Chapter 14
Viruses, Cancer, and Immunology
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SUMMARY
Section 14.1
Viruses are simple genes, made up of RNA or DNA, that infect cells and take
over their replication, transcription, and translation machinery.
Viruses are characterized by their structure, their type of nucleic acid, whether it
Section 14.2
Retroviruses have a genome based on RNA. When they infect a cell, their RNA
is turned into DNA by reverse transcriptase. The DNA is then incorporated into
Section 14.3
Vertebrates have a complicated and elegant system of defense called the
immune system.
One type of immunity, called innate immunity, consists of physical barriers, such
as skin, and cellular warriors, such as dendritic cells. This system is always
present and waiting to attack invading organisms or even cancerous cells.
Another type of immunity, called acquired immunity, is based on two types of T
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Section 14.4
All potentially fatal cancers have several things in common, such as having cells
that are immortal, that divide despite “stop growth” signals from nearby cells, that
stimulate blood-vessel formation near to themselves, and that spread to other
parts of the body.
Section 14.5
Human immunodeficiency virus (HIV) is the most infamous of the retroviruses, as
it is the causative agent of acquired immunodeficiency syndrome (AIDS). This
disease affects more than 40 million people worldwide and has continually
thwarted attempts to eradicate it
HIV offers a classic example of the mode of operation of retroviruses. The HIV
LECTURE NOTES
This chapter begins with a general discussion of viruses and brings in some
LECTURE OUTLINE
I. Viruses
A. Importance of viruses
B. Virus structure
C. Families of viruses
D. Virus life cycles
Viruses, Cancer, and Immunology 3
1. Lytic pathways
2. Lysogeny
II. Retroviruses
A. Reverse transcriptase
B. HIV
III. The immune system
A. Innate immunity
B. Acquired immunity
2. Self and non-self
IV. Cancer
A. Hallmarks of cancer cells
F. Viruses as cancer cures
V. AIDS
A. Virus characteristics
B. HIV confounds the immune system
C. Fighting AIDS
ANSWERS TO PROBLEMS
14.1 Viruses
1. Some viruses have DNA and some have RNA. In some cases, a viral genome is
single-stranded and in others it is double-stranded.
2.
(a) The virion is the entire virus particle.
(b) The capsid is the protein coat that surrounds the viral nucleic acid.
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5. In the lytic pathway, the viral nucleic acid is replicated in the host cell and
packaged into new virus particles that lyse the host cell. In the lysogenic
pathway, the viral DNA is incorporated into the host DNA.
6. There is no correlation. Some viruses, such as Ebola virus, are fast acting and
8. Viruses can often switch from one pathway to another, based on the condition of
the host cells. If the host is healthy, there is sufficient material to allow the virus
to replicate and to produce new virions. If the host cell is starved or unhealthy,
14.2 Retroviruses
10. A retrovirus has an RNA genome that must pass through a stage in which it is
reverse-transcribed to DNA, and this DNA must recombine with the host’s DNA.
11. Reverse transcriptase.
15. The two most common are the Maloney murine leukemia virus (MMLV) and
adenovirus. Both must be manipulated so that the critical genes for replication
are removed and replaced with an expression cassette containing the therapeutic
gene.
16. When retroviruses, such as MMLV, are used, there is the danger that the
therapeutic gene will incorporate in a place that will disrupt another gene. In more
Viruses, Cancer, and Immunology 5
from the body and then replaced. However, if the problem is in an immune cell,
then bone marrow cells can be removed and transformed and later given back to
14.3 The Immune System
19. AIDS is the most well-known problem of a malfunctioning immune system. SCID
20. Innate immunity refers to a variety of protective processes, including skin, mucus,
and tears as a first line of defense, and dendritic cells, phagocytes,
21. One part includes physical barriers, such as skin, mucus, and tears. The cells of
the innate immune system are dendritic cells, macrophages, and natural killer
(NK) cells.
