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Chapter 24
Viruses
Table of Contents
Section 1 Viral Structure and Replication
Section 2 Viral Diseases
In the late 1800’s
• Scientists really didn’t know much about what a virus
was (no tech)
• They found out that it wasn’t a bacteria causing the
disease.
• They knew it was smaller than a bacteria cell
• Scientists began to study viruses
• They wanted to know if they were cellular or not
• So are Viruses made of cells?
Wendell Stanley 1935
• He took a TMV (tobacco
mosaic virus) plant and broke
it down to it’s smallest part
• He was expecting to see a
cell, but instead he saw
crystals!
• So Wendell concluded that
viruses are NOT alive!
Chapter 24
Section 1 Viral Structure and
Replication
Characteristics of Viruses
•
•
•
•
•
•
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Lack cytoplasm
Lack organelles
Cannot do metabolism
Cannot do homeostasis
Do not have cells
Have DNA OR RNA, not both
Cannot reproduce outside their host cell
– They must enter a living cell and use it’s
machinery (ribosomes, ATP & enzymes) in order
to reproduce
Viral Sizes
• Extremely small; cannot be seen using a light
microscope
• Vary in size and shape
Virus Structure
• Capsid – protein coat that
surrounds it’s DNA or RNA
• Envelope – some viruses
have this 2nd layer. It is
made of specific proteins
and help new viruses
recognize host cells (ex. –
chickenpox & HIV)
• Bacteriophages (viruses
that attack bacteria) have
a tail and tail fibers
capsid
Chapter 24
Section 1 Viral Structure and
Replication
Virus
Click below to watch the Visual Concept.
Visual Concept
Chapter 24
Section 1 Viral Structure and
Replication
Parts of a Virus
Click below to watch the Visual Concept.
Visual Concept
Chapter 24
Section 1 Viral Structure and
Replication
Characteristics of Viruses, continued
Viruses can be classified based on:
• whether they have RNA or DNA
• whether the RNA or DNA is single or double stranded
• Whether the RNA or DNA is circular or linear
• capsid shape
• whether or not they have an envelope.
Other things….
• Outside their host, virus’s are lifeless with no control
over its movements
• Viruses are spread through
• Air
• Water
• Food
• Body fluids
• In order to replicate, viruses need to recognize the
appropriate host cell before it can infect it. An
enveloped virus needs to match the host cell’s
membrane receptors precisely.
Chapter 24
Section 1 Viral Structure and
Replication
Replication in DNA Viruses
• The DNA in some DNA viruses enter a host cell and
then it makes mRNA
• The DNA in some DNA viruses insert into the host’s
chromosomes. Then it is called a provirus.
• Some DNA viruses use the host cell enzymes to
make new viral DNA and eventually new viruses!
Chapter 24
Section 1 Viral Structure and
Replication
Replication in RNA Viruses
• Some RNA viruses enter their host and that RNA
serves as mRNA, which then is translated into new
viral proteins immediately
• Some RNA viruses, called retroviruses contain the
enzyme reverse transcriptase in addition to RNA.
• Reverse transcriptase uses RNA as a template to
make DNA, which then inserts into the host cell’s
genome.
• Reverse transcriptase reverses the normal process of
transcription
• HIV is a retrovirus
Chapter 24
Section 1 Viral Structure and
Replication
Replication in Bacteriophages
• Remember bacteriophages are viruses that attack
bacteria
• They have complex capsids
• They replicate by one of both of two different
methods:
– Lytic cycle
– Lysogenic cycle
Chapter 24
Section 1 Viral Structure and
Replication
Lytic Cycle
• Lytic = lyse = break
• This means that the host cell will rupture after
replication is completed.
• Viruses that do this cycle are called virulent.
• There are 5 steps to the lytic cycle
5 Steps to the Lytic Cycle
1. Bacteriophage attaches its tail fibers to specific
receptor molecules on the cell surface of a bacteria
cell. The DNA is injected into the host cell
2. The ends of the viral DNA attach to each other,
forming a circle. It is separate from the host cell’s
DNA
3. The viral DNA takes control of protein synthesis and
the viral genome is copied. Enzymes and
ribosomes are used from the host cell.
4. New Viral pieces are made and put together
5. Lysozyme enzymes digest the cell wall and up to
200 new viruses are released.
Chapter 24
Section 1 Viral Structure and
Replication
Lytic Cycle
Click below to watch the Visual Concept.
Visual Concept
Chapter 24
Section 1 Viral Structure and
Replication
Lysogenic Cycle
• Host cell does not get killed
• It allows viruses to hide in the host cell for days,
months or even years.
• A virus that does this cycle is called temperate.
• 5 steps
5 Steps to Lysogenic Cycle
1. Bacteriophage attaches to host cell and injects it’s
DNA
2. Viral DNA attaches its ends to make a circle
3. The viral DNA then integrates into the host DNA –
now it’s a prophage
4. When the bacteria replicates it’s own DNA, it
replicates the viral DNA. So now each bacteria cell
will be infected with a prophage.
