Chapter 21 End of Chapter ?'s

Question 1

Which of the following statements about virus structure is true?
a. All viruses are encased in a viral membrane.
b. The capsomere is made up of small protein subunits called capsids.
c. DNA is the genetic material in all viruses.
d. Glycoproteins help the virus attach to the host cell.

Answer 1

The correct answer is:

d. Glycoproteins help the virus attach to the host cell.

Here’s why the other options are incorrect:
- a. All viruses are encased in a viral membrane.
Not all viruses have an envelope (viral membrane). Some are naked viruses, lacking a lipid envelope, such as adenoviruses.

• b. The capsomere is made up of small protein subunits called capsids.
  This statement is reversed: the capsid is made up of smaller units called capsomeres.
• c. DNA is the genetic material in all viruses.
  Not true. Some viruses contain RNA as their genetic material, such as influenza and HIV.

Correct Option (d) Explained:
Glycoproteins on the viral envelope (or capsid in some viruses) facilitate attachment to the host cell by binding to specific receptors on the cell’s surface. This is a crucial step for infection to occur.

Question 2

Influenza virus is packaged in a viral envelope that fuses with the plasma membrane. This way, the virus can exit the host cell without killing it. What advantage does the virus gain by keeping the host cell alive?

Answer 2

The advantage of keeping the host cell alive is that it allows the influenza virus to continue using the host cell’s machinery to produce more viral particles. Here are the key benefits:

1. Sustained Viral Production:
   A living host cell can keep synthesizing viral RNA, proteins, and new virions, ensuring that the virus can produce as many particles as possible over time.
2. Stealth and Persistence:
   Non-lytic (non-destructive) exit from the host cell helps the virus evade detection by the immune system. If too many cells were destroyed quickly, it could trigger a stronger immune response.
3. Reduced Inflammatory Response:
   Preserving the host cell minimizes cellular damage, reducing the release of inflammatory signals. This helps the virus avoid immediate clearance by the immune system.
4. Facilitates Continuous Infection:
   A living host cell can continue interacting with neighboring cells, aiding the spread of the virus within the tissue without triggering a massive immune response early on.

By avoiding host cell death, the influenza virus maximizes replication while buying time to infect other cells.

Question 3

Which of the following statements is false?
a. In the lytic cycle, new phage are produced and released into the environment.
b. In the lysogenic cycle, phage DNA is incorporated into the host genome.
c. An environmental stressor can cause the phage to initiate the lysogenic cycle.
d. Cell lysis only occurs in the lytic cycle.

Answer 3

The correct answer is:

c. An environmental stressor can cause the phage to initiate the lysogenic cycle.

Explanation:
This statement is false because an environmental stressor (such as UV radiation or chemicals) typically causes the phage to exit the lysogenic cycle and enter the lytic cycle, not initiate lysogeny. In the lytic cycle, the phage actively replicates and lyses the host cell to release new viral particles.

Here’s why the other statements are true:
- a. In the lytic cycle, new phage are produced and released into the environment.
This is correct. In the lytic cycle, the phage uses the host’s machinery to produce new viral particles, which are released through cell lysis.

• b. In the lysogenic cycle, phage DNA is incorporated into the host genome.
  This is true. In the lysogenic cycle, the phage DNA integrates into the host cell’s chromosome as a prophage, where it remains dormant until activated.
• d. Cell lysis only occurs in the lytic cycle.
  This statement is true. In the lysogenic cycle, the host cell remains intact, while in the lytic cycle, the host cell bursts to release new phage particles.

Question 4

Which statement is true?
a. A virion contains DNA and RNA.
b. Viruses are acellular.
c. Viruses replicate outside of the cell.
d. Most viruses are easily visualized with a
light microscope.

Answer 4

The correct answer is:

b. Viruses are acellular.

Explanation:
- a. A virion contains DNA and RNA.
This is false. A virion contains either DNA or RNA, but never both. Viruses are classified based on their nucleic acid type (DNA or RNA).

• b. Viruses are acellular.
  This is true. Viruses lack cellular structure; they are made up of genetic material (DNA or RNA) enclosed in a protein coat, and sometimes a lipid envelope. They do not have organelles or a metabolism, distinguishing them from living cells.
• c. Viruses replicate outside of the cell.
  This is false. Viruses can only replicate inside a host cell by hijacking the cell’s machinery to reproduce.
• d. Most viruses are easily visualized with a light microscope.
  This is false. Most viruses are too small (20-300 nm) to be seen with a light microscope and require an electron microscope for visualization.

