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AP Biology: Notes and Practice
Viruses
A virus is composed of genetic information (RNA or DNA) and proteins. Viruses can have different structures,
but a common type is a bacteriophage, a virus that infects bacteria. It is shaped like a moon lander and has three
distinct regions (Figure 1). The head contains the genetic material. Proteins form a protective coat around it,
giving the head a roundish appearance. The neck and helical sheath are also composed of proteins and serve as a
conduit for injecting the genetic material into a bacterium – like a syringe. Finally, the base plate and the tail
fibers are responsible for recognizing and binding to target bacteria. Since they are composed of proteins, they
have unique three dimensional shapes that fit into specific targets on a bacteria’s surface, in a lock and key
fashion.
Viruses are inert, meaning they can only replicate inside a host cell, using the cell’s energy and machinery.
Figure 2 displays a bacteriophage infecting the bacterium E. coli. Infection begins when the virus comes into
contact with a specific protein receptor on the surface of a cell. The virus attaches to the cell, makes a hole in
the cell wall or cell membrane, and injects its genetic material into the cell. The genetic material can then follow
one of two fates:
1. It may use the cell’s machinery to replicate its genome and make more of its proteins. In effect, the host
cell becomes a virus-producing factory, though the process happens quickly, so there is a high frequency
of mutation in producing new viral genomes. Eventually, the virus directs the production of proteins that
cause the cell membrane of the host cell to rupture, killing the cell and releasing the newly synthesized
viral particles into the environment to infect other cells. This process is called the lytic cycle.
2. Other times, once inside the cell the virus’ genetic material pursues another path called the lysogenic
cycle. Here, the viral genome integrates into the host cell genome. Each time the host cell replicates, its
progeny (offspring) contains a copy of the viral genome. In time, perhaps when the virus detects that
nutrients are depleted in the host cell, it resumes a lytic life cycle and exits the cell.
Figure 1: Bacteriophage structure. (1) Head, (2) Tail, (3) Nucleic acid, (4) Capsid, (5) Collar,
(6) Sheath, (7) Tail fiber, (8) Spikes, (9) Baseplate
Credit: Wikimedia contributor User:Y_tambe, licensed under the Creative Commons AttributionShare Alike 3.0 Unported license. http://commons.wikimedia.org/wiki/File:Bacteriophage_structure.png.
AP Biology: Notes and Practice
Figure 2: Virus life cycle.
Credit: Reprinted by permission from Macmillan Publishers Ltd: Nature Reviews Genetics 4, 471–477, "The future
of bacteriophage biology," by Allan Campbell, copyright 2003.
1. A bacteriophage is a specific type of virus that infects bacteria. Describe the structure of the bacteriophage
and explain how structural features support its function.
2. The viral life cycle consists of two specific phases:
a. What is the purpose of the lytic cycle?
b. What is the purpose of the lysogenic cycle?
AP Biology: Notes and Practice
c. Which of the two viral life cycle phases directly results in death of the host cell? Explain your
reasoning.
3. A student argues that viruses contain their own RNA polymerase for transcription and DNA polymerase for
replication. Is this statement supported or refuted by the information in the passage and Figure 2? Explain
your reasoning.
4. According to the passage and Figure 2:
a. How does a virus recognize and bind to a specific host cell?
b. Provide another biological example of two structures that bind specifically with each other the way a
virus binds specifically with its host cell.
c. In 1926, there was a large outbreak of Cholera in many villages in India. Cholera is an infection
caused by E. coli bacteria which causes severe abdominal pain, vomiting, and diarrhea, and
ultimately death if dehydration caused by these symptoms persists for several days. In the 1920s, it
was common for the bacteria to be grow in dirty wells, so people could get sick from drinking water
containing the E. coli. A group of Europeans touring the region at this time treated the water by
pouring in a bacteriophage that targeted E. coli. In a few days, many people suffering from Cholera
began to get better and no new cases of Cholera were reported.
 Why was the bacteriophage treatment effective in treating Cholera?

Based on the information in the passage, would the bacteriophage added to the water have
been harmful to normal flora, intestinal bacteria that are naturally present in the human gut
and contribute positively to intestinal health? Why or why not?
AP Biology: Notes and Practice
5. When viruses assemble during the lytic cycle just before rupturing the host cell, they may incorporate some
of the host DNA into the newly made viruses. This is shown in Figure 3 below.
Virus containing viral DNA
Virus containing host DNA
Figure 3
a. Though some viruses produced contain viral DNA and others contain host DNA, all of the protein
structures that build the viruses are coded for by viral DNA. According to this new information,
how will the protein structure for the capsid and spikes compare between the viruses containing the
two different types of DNA? Explain your reasoning.
b. A student claims that if a virus contains host DNA, the virus will not be able to attach to and infect
another host cell. Is this conclusion supported or refuted by your answer to part a? Explain your
reasoning.
c. Genetic recombination is the process by which two DNA molecules exchange genetic information,
resulting in the production of a new combination of alleles. Explain how the process shown in
Figure 3 could lead to genetic recombination in bacteria and provide an example.
6. Antivirals are drugs that are specifically used for treating viral infections. Many antivirals work by
attaching to and disabling an enzyme coded for by viral genes that aids in virus assembly during the lytic
cycle. Some antivirals are only effective against a virus for a short period of time and are unable to target
the same virus in future generations. According to the passage, propose an explanation for why this is.