5. The prophage will exit the host cell – usually caused
by radiation or chemicals
Section 1 Viral Structure and
Replication
Chapter 24
Lysogenic Cycle
Click below to watch the Visual Concept.
Visual Concept
Chapter 24
The Lytic and
Lysogenic
Cycles
Section 1 Viral Structure and
Replication
Chapter 24
Section 1 Viral Structure and
Replication
The Origin of Viruses
• Most scientists think viruses originated from naked
pieces of nucleic acid and evolved from that
• Viruses mutate quickly, such as influenza and HIV
• It is difficult for the immune system to recognize and
destroy them, which makes it hard to make good
vaccines.
• A vaccine is a live virus that is injected so that the
body can make antibodies to kill it
• Therefore each year, there is a new “flu shot”
available,
Chapter 24
Section 2 Viral Diseases
Vectors of Viral Diseases
• Vectors, or hosts, of viral diseases include humans,
animals, and insects.
• They transfer a pathogen or parasite or virus to
another organism
Chapter 24
Section 2 Viral Diseases
Human Viral Diseases
• Viruses cause many human diseases, including the
common cold, flu, hepatitis, rabies, chickenpox,
certain types of cancer, and AIDS.
• They can affect various human organs, such as the
brain, liver, heart, lungs and skin
Chapter 24
Section 2 Viral Diseases
Human Viral Diseases, continued
• Chickenpox and Shingles
• Chickenpox and shingles are caused by the same
varicella-zoster herpesvirus.
• It multiplies in the lungs and travels to blood vessels
in the skin
• Symptoms are fever and skin rash
• It is spread thru direct contact with the skin rash and
through the air
• After having it, people usually won’t get it again, but it
can stay in the nerve cells are a provirus. Then if it
gets activated, it’s called shingles.
Chapter 24
Section 2 Viral Diseases
Human Viral Diseases, continued
• Viral Hepatitis
• Hepatitis, or inflammation of the liver, can be caused
by at least five viruses.
• Hepatitis A and hepatitis E can be spread by fecally
contaminated food and water.
• Hepatitis B, C, and D are spread by sexual contact,
by contact with infected blood and serum,and by the
use of contaminated needles.
• Symptoms are fever, nausea, jaundice & liver failure
Chapter 24
Section 2 Viral Diseases
Human Viral Diseases, continued
• Acquired Immune Deficiency Syndrome (AIDS)
• The human immunodeficiency virus (HIV) is an RNA
virus spread by sexual contact, by contact with
infected body fluids, and from mother to fetus.
• HIV targets macrophages, which are immune system
cell, and thus damages the body’s immune system.
The disease called acquired immune deficiency
syndrome (AIDS) results.
HIV
• Virus attaches to the CD4 and CCR5 receptors on
cell surface. Binding must occur.
• The viral envelope fuses with the membrane and
releases the capsid into the host cell
• Viral RNA and reverse transcriptase are released and
used to make a template to make a double stranded
DNA version of the viral genome
• The HIV DNA enters the cell’s nucleus and integrates
into the cell’s DNA, thus making a provirus
• Transcription and translation occurs and viral proteins
are made
• Then HIV assembles and a new envelope forms
around it and eventually is released from the cell
Chapter 24
Section 2 Viral Diseases
AIDS (Acquired Immunodeficiency Syndrome)
Chapter 24
Section 2 Viral Diseases
Human Viral Diseases, continued
• Viruses and Cancer
• Some viruses contain oncogenes that can cause
cancer
• other viruses convert proto-oncogenes, which
usually control cell growth, to oncogenes.
• Human papillomavirus (HPV) can cause cervical
cancer and hepatitis B virus can cause liver cancer
Chapter 24
Section 2 Viral Diseases
Emerging Viral Diseases
• Emerging viruses usually infect animals isolated in
nature but can jump to humans when contact occurs
in the environment.
• Ebola virus – animals in tropical forests of central
Africa
• SARS – civet cats
• Vaccines are expensive to develop for these
infectious diseases
Chapter 24
Section 2 Viral Diseases
Prevention and Treatment
• Vaccinations
• A vaccine contains a harmless version of a virus,
bacterium, or a toxin that causes an immune
response when introduced to the body.
• Vaccines have helped to greatly reduce certain viral
diseases.
• Viral vaccines are made from inactivated viruses,
which are viruses that are unable to replicate in a
host
Attenuated Viruses
• Weakened forms of a virus that cannot cause a
disease
• When made into a vaccine, they provide greater
protection from disease
• Ex., measles, mumps, rubella, polio, hepatitis a and b
and chicken pox
Chapter 24
Section 2 Viral Diseases
Virods and Prions
• Viroids are short, circular, single strands of RNA
lacking a capsid that infect plant cells.
• Prions are infectious particles containing protein but
no nucleic acids.
– Prions cause mad cow disease and similar
degenerative brain diseases.
Chapter 24
Section 2 Important Viral
Diseases