Question 5

The viral ________ plays a role in attaching a virion to the host cell.
a. core b. capsid
cell without killing it. What advantage does the virus gain by keeping the host cell alive?
3. Figure 21.10 Which of the following statements is false?
a. In the lytic cycle, new phage are produced and released into the environment.
b. In the lysogenic cycle, phage DNA is incorporated into the host genome.
c. An environmental stressor can cause the phage to initiate the lysogenic cycle.
d. Cell lysis only occurs in the lytic cycle.
c. envelope
d. both b and c

Answer 5

The correct answer is:

d. both b and c

Explanation:
Capsid: The capsid is the protein shell surrounding the viral genome. In some non-enveloped viruses, specific proteins on the capsid surface help the virus attach to the host cell by interacting with host receptors.
Envelope: In enveloped viruses (like the influenza virus), the envelope contains glycoproteins that play a crucial role in attaching to host cells.
Thus, both the capsid (in non-enveloped viruses) and the envelope (in enveloped viruses) contribute to the attachment of virions to host cells, making “d. both b and c” the best answer.

Question 6

Viruses_______.
a. all have a round shape
b. cannot have a long shape
c. do not maintain any shape
d. vary in shape

Answer 6

The correct answer is:

d. vary in shape

Explanation:
Viruses come in a wide variety of shapes, depending on the structure of their capsid and whether or not they have an envelope. Some common viral shapes include:
- Helical (long, rod-like): e.g., tobacco mosaic virus
- Icosahedral (spherical or polyhedral): e.g., adenovirus
- Complex (combination of shapes): e.g., bacteriophages, which have a head-and-tail structure
- Spherical (enveloped viruses): e.g., influenza virus

Thus, viruses do not all have a single shape and can appear in many different forms.

Question 7

Which statement is not true of viral replication?

A. A lysogenic cycle kills the host cell.

B. There are six basic steps in the viral replication cycle.

C. Viral replication does not affect host cell function.

D. Newly released virions can infect adjacent cells.

Answer 7

The correct answer is:

A. A lysogenic cycle kills the host cell.

Explanation:
- A. A lysogenic cycle kills the host cell.
Not true. The lysogenic cycle allows the virus to integrate its genetic material into the host genome without immediately killing the host cell. The viral DNA (now called a prophage) can remain dormant until triggered to enter the lytic cycle, which causes the host cell to burst.

• B. There are six basic steps in the viral replication cycle.
  True. The six general steps of viral replication are:
  
  1. Attachment
  2. Penetration
  3. Uncoating
  4. Replication and Transcription
  5. Assembly
  6. Release
• C. Viral replication does not affect host cell function.
  Not true. Viral replication affects host cell function by diverting cellular resources to produce viral components, often impairing normal cell processes.
• D. Newly released virions can infect adjacent cells.
  True. After being released from the host cell, virions are capable of infecting nearby cells, continuing the cycle of infection.

Thus, the false statement is A. A lysogenic cycle kills the host cell.

Question 8

Which statement is true of viral replication?

A. In the process of apoptosis, the cell survives.

B. During attachment, the virus attaches at specific sites on the cell surface.

C. The viral capsid helps the host cell produce more copies of the viral genome.

D. mRNA works outside of the host cell to produce enzymes and proteins.

Answer 8

The correct answer is:

B. During attachment, the virus attaches at specific sites on the cell surface.

Explanation:
- B. During attachment, the virus attaches at specific sites on the cell surface.
True. Viral attachment is highly specific, with the virus binding to receptors on the host cell’s surface. This interaction determines the virus’s ability to infect specific cells (host specificity).

Why the other options are false:
- A. In the process of apoptosis, the cell survives.
False. Apoptosis is a programmed cell death process. It leads to the controlled death of the cell to prevent damage or spread of infections, often as a defense mechanism.

