Viruses
Viruses: “Non-living” Entities
• Small packages of nucleic acids in a protein
coat
• Are NOT cells—no cytoplasm and do not
perform metabolic reactions
• Obligate intracellular parasites —dependent
upon other cells for replication
Viruses Replicate Inside Living
Cells
• Viruses lack enzymes needed for
metabolism and have no structures to make
proteins
• Use cells own machinery to replicate
viruses
• Pathogen: agent that causes disease
– Viruses damage cells during replication
Types of Viruses
• DNA viruses
– Genome is DNA
• RNA viruses
– Genome is RNA
– Smaller than DNA
viruses
– Lack of
proofreading—leads to
 evolution rate
Bacteriophages
• Virus that infects
bacteria
• Two replication cycles
– Lytic cycle
– Lysogenic cycle
Replication in Bacteriophages
Virulent Virus
Temperate Virus
Animation: Phage T4 Lytic Cycle
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Animation: Phage Lambda Lysogenic and Lytic Cycles
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Plant Viruses
• Viruses that infect
plants
– Can stunt growth and
diminish plant yields.
– Can spread throughout
the entire plant.
Animal Viruses
• Viruses that infect
animals are common
causes of disease.
Replication cycle of an enveloped
virus
HIV, the AIDS Virus
• HIV is a retrovirus.
– A retrovirus is an RNA
virus that relicates by
means of a DNA
molecule.
– It copies its RNA to
DNA using reverse
transcriptase.
– Infects white blood cells
which are involved with
immunity
Animation: HIV Reproductive Cycle
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AIDS
• Acquired immune deficiency syndrome.
• The disease caused by HIV infection.
• Treated with the drug AZT.
Vaccinations
• Antibiotics don’t work—no metabolic
reactions to interfere with
• Vaccinations—parts of viruses, modified or
killed viruses are injected into the body
– Allows immune system to make antibodies
against specific markers on the viral coat
• HIV mutates too fast for immune system to keep up
with
Influenza Vaccine
• Influenza, also known as the flu,
is a contagious disease that is
caused by the influenza virus. It
attacks the respiratory tract in
humans (nose, throat, and
lungs). The flu is different from
a cold. Influenza usually comes
on suddenly and may include
these symptoms:
– Fever
– Headache
– Tiredness (can be extreme)
– Dry cough
– Sore throat
– Nasal congestion
– Body aches
Evolution Connection:
Emerging Viruses
• Viruses that have appeared suddenly or recently come to
scientists’ attention
• Many new viruses have emerged in recent years:
– HIV
– Ebola
– West Nile
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Evolution Connection:
Emerging Viruses
• How do new viruses arise?
– Mutation of existing viruses
– Spread to new host species
• ¾ of new human diseases
originated in other animals
• Widespread epidemics possible
because of technology and social
factors
–
–
–
–
Affordable international travel
Blood transfusions
Sexual promiscuity
Abuse of intravenous drugs
Evolution Connection:
Emerging Viruses
• Early reports in 2004
brought news of people in
Southeast Asia
– Infected with a flu virus
previously seen only in
birds.
• As of early 2006,
– Over 140 people in six
Asian countries have
become infected.