 Very
small
 Non-living
 Made
 Can
 Do
particle
up of nucleic acid and proteins
only reproduce by infecting living cells
not carry out respiration, grow or develop
 Viruses
have an inner core which contains
either RNA or DNA
 The
core consists of the viruses genetic
material.
 Has
outer coat called a capsid that is made
up of proteins
Capsid
Nucleic
Acid: DNA
or RNA
 Viruses
have
different shapes and
sizes.
 The
arrangement of
the proteins in the
capsid of a virus
determines the
virus’s shape
 The
cell in which a virus replicates
 Before
a virus can replicate it must enter a
host cell
 Before
it can enter a host cell it must first
recognize and attach to a receptor site on
the plasma membrane of the host cell.
 Most
viruses are highly specific to the cells
they infect.

Ex: Plant viruses only infect plants
 Bacteriophages
bacteria

Ex: bacteriophage T4
 Viruses
cell:


are viruses that infect
have two ways of infecting the host
-Lytic Infection
-Lysogenic Infection
 Lytic





Cycle
virus injects its nucleic acid into the host
takes over the host cells metabolism
causes host cell to start replicating the viruses
proteins and nucleic acids
virus secretes enzymes that “lyse” the host cell’s
cell wall and releases new virus particles
these particles can attack other host cells
Lytic Infection causes host cell to be “lysed” and
destroyed.
 Lysogenic



cycle
Lysogenic virus injects its nucleic acid into a host
cell
Viral DNA is integrated into the host cell’s
chromosome→ provirus
Every time host cell reproduces, the provirus is
replicated along with it → every cell that
originates from the infected host cell has a copy
of the provirus
 The
lysogenic cycle
can take place for
many years
 At
any time the
provirus can be
activated and enter
a lytic cycle
↓
host cell is killed
 Viruses
cause human diseases such as polio,
measles , influenza, and AIDS.
 Viruses
attack and destroy certain cells in
the body → causing symptoms of the disease
 The
best way to prevent against viral
diseases is to prevent infection through
vaccination.
A
preparation of a weakened or killed virus
or viral protein
 When
injected into the body, it stimulates
the immune system in an effort to create
permanent immunity to the disease
 For
vaccines to be most effective it must be
used before an infection begins.
 Vaccines
are used for the prevention of viral
infections
 Vaccines
are NOT used for bacterial
infections
The symptoms and incubation time of a disease can indicate how
the virus acts inside its host cell.
Disease
Symptom
Incubation
Measles
Rash, fever
9-11 days
Shingles
Pain, itching on skin
Years
Warts
Bumpy areas on skin
Months
Coryza (cold)
Sneezing, runny
nose, fever
2-4 days
HIV
Fatigue, weight loss
2-5 years
Which diseases may be caused by lytic viruses?
Which diseases may be caused by lysogenic viruses?
 Viruses
that contain RNA as their genetic
material
 When
retroviruses infect a cell they produce
a DNA copy of their RNA.
 Called
retroviruses because their genetic
information is copied backwards

From RNA to DNA instead of DNA to RNA
 Retroviruses
are responsible for some types
of cancer and HIV
 Retroviruses
can cause some cancers
 Retroviruses
that convert, or transform,
normal cells → tumor viruses
 These
viruses carry genes that disrupt the
normal controls over cell growth and division
 Was
the first virus
to be identified
 Causes
disease in
tobacco plants
What characteristic do viruses share with all
living organisms?
a.
b.
c.
d.
Respiration
Metabolism
Replication
Movement
A lytic infection occurs when a
a.
b.
c.
d.
Virus infects a bacterium and kills it
immediately
Virus embeds its genome into the DNA of
the host cell
Virus enters a cell, makes copies of itself,
and causes the cell to burst
Virus inserts its DNA into the DNA of the
host cell and remains part of the host cell
for many generations
One group of viruses that contain RNA as their
genetic information are
a.
b.
c.
d.
Oncogenetic viruses
Retroviruses
Capsids
Prophages
Fill in the blanks
The nucleic acid core of a virus contains
________ or __________
All viruses contain a coat of ________ and a
core of _________
 http://www.pbs.org/wnet/secrets/previous_
seasons/lessons/lp_virus_videos.html#