• C. The viral capsid helps the host cell produce more copies of the viral genome.
  False. The capsid is the protective protein coat of the virus, but it does not directly help in viral genome replication. Replication is carried out by host cell machinery (or viral enzymes in some cases).
• D. mRNA works outside of the host cell to produce enzymes and proteins.
  False. mRNA functions inside the host cell’s cytoplasm, where it directs the production of viral proteins through the host’s ribosomes.

Thus, the only correct statement is B.

Question 9

Which transcriptase?
statement is true of reverse
a. It is a nucleic acid.
b. It infects cells.
c. It transcribes RNA to make DNA.
d. It is a lipid.

Answer 9

The correct answer is:

B. During attachment, the virus attaches at specific sites on the cell surface.

Explanation:
- B. During attachment, the virus attaches at specific sites on the cell surface.
True. Viral attachment is highly specific, with the virus binding to receptors on the host cell’s surface. This interaction determines the virus’s ability to infect specific cells (host specificity).

Why the other options are false:
- A. In the process of apoptosis, the cell survives.
False. Apoptosis is a programmed cell death process. It leads to the controlled death of the cell to prevent damage or spread of infections, often as a defense mechanism.

• C. The viral capsid helps the host cell produce more copies of the viral genome.
  False. The capsid is the protective protein coat of the virus, but it does not directly help in viral genome replication. Replication is carried out by host cell machinery (or viral enzymes in some cases).
• D. mRNA works outside of the host cell to produce enzymes and proteins.
  False. mRNA functions inside the host cell’s cytoplasm, where it directs the production of viral proteins through the host’s ribosomes.

Thus, the only correct statement is B.

Question 10

Oncogenic virus cores can be_______.
a. RNA
b. DNA
c. neither RNA nor DNA
d. either RNA or DNA

Answer 10

The correct answer is:

d. either RNA or DNA.

Explanation:
Oncogenic viruses, also known as oncoviruses, are capable of causing cancer and can have either RNA or DNA as their genetic material. Here are some examples:

• RNA Oncogenic Viruses:
  
  • Retroviruses (e.g., Human Immunodeficiency Virus, HIV) can integrate into the host genome and lead to cancer development.
• DNA Oncogenic Viruses:
  
  • DNA viruses such as human papillomavirus (HPV) and hepatitis B virus (HBV) can also cause cancer through mechanisms like integration into the host’s DNA and disrupting normal cell cycle regulation.

Therefore, oncogenic virus cores can indeed be either RNA or DNA.

Question 11

Which is true of DNA viruses?
a. They use the host cell’s machinery to
produce new copies of their genome.
b. They all have envelopes.
c. They are the only kind of viruses that
can cause cancer.
d. They are not important plant pathogens.

Answer 11

The correct answer is:

a. They use the host cell’s machinery to produce new copies of their genome.

Explanation:
- a. They use the host cell’s machinery to produce new copies of their genome.
True. DNA viruses rely on the host cell’s replication and transcription machinery to synthesize their DNA and produce viral proteins. They often utilize host polymerases for these processes.

• b. They all have envelopes.
  False. Not all DNA viruses have envelopes. For example, adenoviruses are non-enveloped DNA viruses.
• c. They are the only kind of viruses that can cause cancer.
  False. While some DNA viruses (e.g., HPV, HBV) are oncogenic, RNA viruses (like HIV and HTLV) can also cause cancer.
• d. They are not important plant pathogens.
  False. There are several DNA viruses that are significant plant pathogens, such as Tobacco mosaic virus (though it’s primarily an RNA virus, there are DNA plant viruses like Gemini viruses).

Thus, the only accurate statement is a.

Question 12

A bacteriophage can infect ________.
a. the lungs b. viruses c. prions
d. bacteria

Answer 12

The correct answer is:

d. bacteria.

Explanation:
- Bacteriophages, often referred to as phages, are viruses specifically designed to infect bacteria. They attach to bacterial cells, inject their genetic material, and can either enter the lytic cycle (leading to cell lysis and death) or the lysogenic cycle (where their DNA integrates into the bacterial genome).

Why the other options are incorrect:
- a. the lungs:
Bacteriophages do not infect lungs; they target bacteria.

• b. viruses:
  Bacteriophages do not infect other viruses; they specifically infect bacterial cells.
• c. prions:
  Prions are misfolded proteins that cause disease but are not living organisms, and bacteriophages do not infect prions.

Thus, the only correct answer is d. bacteria.

