VIRUSES
CHAPTER 19
TED talks: Nathan Wolfe's jungle search for viruses
BIG IDEA 3: LIVING SYSTEMS
STORE, RETRIEVE, TRANSMIT, AND
RESPOND TO INFORMATION
ESSENTIAL TO LIFE PROCESSES
EU 3.A: Heritable information provides for
continuity of life.
•EK 3.A.1: DNA, and in some cases RNA, is the
primary source of heritable information.
EU 3.C: The processing of genetic information is
imperfect and is source of genetic variation
•EK 3.C.3: Viral replication results in genetic
variation and viral infection can introduce genetic
variation into the hosts.
WHAT YOU MUST KNOW:
• The components of a virus.
• The differences between lytic and
lysogenic cycles.
BACTERIA VS. VIRUSES
BACTERIA
• Prokaryotic cell
• Most are free-living
(some parasitic)
• Relatively large size
• Antibiotics used to kill
bacteria
VIRUS
• Not a living cell (genes
packaged in protein shell)
• Obligate Intracellular
parasite
• 1/1000 size of bacteria
• Vaccines used to prevent
viral infection
• Antiviral treatment
VIRUSES
Video: Understanding viruses
Very small (<ribosomes)
Components = nucleic acid + capsid
•Nucleic acid: DNA or RNA (double
or single-stranded)
•Capsid: protein shell
•Some viruses also have viral
envelopes that surround capsid and
helps them infect hosts
Limited host range (eg. human cold
virus infects upper respiratory tract)
Reproduce within host cells
SIMPLIFIED VIRAL REPLICATIVE CYCLE
BACTERIOPHAGE
• Infects bacterial cells
• Complex capsids that
encloses their DNA
• A protein tailpiece that
attaches the phage to the host
• They inject phage DNA inside
host cell
• Bacteria have defenses
against phages
• use restriction enzymes that
recognize and cut up certain
phage DNA
LYTIC CYCLE OF
T4 PHAGE
BACTERIOPHAGE
REPRODUCTION
●Lytic Cycle:
◦Use host machinery to make copies of virus
◦Death of host cell by rupturing it (lysis)
◦Virulent phages replicate by this method
●Lysogenic Cycle:
◦Phage DNA incorporated into host DNA and
replicated along with it… without destroying host
◦Integrated viral “phage” DNA = prophage
◦Host cell division copies viral DNA
Temperate Phage: uses both methods of replication
LYTIC CYCLE VS. LYSOGENIC CYCLE
Viral Life Cycles
(video)
ANIMAL VIRUSES HAVE A
MEMBRANOUS ENVELOPE
Host membrane
forms around
exiting virus
Difficult for host
immune system
to detect virus
RETROVIRUS
• RNA virus that uses reverse transcriptase
(RNA → DNA)
• Newly made viral DNA inserted into
chromosome of host
• Host transcribes viral DNA (= provirus) into
RNA
• RNA uses to make new virus parts
• Example: HIV
• The broadest variety of RNA genomes is
found in viruses that infect animals
HIV =
Retrovirus
HIV
• Infects white blood cells
• HIV+: provirus (DNA inserted)
• AIDS: active viral reproduction
VACCINES
Weakened virus or part of pathogen that
triggers immune system response
EMERGING VIRUSES = MUTATION
VIDEO:
OF EXISTING VIRUSES
How Flu Viruses
Attack
VIROIDS
• Small, circular RNA
molecules that infect
plants
• Cause errors in
regulatory systems that
control plant growth
• Eg. coconut palms in
Philippines
PRIONS
• Misfolded, infectious proteins that cause
misfolding of normal proteins
• Eg. mad cow disease (BSE),CreutzfeldtJakob disease (humans), scrapie (sheep)
DISEASES CAUSED BY PRIONS
• Prions act slowly – incubation period of at
least 10 years before symptoms develop
• Prions are virtually indestructible (cannot be
denatured by heating)
• No known cure for prion diseases