Transcript Chapter 18 - Virus Notes

Chapter 18 - Viruses
Structure
A
virus is a non-living particle
 Composed of DNA or RNA and a
protein coat
• The outer protein coat is called a
capsid
Common Diseases Caused by Viruses
Grouping
Viruses
Section 19-3
Type of Virus
Go to
Section:
Nucleic Acid
Disease
Oncogenic viruses DNA
cancer
Retroviruses
RNA
cancer, AIDS
Adenoviruses
DNA
respiratory infections
Herpesviruses
DNA
chickenpox
Poxviruses
DNA
smallpox
Viral Infection

Scientists learned about viral
replication by studying bacteriophages,
A bacteriophage is a virus that invades
bacteria. It attacks by adhering to a
cell wall & injecting its nucleic acids
that alter the bacterium’s genetic
code. Animal viruses enter their host
through endocytosis.


The reproductive cycle of a virus consists of
2 cycles: the lytic cycle & the lysogenic
cycle. The lytic cycle consists of the virus
killing the host. It has 5 stages.
1. Adsorption- the phage attaches itself to
the cell wall – a chemical bond forms between
sites on the tail of the virus & sites on the
cell wall – this is like a lock & key

2. Entry- The phage releases enzymes that
break down the wall – the tail is forced
through the weakened wall – the nucleic acid
of the phage passes through the hollow tail
into the host – the empty capsid is left
outside
3. Replication- the virus copies itself in the
cell – DNA or RNA enters the host’s
chromosomes & makes a template from its
own – the organism now makes viral proteins &
viral RNA
4. Assembly - viral nucleic acids &
proteins are assembled into new
particles called virions
 5. Release - new phages release an
enzyme that weakens the cell wall – the
host cell lyses & releases newly created
viruses- up to 300 new viruses can be
produced in 1 cell

The Lysogenic cycle is considered to be an
inactive cycle & may stay this way for several
generations. It has 3 stages.
1. Attachment - the nucleic acids of the
invading phage attach to the DNA of the host
cell – this is now called a prophage
 2. Replication - the prophage is now
replicated along with the DNA of the host
 3. Activation - the prophage enters the lytic
cycle & makes new viral parts


Pathogens are disease-causing
organisms. Virulence refers to the
ability to cause disease. A virus’
virulence depends on 1) Ability to
attach to host, 2) speed of
reproduction, & 3) response of the
host’s cells. The body’s natural defense
against disease is referred to as
immunity.

Cells called phagocytes engulf & destroy
invading organisms. An antigen is a foreign
protein or virus while antibodies are produced
by the body to fight them. Active immunity
is achieved when the body has actually been
exposed to antigens through the pathogen
itself or through a vaccine. This process
stimulates the production of
antibodies. Passive immunity is received
from the antibodies produced in another
person or animal that has developed
immunity. This is only temporary.
Retroviruses & Others
•
A retrovirus is deadly because it transcribes
DNA from an RNA template & inserts this into
the host. This is made possible by reverse
transcriptase. These viruses can have a lengthy
latent period. Examples are HIV & Herpes.
Prions are “protein infectious particles” that
contain no DNA or RNA. These typically infect
the brain (mad cow disease).
Prevention & Treatment
•
Control of viral diseases is
accomplished in one of two ways:
• Vaccination to prevent disease
• Administration of antiviral drugs to
infected patients
• The drugs interefere with nucleic
acid synthesis
Viruses and Cancer

Cancer genes may be triggered by
certain lysogenic viruses.
Any
Questions ???