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Name______________________________
Per________
Chapter 20.1 – VIRUSES
Study Guide
Use your PowerPoint notes, the textbook, and the notes at the end of this study
guide to answer the following questions.
Viral Reproduction starts on page 436.
1. Viruses must rely on living cells (____________________) for replication. It must first infect a living
cell. A plant virus enters through tiny tears in the ___________________. An animal virus enters through
the process of ___________________. A bacterial virus ________________ a hole in the cell wall.
2. Define Lytic cycleDefine Lysogenic cycle-
3. Draw and label the lytic and lysogenic cycles.
4. Which cycle harms/destroys the host cell? ________________________ At which step does this
occur? _________________________________________________________
HIV discussion starts on page 437 and is also in your notes from class and the notes at the end of this study
guide.
5. What does AIDS stand for? _____________________________________________
6. What does HIV stand for? ______________________________________________
7. HIV is a type of retrovirus since it contains _________ for its nucleic acid. Since HIV has RNA, the
genetic material must work in reverse to create the viral DNA. In essence, HIV uses the cell’s machinery to
direct the production of more HIVs. (Lysogenic cycle)
8. AIDS is a disease in which a person’s ________________ system is unable to defend against infections
that do not normally create health problems. This is because the HIV virus attacks ___________ cells.
Virus Self-Quiz
_____1.
_____2.
_____3.
_____4.
_____5.
_____6.
capsid
endocytosis
prion
provirus
bacteriophage
viroid
Match the term to the correct phrase.
A. a viral gene that is inserted into
a host chromosome
B. a virus that infects bacteria
C. the protein coat of a virus
D. the process by which HIV
enters a host cell
E. infectious single strand of RNA
with no capsid
F. infectious protein with no
nucleic acid
Choose the correct term or phrase that completes each sentence AND EXPLAIN WHY you chose
the answer you did.
_____7. Scientists think that there are so many different kinds of viruses because the viruses
a. are so highly evolved
7. Why:______________________
b. originated from a variety of host cells
____________________________
c. originated from several selected areas of the world
d. have been reduced due to natural selection
_____8. The nucleic acid of a virus consists of which of the following?
a. DNA
8. Why:______________________
b. RNA
____________________________
c. Both DNA and RNA
d. Either DNA or RNA
Draw a bacteriophage and label the nucleic acid, capsid, collar, tail sheath, tail fiber, and base plate.
Next to the bacteriophage, draw one of the other 4 shapes of viruses and label the nucleic acid and
protein coat (capsid).
If you were a scientist developing a drug that would block viral replication, which steps would you
choose to block? Explain.
Tell me the shape of the virus and an example of each:
Name of Shape
What illness it causes
Virus Review Puzzle
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2. virus shape that has its nucleic acid coiled tightly
inside the long, narrow capsid – ex) tobacco mosaic
virus
5. virus type that does not have a distinct, uniform
shape – ex) Ebola
10. a single strand of pure RNA, with no capsid;
cause plant diseases
11. the type of viral replication in which the virus
infects the host and immediately begins to make
many new viruses
13. an organism that shelters and nourishes
something
14. virus shape that consists of two distinct parts:
a polyhedral capsid and a helical tail
15. viruses that only attack bacteria
16. protein coat around the nucleic acid of a virus
1. an infectious protein molecule with no nucleic
acid, which can cause disease in animals
3. an additional protective coat of a virus made of
lipids, proteins and carbohydrates
4. the piece of viral DNA that has been inserted in
a bacterial cell’s chromosome
6. the process of injecting an organism with a
harmless form of a virus to stimulate the immune
system to make antibodies to the virus
7. a virus that contains an RNA core that replicates
by first transcribing its RNA into DNA – ex) HIV
8. infectious agent made up of a core of nucleic
acid and a protein coat
9. virus shape that has a multifaceted geometric
appearance – ex) adenovirus that causes the
common cold
12. the type of viral replication in which a virus
does not immediately kill a host cell, because the
viral DNA becomes incorporated into the host DNA
Ch. 20 Viruses and Bacteria
Section 1: Viruses pp. 434 - 441
 Is a virus alive?