22. B cells, which make antibodies, killer T cells, which attack infected cells, and
25. The cells of the innate system initially attack a pathogen, such as a virus,
bacteria, or even a cancerous cell. They then present antigens from the
pathogen on their surfaces via their MHC proteins. The acquired immunity cells
then recognize the MHC/antigen complex, bind to it, and begin the involvement
of the acquired immunity system.
26. Interferon is a cytokine produced in very small quantities that stimulates natural
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28. Macrophages, part of the innate immune system, are the “double–edged sword.”
Their presence is important to attack cancer cells, and if they do a thorough job,
29. The small noncoding RNA (ncRNA) of the herpes virus has been linked with its
ability to evade the immune system.
30. The herpes virus produces a ncRNA that stabilizes the respiratory chain of the
mitochondria of the host cell. This prevents the early destruction of the infected
14.4 Cancer
31. Cancer cells continue to grow and divide in situations in which normal cells do
not, such as when they are not receiving growth signals from surrounding cells.
32. A tumor suppressor is a molecule that restricts the ability of a cell to grow and to
divide. An oncogene is a gene whose product stimulates a cell to grow and to
divide.
33. The protein called p53 is a tumor suppressor. Mutations of p53 have been found
in more than half of all human cancers. Ras is involved in cell division, and
mutations in this protein are involved in 30% of human tumors.
34. Viruses have been implicated in many cancers. Retroviruses are particularly
dangerous because they insert their DNA into the host’s DNA. When this
35. Virotherapy is the process of using a virus to attempt to treat cancer. There are
two strategies for virotherapy. One is to use the virus to attack and kill cancer
36. If smoking caused cancer, then everyone who smokes would have cancer, but
37. A tumor suppressor is a protein that helps control cell growth and division. It is
like the brakes on a car, trying to slow down a process. Many cancers are related
Viruses, Cancer, and Immunology 7
to mutation of tumor suppressors. An oncogene produces something that
stimulates growth and division. This is like the accelerator of the car. Many other
cancers are caused ultimately by overactivation of an oncogene.
38. Ras, Jun, and Fos are all considered oncogenes. In the process of cell division,
Ras is a necessary component, but it is usually active only when the cell should
39. Many of the early trials involved specific delivery of an active p53 gene via gene
therapy. However, such delivery is impractical for human patients in many cases.
Now researchers are looking for drugs that can be taken that will increase the
levels of p53.
40. Two drugs, Prima-1 and CP-31398 reactivate mutant p53; nutlins inhibit a protein
called MDM2, which is itself a natural inhibitor of p53.
41. p53 can be restored in several ways. One way would be through gene therapy to
give the patient functioning copies of the p53 gene if he or she lacks it. Another is
42. The innate immune system is instrumental in fighting cancer cells. Cell that turn
cancerous display specific molecules on their surfaces that act as a help signal.
Cells of the innate immune system such as macrophages and natural killer cells
43. The realization that cancer’s progression is fueled by inflammation has led to a
theory by some scientists that we should spend more time focusing on the
44. They found more than 30,000 mutations in the melanoma genome and more
than 23,000 in lung cancer. This information will make it possible to diagnose
cancer much earlier and lead to more effective treatment. For individual patients,
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48. HIV is difficult to kill because it is difficult to find. The reverse transcriptase of
HIV is very inaccurate in replication. The result is rapid mutation of HIV. There is
49. The two main approaches are creating vaccines to the virus and using anti-
retroviral therapy
50. No, the virus does not kill its host directly. It weakens the immune system via its
51. The common flu is the least lethal, in general, although there have been flu
epidemics in the past that were very lethal. SARS is also very lethal. A high
52. The gp120 protein on the HIV virus is the molecule that docks with the CD4
receptor on the helper T cell, so it is very important to the action of HIV. The
53. HAART stands for highly active anti-retroviral therapy. It is based on using a
combination of drugs to attack multiple parts of the HIV lifecycle.
54. To be 100% cured, there would have to be zero virus particles in the patient or
zero helper T cells that were susceptible to the virus. It always hard to say with