Download Ch 24 Viruses

Chapter 24
Viruses
Table of Contents
Section 1 Viral Structure and Replication
Section 2 Viral Diseases
Chapter 24
Section 1 Viral Structure and Replication
Objectives
• Summarize the discovery of viruses.
• Describe why viruses are not considered living organisms.
• Describe the basic structure of viruses.
• Compare the lytic and lysogenic cycles of virus replication.
• Summarize the origin of viruses.
What is happening
in the photograph?
Newly assembled
HIV viruses are
leaving the T cell.
How did the
viruses, pictured
leaving the cell,
enter the cell?
The genetic
material from a
single HIV particle
entered the cell and
was responsible for
the production of
all the viruses
pictured.
Chapter 24
Section 1 Viral Structure and
Replication
Discovery of Viruses
• Researchers in the late 1800s
discovered that something
smaller than bacteria could cause
disease.
• In 1935, Wendell Stanley
demonstrated that viruses were
not cells when he crystallized
TMV, the virus that causes
tobacco mosaic disease in
tobacco and tomato plants.
• Virus
Chapter 24
Section 1 Viral Structure and
Replication
Characteristics of Viruses
• What are the characteristics of living
organisms?
– Cellular organization, reproduction, metabolism,
homeostasis, heredity, responsiveness, and
growth and development.
• Viruses do not have all of the characteristics of
life and are therefore not considered to be
living.
Chapter 24
Section 1 Viral Structure and
Replication
Characteristics of Viruses, continued
• Viral Size and Structure
– Viruses are nonliving particles containing DNA or
RNA and are surrounded by a protein coat called a
capsid.
– Some viruses also have an envelope that is
derived from a host cell’s nuclear membrane or
cell membrane.
– Parts of a Virus
Chapter 24
Section 1 Viral Structure and
Replication
Characteristics of Viruses, continued
• Classification of Viruses
– Viruses can be classified based on whether they
have RNA or DNA, whether the RNA or DNA is
single or double stranded and circular or linear, by
capsid shape, and whether or not they have an
envelope.
Review
• What have we learned about the size and
structure of viruses?
– Very small
– Various sizes and shapes (helical, polyhedral,
enveloped)
• How are viruses classified?
–
–
–
–
–
RNA or DNA
Single or double stranded
Linear or circular
Capsid characteristics
Presence or absence of envelope
Chapter 24
Section 1 Viral Structure and
Replication
Viral Replication, continued
• Replication in DNA Viruses
– DNA viruses can enter host cells and directly
produce RNA, or they can insert into a host’s
chromosome, where they are transcribed to RNA
along with the host’s DNA.
Chapter 24
Section 1 Viral Structure and
Replication
Viral Replication, continued
• Replication in RNA Viruses
– The RNA genome of some RNA viruses can be
directly translated to make viral proteins.
– Retroviruses use reverse transcriptase and RNA
as a template to make DNA, which is then used to
produce viral RNA and proteins.
• HIV, murine leukemia virus, feline immunodeficiency
virus.
Reverse Transcriptase
• Why is the enzyme contained in retroviruses
called reverse transcriptase?
– Catalyzes the transcription of DNA from RNA
Chapter 24
Section 1 Viral Structure and
Replication
Viral Replication
• Replication in Viruses That
Infect Prokaryotes
– Bacteriophages are viruses that
infect bacteria.
Chapter 24
Section 1 Viral Structure and
Replication
Viral Replication, continued
• Lytic Cycle
– Viruses can follow a lytic cycle, making new viral
particles immediately.
– Occurs when virus is active
– Lytic Cycle
Chapter 24
Section 1 Viral Structure and
Replication
Viral Replication, continued
• Lysogenic Cycle
– Viruses can follow a lysogenic cycle, becoming
part of the host genome and making new particles
later.
– Not active
– Lysogenic Cycle
Chapter 24
Section 1 Viral Structure and
Replication
The Lytic and Lysogenic Cycles
Chapter 24
Section 1 Viral Structure and
Replication
Viral Replication, continued
• Viruses: Tools for Biotechnology
– Viruses are important tools for biotechnology.
Review
• How do viruses reproduce?
– Directly produce RNA
– Insert into host DNA
– Lytic or lysogenic cycles
Chapter 24
Section 1 Viral Structure and
Replication
The Origin of Viruses
• Most scientists think viruses originated from
fragments of host-cell nucleic-acid material.
Chapter 24
Section 2 Viral Diseases
Objectives
• Name several vectors of viral diseases.
• Identify four viral diseases that result in serious human illnesses.
• Discuss the relationship between viruses and cancer.
• Name three examples of emerging viral diseases.
• Compare the effectiveness of vaccination, vector control, and drug
therapy in fighting viruses.
• Contrast viroids, prions, and viruses.
Chapter 24
Section 2 Viral Diseases
Vectors of Viral Diseases
• Vectors, or hosts, of viral diseases include
humans, animals, and insects.
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.
Chapter 24
Section 2 Viral Diseases
Human Viral Diseases, continued
• Chickenpox and Shingles
– Chickenpox and shingles are caused by the same
varicella-zoster herpesvirus.
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.
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 and thus damages the
body’s immune system. The disease called
acquired immune deficiency syndrome (AIDS)
results.
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, while other viruses convert protooncogenes, which usually control cell growth, to
oncogenes.
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.
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.
Chapter 24
Section 2 Viral Diseases
Prevention and Treatment,
• Vector Control continued
– Control efforts, including killing mosquitoes and
other vectors and quarantining ill patients, have
helped reduce the spread of certain viral diseases.
Chapter 24
Section 2 Viral Diseases
Prevention and Treatment,
• Drug Therapy continued
– Antibiotics are ineffective against viral diseases.
– Viral drugs, such as acyclovir, block specific steps
in viral replication.
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 Viral Diseases
Important Viral Diseases