Download AP Biology Notes Outline Chapter 19: Viruses OVERVIEW OF

AP Biology Notes Outline
Chapter 19: Viruses
OVERVIEW OF VIRAL GENETICS
A virus is a parasitic infectious agent that is UNABLE to survive outside a host organism. This is because
viruses DO NOT CONTAIN RIBOSOMES FOR PROTEIN SYSNTHESIS, NOR DO THEY CONTAIN
ENZYMES FOR METABOLISM. They are completely dependent on their host.
 Once a virus infects a cell, it takes over the cell’s machinery and uses it to produce whatever it needs to
survive and reproduce.
 How a virus acts after it enters a cell depends on what type of virus it is. Classification of viruses is
based on several factors:
o Genetic Material: DNA, RNA, protein, etc?
o Capsid: Type of capsid?
o Viral Envelope: present or absent?
o Host Range: what type of cells does it affect?
All viruses have a genome (DNA or RNA) and a protein coat (capsid).
 Genome is an entire set of genes (DNA or RNA)
 Capsid is the protein coat that encloses the viral genome. Capsids are built from a large number of
protein subunits called capsomeres.
 Viral Envelopes: accessory structures that help viruses attach their host – these are membranes
cloaking the capsid. Envelopes are derived from the membrane of the host cell and will always
contain some components of the virus and some of the host cell.
 Each virus has a host range…the range of cells that the virus is able to infect. For example, HIV
infects the T cells of our body, and bacteriophages only infect bacteria.
A) Tobacco mosaic viruses have a helical capsid with the overall shape of a rigid rod.
B) Adenoviruses have a polyhedral capsid with a glycoprotein spike at each vertex.
C) Influenza viruses have an outer envelope studded with glycoprotein spikes. The genome consists of
eight different RNA molecules, each wrapped in a helical capsid.
D) T4 Bacteriophages have a complex capsid consisting of a polyhedral head and tail apparatus.
AP Biology Notes Outline
Chapter 19: Viruses
RETROVIRUSES
http://www.sumanasinc.com/webcontent/animations/content/lifecyclehiv.swf
A very special type of virus is one called a
retrovirus. RETROVIRUSES ARE RNA
VIRUSES THAT CARRY AN ENZYME
CALLED REVERSE TRANSCRIPTASE.
 Once in the cytoplasm of the host cell, the
RNA virus uses this enzyme and “reverse
transcribes” its genetic information from
RNA into DNA, which then enters the
nucleus of the cell.
 In the nucleus, the newly transcribed DNA
incorporates into the host DNA and is
transcribed into RNA when the host cell
undergoes normal transcription.
 The mRNA produced from this process
gives rise to new retrovirus offspring, which
can then leave the cell in a lytic pathway. A
well-known example of a retrovirus is the
HIV virus of AIDS.
VIRAL REPRODUCTION: THE LYTIC AND LYSOGENIC CYCLES
http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter17/animation_quiz_2.html
Once inside the cell, a DNA virus can take one of two pathways – a lytic or lysogenic pathway.
 In a lytic cycle, the cell actually produces many viral offspring, which are released from the cell, thus
killing the host cell in the process.
 In a lysogenic cycle, the virus falls dormant and incorporates its DNA into the host DNA as an entity
called a provirus OR prophage. The viral DNA is quietly reproduced every time the cell reproduces
itself, and this allows the virus to stay alive from generation to generation without killing the host cell.
Viruses in the lysogenic cycle can sometimes separate out from the host DNA and enter the lytic cycle
(like a bear waking from hibernation).
 http://biology.about.com/gi/o.htm?zi=1/XJ&zTi=1&sdn=biology&cdn=education&tm=48&gps=195_41
3_1276_493&f=11&su=p897.9.336.ip_&tt=2&bt=1&bts=1&zu=http%3A//student.ccbcmd.edu/courses/
bio141/lecguide/unit3/viruses/lytsum.htmL
AP Biology Notes Outline
OVERVIEW OF VIRAL REPLICATION
1. Attachment
2. Penetration
3. Replication and Synthesis
4. Assembly
5. Release
 Virus uses host’s nucleotides and enzymes to replicate itself.
 At the same time, other host resources are used to make new
capsid proteins by transcription and translation.
 The new viral genomes and capsids are assembled into new
virus particles; when the number exceeds the cell’s surface
area limitations, the cell bursts open and new viruses are
released to infect other cells – exponential increase.
Chapter 19: Viruses
AP Biology Notes Outline
Chapter 19: Viruses
OTHER IMPORTANT INFORMATION
Viruses can come in many shapes and sizes.
 viroids are plant viruses that are only a few hundred nucleotides in length
 prions are incorrectly folded forms of brain cell proteins that can convert other normal host proteins into
misshapen proteins…mad cow disease is caused by a prion – diseases of this type are degenerative and
tend to cause brain disfunction (dementia, muscular control problems, and loss of balance).
CONTROLLING VIRUSES
Diseases causes by viruses are difficult to treat. Drugs are only used to treat SYMPTOMS, not cure the disease
(just make patient feel better for short duration). The only methods to control viruses are to PREVENT illness
via vaccines and antibody production, use of interferon in body.
 Antibodies are made by host’s immune system after infection occurs (if host survives the infection).
Help inactivate viruses and destroy harmful bacteria - Are specific for viruses or bacteria - Once an
antibody is produced that recognizes a specific virus or bacteria, then that strain will be ineffective on
that individual organism
 Vaccines are harmless variants or derivatives of pathogenic microbes - Stimulate the immune system to
mount defenses against a specific pathogen - Developed by Edward Jenner – cowpox used to develop
smallpox vaccination - Vaccinated or immunized again disease
Ex. MMR, DPT, polio, smallpox, influenze,rabies, hepatitis C
 Interferons are chemicals in the body that are activated when cells are attacked
Cell under seige produces interferon which binds to neighboring cells’ cell membranes to warn them of
the dangerous pathogen
Antibiotics are powerless against viruses! Antibiotics kill bacteria by inhibiting enzymes or processes specific
to the pathogens; since viruses have no metabolism of their own, the antibiotics do not work. Only drugs that
have any effect on viruses are ones that interfere with nucleic acid synthesis – AZT (with HIV), acyclovir (with
herpesvirus)…or with protein production (protease inhibitors with AIDS).
Emerging viruses that cause new outbreaks of disease are usually existing viruses that manage to expand their
host territory.
o AIDS
o Hantavirus
o Ebola (hemorrhagic fever)
o Nipah virus
o Influenza
What contributes to spread of emerging viruses?
 Mutation
 Spread from one species to another
 Dissemination from small, isolated population