Question 13

Which of the following is NOT used to treat active viral disease?
a. vaccines
b. antiviral drugs
c. antibiotics
d. phage therapy

Answer 13

The correct answer is:

c. antibiotics.

Explanation:
- a. vaccines:
Vaccines are used to prevent viral infections but are not typically used to treat active viral diseases once an infection has occurred. However, they play a key role in preventing diseases.

• b. antiviral drugs:
  Antiviral drugs are specifically designed to treat active viral infections by inhibiting viral replication and helping manage symptoms.
• c. antibiotics:
  NOT used for viral infections. Antibiotics are effective against bacterial infections but do not work on viruses.
• d. phage therapy:
  Phage therapy involves using bacteriophages to target bacterial infections, but it is not a standard treatment for viral diseases. However, research is ongoing regarding its application.

Thus, the only option that is not used to treat active viral disease is c. antibiotics.

Question 14

Vaccines_______.
a. are similar to viroids
b. are only needed once
c. kill viruses
d. stimulate an immune response

Answer 14

The correct answer is:

d. stimulate an immune response.

Explanation:
- a. are similar to viroids:
False. Vaccines and viroids are not similar. Vaccines are preparations made from weakened or inactivated viruses, or parts of the virus, designed to provoke an immune response. Viroids are small, circular pieces of RNA that can infect plants and do not produce proteins or elicit immune responses.

• b. are only needed once:
  False. Many vaccines require multiple doses or booster shots to ensure long-lasting immunity. For example, the measles, mumps, and rubella (MMR) vaccine typically requires two doses.
• c. kill viruses:
  False. Vaccines do not kill viruses; rather, they expose the immune system to a harmless form of the virus (or parts of it) to stimulate an immune response without causing the disease.
• d. stimulate an immune response:
  True. This is the primary function of vaccines. They help the body recognize and fight off future infections by training the immune system to recognize specific pathogens.

Therefore, the correct statement about vaccines is d. stimulate an immune response.

Question 15

Which of the following is not associated with prions?
a. replicating shapes
b. mad cow disease
c. DNA
d. toxic proteins

Answer 15

The correct answer is:

c. DNA.

Explanation:
- a. replicating shapes:
True. Prions are misfolded proteins that can induce other proteins to misfold as well, which can lead to a chain reaction of misfolding. This process is sometimes described as replicating shapes.

• b. mad cow disease:
  True. Mad cow disease, or bovine spongiform encephalopathy (BSE), is a well-known prion disease affecting cattle. It can also affect humans who consume contaminated beef, leading to variant Creutzfeldt-Jakob disease (vCJD).
• c. DNA:
  Not associated. Prions are solely composed of protein and do not contain DNA or RNA. They replicate by causing normal proteins to misfold into the abnormal prion form.
• d. toxic proteins:
  True. Prions are considered toxic because their accumulation in neural tissue can lead to neurodegeneration and cell death.

Therefore, the statement that is not associated with prions is c. DNA.

Question 16

Which statement is true of viroids?
a. They are single-stranded RNA particles.
b. They reproduce only outside of the cell.
c. They produce proteins.
d. They affect both plants and animals.

Answer 16

The correct answer is:

a. They are single-stranded RNA particles.

Explanation:
- a. They are single-stranded RNA particles.
True. Viroids are composed of short, circular, single-stranded RNA and do not encode proteins.

• b. They reproduce only outside of the cell.
  False. Viroids replicate inside the host cell. They hijack the host’s cellular machinery to replicate their RNA.
• c. They produce proteins.
  False. Viroids do not produce proteins. They lack the coding capacity for protein synthesis, which distinguishes them from viruses.
• d. They affect both plants and animals.
  False. Viroids primarily affect plants. There are currently no known viroids that infect animals.

Therefore, the true statement about viroids is a. They are single-stranded RNA particles.

Question 17

The first electron micrograph of a virus (tobacco mosaic virus) was produced in 1939. Before that time, how did scientists know that viruses existed if they could not see them? (Hint: Early scientists called viruses “filterable agents.”)

