the cell’s cytoplasm
2. Synthesis of protein and nucleic acid: The virus tricks
the cell into copying the virus DNA (or RNA)
How Do Viruses Replicate?
Virus replicates through four steps:
3. Assembly of the virus: All the virus components (i.e.
DNA (or RNA), enzymes, capsid proteins, and other
proteins are brought together
4. Release of new viruses: the new viruses burst out of the
cell in a process known as lysis
Lysogenic Cycle
The virus enters the cell just as the lytic cycle,
but instead of it replicating, its nucleic acid
becomes part of the cell’s nucleic acid
When the cell replicates, so does the virus
nucleic acid
https://www.youtube.com/watch?v=3vSRfVC4OIo
Viruses and Human Health
Viruses destroys cells as they replicate  This
causes the symptoms
Vaccines are used to prevent virus infection
Vaccines: are made of dead virus or virus
components to activate the body’s immune cells
to prepare them for battle
VIRUSES AND CANCER
Some viruses can cause cancer by adding
specific genes to an infected cell
transforming it into a cancer cell
Virus
Human
papillomavirus
Hepatitis B/C
Epstein-Barr virus
Cancer Type
Cervical, skin, etc.
Liver cancer, lymphomas
Burkitt’s lymphoma,
Hodgkin’s lymphoma
INFLUENZA VIRUS
Various strands of the flu
virus exist in the
environment
Flu Vaccines only treat the
most common strain of the
season
Virus destroys cells in
upper respiratory tract so
there is no sweeping of
foreign particles
Symptoms – sore throat,
congested lungs
Human
Immunodeficiency
Virus (HIV)
Virus that causes AIDS
(Aquired Immunodeficiency
Syndrome)
Retrovirus is transmitted
through variety of bodily
fluids: blood, semen, vaginal
fluid, breast milk
HIV attacks the immune
system
Since 1981, almost 25
million people have died
from AIDS
Other Viral Diseases
Varicella zoster virus:
chickenpox (children)
and shingles (adults)
Poliovirus - Polio
Variola major virus:
smallpox
SARS coronovirus:
Severe Acute
Respiratory Syndrome
(SARS)
Ebola:
https://www.youtube.com/watch?v=sRv19gkZ4E0
Homework
Read pages 52-57
#1-3, 5, 7, 8, 11-14 (p.58)