- living things are made of cells, grow, reproduce, and have DNA
- viruses are only protein and nucleic acid & are smaller than cells; need electron microscope to see
- pathogens – cause disease
- use other cells to make more viruses
- So, is a virus alive? NO, because viruses do not grow, do not have homeostasis, do not metabolize
Discovery of Viruses
 Searching for cause of tobacco mosaic disease
 Isolated sap of infected plants & filtered for bacteria – sap could still infect plants
 Not bacterium
 Virus = poison
 Wendell Stanley, 1935, purified tobacco mosaic virus (TMV) and crystallized it – chemical
not tiny cell
 TMV = RNA and protein
Viral Structure
capsid – protein coat
contains RNA or DNA, not both
envelope – membrane surrounding the capsid
o helps the virus enter cells
o made of protein, lipid, and glycoprotein
 glycoprotein comes from host cell
o some contain enzymes
shapes
o filovirus - filaments (Ebola virus)
o spherical (influenza) with receptors to help virus enter cells
o helical (TMV) is rodlike
o polyhedral – rough, many-sided spheres (adenoviruses)
o binal (bacteriophage) – infects bacteria – polyhedron tail and helical tail
Viral Reproduction
lack enzymes and protein synthesis
rely on living cells for replication
1. enter plant cells through tears in cell wall
2. enter animal cells through endocytosis
3. bacteriophages punch holes in bacteria cell wall and inject DNA
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Lytic Cycle – one path of reproduction of viruses in bacterial cells
 Infection, replication, cell destruction
Lysogenic Cycle – second path of reproduction of viruses in bacterial cells
 Infection, viral genome (provirus) inserted into host chromosome, provirus divides with host
cell, change triggers lytic cycle
Host Cell Specificity
 Viruses restricted to certain kinds of cells
 Due to virus’ origin – possibly fragments of host genes escaped or expelled from cells
Structure of HIV – an Enveloped Virus
 Envelope with glycoprotein
 2 molecules of single-stranded
 Capsid
RNA (9 genes)
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How HIV Infects Cells
entry is 2-step process
1. virus attaches to cells at receptors
2. attachment triggers endocytosis
Attachment – glycoproteins fit cell surface receptor called CD4 found in lymphocytes and
macrophages as well as some brain cells
o Entry into Macrophages – activates second co-receptor CCR5 to start endocytosis; CCR5
not found in lymphocytes, so do not infect
o Replication – shed capsule inside, releases reverse transcriptase (copies RNA to DNA; lots
of mutations), host cell translates the DNA and makes more virus, viruses released by
budding and now are covered with envelope from the cell membrane
o AIDS
 HIV replicates for years
 Mutation of glycoproteins allows infection of T cell lymphocytes
 T cells ARE destroyed by HIV
 Immune system impaired – onset of AIDS – body unable to defend against
infections that do not normally occur in healthy individuals
 Takes years, therefore virus spreads before symptoms begin
 Spreading is via body fluids (semen, vaginal fluid, blood, breast milk or across
placenta)
Viral Diseases
most lethal in history = influenza, infects cells of respiratory tract
can cause cancers, including hepatitis B  liver cancer, Epstein-Barr  lymphoma, human
papilloma virus (HPV)  cervical cancer
o Table 1 Important Viral Diseases
 AIDS, cold, Ebola, hepatisis A & B, influenza, mumps, polio, rabies, smallpox, yellow
fever
o Emerging Viruses
 Geographically isolated & pathogenic
 West Nile – from probably brought by infected bird and spread by mosquitoes; flulike usually; lethal in elderly
 Hanta virus – SW U.S.; spread in rodent droppings; lethal 38% victims
o Prions and Viroids
 Stanley Pruisner described new class of pathogen = prion
 Prion – composed of protein only
 Infect animals
 Scrapie, mad cow
 Crutzfeldt-Jakob
 Eating meat contaminated with prions spreads the disease
 viroid – single strand of RNA with no capsid
 infect plants
 cucumbers, potatoes, avocados, oranges
p. 434 – Ebola virus
p. 435 – viral structures*
p. 436 – bacteriophages
- lytic and lysogenic cycles*
p. 437 – HIV structure*
p. 438 – HIV photo
p. 439 – HIV infection*
p. 440 – Table 1 – Important Viral Diseases*
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