Answer 17

Before the advent of electron microscopy, scientists inferred the existence of viruses through several key observations and experiments:

1. Disease Transmission:
   Scientists observed that certain diseases could be transmitted from one plant or animal to another even when no bacteria or fungi could be found. For example, the tobacco mosaic disease could spread from infected plants to healthy ones, suggesting the presence of a pathogen smaller than bacteria.
2. Filtration Experiments:
   The term “filterable agents” arose from experiments in which scientists used fine filters to remove bacteria from infected sap. When they filtered the sap and then inoculated healthy plants with the filtrate, the plants still became infected. This indicated that the infectious agent was smaller than the pores of the filters (which could trap bacteria), suggesting the existence of a new type of pathogen.
3. Symptoms and Disease Patterns:
   Scientists carefully studied the symptoms and patterns of viral infections. They noted that these infections often had specific symptoms that could be correlated with certain “filterable agents,” further supporting the idea that there were smaller infectious agents causing these diseases.
4. Infectivity and Biological Properties:
   The infectious nature of these agents was demonstrated through biological assays. The ability of the agent to reproduce in host cells, despite not being visible under a light microscope, indicated that it had characteristics of living organisms.

Through these methods, early scientists built a strong case for the existence of viruses, even without the direct visualization capabilities that would later be provided by electron microscopy.

Question 18

Why can’t dogs catch the measles?

Answer 18

Dogs cannot catch the measles virus because measles is a disease caused by a specific virus (the measles virus, or Morbillivirus) that primarily infects humans. Here are the key reasons why dogs are not susceptible to measles:

1. Host Specificity:
   The measles virus is adapted to infect human cells. It binds to specific receptors on human immune cells (like CD46 and SLAM) that are not present or are significantly different in dogs. This means that the virus cannot effectively attach to and enter dog cells.
2. Different Immune Responses:
   Dogs have a different immune system and immune response compared to humans. Even if a dog were to somehow come into contact with the measles virus, its immune system would not support the virus’s replication.
3. Lack of Viral Tropism:
   Viral tropism refers to the specific types of cells or tissues that a virus can infect. The measles virus is tropic to human epithelial cells, meaning it specifically targets human tissues and not canine tissues.
4. Evolutionary Adaptation:
   Over time, viruses have evolved to infect particular host species. The measles virus has adapted to infect humans and does not have the necessary mechanisms to infect dogs.

While dogs can suffer from their own viral diseases, such as canine distemper, these are caused by different viruses that have evolved to infect dogs.

Question 19

One of the first and most important targets for drugs to fight infection with HIV (a retrovirus) is the reverse transcriptase enzyme. Why?

Answer 19

The reverse transcriptase enzyme is a crucial target for drugs designed to fight HIV (a retrovirus) for several reasons:

1. Key Role in Viral Replication:
   HIV is a retrovirus, meaning it converts its RNA genome into DNA after entering a host cell. The reverse transcriptase enzyme is responsible for this conversion, which is essential for the viral life cycle. By targeting this enzyme, drugs can effectively inhibit the replication of the virus.
2. Viral Dependence on Host Cellular Machinery:
   Unlike many other viruses, retroviruses like HIV rely heavily on reverse transcriptase to replicate their RNA into DNA, which is then integrated into the host cell’s genome. If reverse transcriptase is inhibited, the virus cannot produce the necessary DNA to replicate and spread.
3. Lack of Similarity to Human Enzymes:
   Reverse transcriptase is significantly different from human DNA polymerases and other enzymes involved in DNA synthesis. This difference allows for selective targeting by antiviral drugs (such as NRTIs - nucleoside reverse transcriptase inhibitors and NNRTIs - non-nucleoside reverse transcriptase inhibitors) with fewer side effects on human cells.
4. Resistance Management:
   HIV is known for its high mutation rate, which can lead to resistance against treatments. Targeting reverse transcriptase can be part of a combination therapy strategy (known as HAART - Highly Active Antiretroviral Therapy), making it more difficult for the virus to develop resistance when multiple drugs are used simultaneously.
5. Impact on Viral Load:
   By inhibiting reverse transcriptase, these drugs can significantly reduce the viral load in an infected individual, which is crucial for managing HIV infection and preventing the progression to AIDS. Lowering the viral load also helps reduce the transmission risk to others.

Overall, targeting reverse transcriptase is vital for effectively controlling and treating HIV infections, as it is a critical step in the virus’s replication process.

Question 20

Although plant viruses cannot infect humans, what are some of the ways in which they affect humans?

Answer 20

While plant viruses do not infect humans directly, they can have significant indirect effects on human health and well-being in various ways:

1. Impact on Food Supply:
   
   • Crop Diseases: Plant viruses can cause serious diseases in agricultural crops, leading to reduced yields and crop failures. This can result in food shortages, increased food prices, and economic instability for farmers and communities reliant on agriculture.
   • Quality Degradation: Infected plants may produce lower-quality fruits and vegetables, affecting the nutritional value and appearance of food products. This can impact food choices and dietary health for consumers.
2. Economic Consequences:
   
   • Agricultural Losses: The economic burden on farmers due to crop losses from viral infections can be substantial, leading to reduced income and potential bankruptcy for some agricultural operations.
   • Increased Control Costs: Farmers may need to invest more in pest management and disease control measures, including purchasing resistant plant varieties, using pesticides, or employing other costly farming practices.
3. Trade and Market Access:
   
   • Trade Restrictions: The presence of plant viruses can lead to trade barriers. Countries may impose restrictions on the import of crops from regions known to harbor certain plant viruses, affecting international trade dynamics and economic relationships.
4. Food Security:
   
   • Nutritional Impact: Reduced agricultural productivity can lead to food insecurity in vulnerable populations, affecting access to nutritious food and increasing malnutrition rates.
5. Environmental Effects:
   
   • Biodiversity: Plant viruses can disrupt ecosystems by affecting plant populations. This can have cascading effects on other organisms, including insects and animals that rely on these plants for food and habitat.
6. Research and Biotechnology:
   
   • Viral Research: Studying plant viruses contributes to scientific knowledge and can lead to advancements in biotechnology, including the development of genetically modified organisms (GMOs) that are resistant to viral infections. This can improve agricultural resilience and food production.
7. Cultural and Social Impacts:
   
   • Local Livelihoods: In many regions, agriculture is a primary livelihood. Plant viruses that devastate crops can affect the social fabric of communities, leading to migration, changes in local economies, and shifts in traditional practices.

In summary, while plant viruses do not infect humans, their impact on agriculture, economies, food security, and ecosystems can have significant implications for human health and well-being.

Question 21

Why is immunization after being bitten by a rabid animal so effective and why aren’t people vaccinated for rabies like dogs and cats are?

Answer 21

Immunization after being bitten by a rabid animal is effective due to the following reasons:

1. Post-Exposure Prophylaxis (PEP):
   
   • Timing: Rabies vaccination after exposure (known as post-exposure prophylaxis) involves administering a series of rabies vaccines as soon as possible after a bite. This is critical because rabies has a long incubation period (weeks to months) during which the virus travels through the nervous system to the brain, giving the immune system time to respond to the vaccine.
   • Boosting Immune Response: The rabies vaccine stimulates the immune system to produce antibodies against the rabies virus. If administered promptly after exposure, it can effectively prevent the virus from causing disease by neutralizing the virus before it can reach the central nervous system.
2. Combination of Vaccine and Rabies Immune Globulin (RIG):
   
   • In addition to the rabies vaccine, rabies immune globulin may also be administered. This provides immediate passive immunity by supplying pre-formed antibodies against the rabies virus, further enhancing protection during the critical period before the body develops its immune response.
3. Effectiveness in Preventing Disease:
   
   • When the rabies vaccine is given quickly after a bite from a rabid animal, it is highly effective (over 99% effective) in preventing the onset of rabies, which is almost always fatal once symptoms appear.

Why Aren’t People Vaccinated for Rabies Like Dogs and Cats?

1. Risk of Exposure:
   
   • Unlike pets, which are often in contact with wildlife and have a higher risk of rabies exposure, the general human population has a much lower risk of exposure to rabies. Most people do not interact with wild animals or are not in high-risk occupations (like veterinarians or animal handlers) that would necessitate preemptive vaccination.
2. Cost and Resources:
   
   • Vaccinating the entire population against rabies would be logistically challenging and expensive. It is more practical to provide vaccination on an as-needed basis, especially for high-risk groups.
3. Public Health Strategy:
   
   • The strategy for rabies management in humans focuses on prompt treatment after exposure rather than widespread vaccination. This approach has proven effective in controlling rabies outbreaks in humans.
4. Natural Immunity:
   
   • In many cases, human immune systems are capable of mounting a strong response to the rabies vaccine if administered shortly after exposure, making universal vaccination unnecessary for the general population.

Conclusion
In summary, the effectiveness of post-exposure rabies vaccination stems from the timely administration of vaccines and immune globulin after exposure. Meanwhile, the lack of widespread vaccination in humans is due to lower exposure risk, practical considerations, and the effectiveness of the post-exposure treatment strategy.

Question 22

Prions are responsible for variant Creutzfeldt- Jakob Disease, which has resulted in over 100 human deaths in Great Britain during the last 10 years. How do humans obtain this disease?

Answer 22

Humans can obtain variant Creutzfeldt-Jakob Disease (vCJD) primarily through the consumption of beef products that are contaminated with the prions responsible for the disease. Here are the key ways humans are exposed to the prions that cause vCJD:

1. Consumption of Infected Beef:
   
   • Bovine Spongiform Encephalopathy (BSE): vCJD is associated with the consumption of beef from cattle infected with bovine spongiform encephalopathy (BSE), commonly known as “mad cow disease.” Prions in the brain and nervous tissue of infected cattle can contaminate beef products, particularly those that involve the central nervous system.
   • Food Products: Risky food products include beef-derived items such as ground beef, beef sausages, or any products made from parts of the cow that may contain nervous tissue.
2. Infection of Humans:
   
   • Once a person ingests prion-contaminated meat, the prions can survive digestion and enter the body. They then misfold normal proteins in the human brain, leading to the characteristic neurodegenerative effects of vCJD.
3. Medical Procedures:
   
   • Although much less common, there is a potential risk of transmission through medical procedures involving contaminated surgical instruments or blood transfusions from infected donors. In the past, there have been a few reported cases of vCJD transmission through blood products.
4. Geographical and Dietary Factors:
   
   • The outbreaks of vCJD have been particularly associated with regions experiencing BSE outbreaks. The majority of cases in humans have been linked to dietary exposure to BSE-infected cattle, primarily in the UK during the 1980s and 1990s when the BSE epidemic peaked.

Conclusion
In summary, humans obtain variant Creutzfeldt-Jakob Disease mainly through the consumption of beef products contaminated with prions from BSE-infected cattle. Preventive measures have been implemented in many countries to reduce the risk of BSE transmission to humans, including banning the use of certain cattle parts in food products and improving animal feed regulations.

Question 23

How are viroids like viruses?

Answer 23

Viroids and viruses share several similarities, despite their differences in structure and function. Here are the key ways in which viroids are like viruses:

1. Infectious Agents:
   
   • Both viroids and viruses are considered infectious agents that can cause diseases in their respective hosts. Viroids primarily infect plants, while viruses can infect a wide range of organisms, including animals, plants, and bacteria.
2. Genetic Material:
   
   • Viroids and viruses contain genetic material. Viroids consist solely of nucleic acid (specifically, circular RNA), while viruses typically have either DNA or RNA as their genetic material. In both cases, this genetic material carries the information necessary for the agent to replicate and propagate within the host.
3. Dependence on Host Cells:
   
   • Both viroids and viruses rely on the host cell machinery for replication. They cannot replicate independently; instead, they hijack the cellular mechanisms of their host organisms to reproduce and spread.
4. Transmission:
   
   • Both can be transmitted between hosts, although the modes of transmission may vary. Viroids can spread through various means, such as mechanical transmission (e.g., through gardening tools) or via insect vectors, while viruses can be transmitted through contact, vectors (like mosquitoes), or through respiratory droplets.
5. Pathogenicity:
   
   • Both viroids and viruses can lead to disease symptoms in their respective hosts. They can cause a range of health effects, from mild symptoms to severe damage, depending on the specific agent and host organism.
6. Lack of Cellular Structure:
   
   • Both viroids and viruses are acellular, meaning they lack a cellular structure. They do not have cellular components like membranes or organelles, which differentiates them from living organisms.

Conclusion
In summary, viroids and viruses are alike in that they are both infectious agents that rely on host cells for replication, contain genetic material, and can cause diseases in their hosts. However, they differ significantly in structure, with viroids being much simpler and composed solely of RNA, while viruses have more complex structures and can contain either RNA or DNA as their genetic material.