6.5: Acellular Pathogens (Exercises) - Biology

6.5: Acellular Pathogens (Exercises) - Biology

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6.1: Viruses

Viruses are generally ultramicroscopic, typically from 20 nm to 900 nm in length. Viruses are obligate intracellular parasites.

Multiple Choice

The component(s) of a virus that is/are extended from the envelope for attachment is/are the:

A. capsomeres
B. spikes
C. nucleic acid
D. viral whiskers


Which of the following does a virus lack? Select all that apply.

A. ribosomes
B. metabolic processes
C. glycoprotein

A and B

The envelope of a virus is derived from the host’s

A. nucleic acids
B. membrane structures
C. cytoplasm
D. genome


In naming viruses, the family name ends with ________ and genus name ends with _________.

A. −virus; −viridae
B. −viridae; −virus
C. −virion; virus
D. −virus; virion


What is another name for a nonenveloped virus?

A. enveloped virus
B. provirus
C. naked virus
D. latent virus



True or False: Scientists have identified viruses that are able to infect fungal cells.


Fill in the Blank

A virus that infects a bacterium is called a/an ___________________.


A/an __________ virus possesses characteristics of both a polyhedral and helical virus.


A virus containing only nucleic acid and a capsid is called a/an ___________________ virus or __________________ virus.

naked or nonenveloped

The ____________ _____________ on the bacteriophage allow for binding to the bacterial cell.

tail fibers

Short Answer

Discuss the geometric differences among helical, polyhedral, and complex viruses.

What was the meaning of the word “virus” in the 1880s and why was it used to describe the cause of tobacco mosaic disease?

Critical Thinking

Name each labeled part of the illustrated bacteriophage.

In terms of evolution, which do you think arises first? The virus or the host? Explain your answer.

Do you think it is possible to create a virus in the lab? Imagine that you are a mad scientist. Describe how you would go about creating a new virus.

6.2: The Viral Life Cycle

Many viruses target specific hosts or tissues. Some may have more than one host. Many viruses follow several stages to infect host cells. These stages include attachment, penetration, uncoating, biosynthesis, maturation, and release. Bacteriophages have a lytic or lysogenic cycle. The lytic cycle leads to the death of the host, whereas the lysogenic cycle leads to integration of phage into the host genome.

Multiple Choice

Which of the following leads to the destruction of the host cells?

A. lysogenic cycle
B. lytic cycle
C. prophage
D. temperate phage


A virus obtains its envelope during which of the following phases?

A. attachment
B. penetration
C. assembly
D. release


Which of the following components is brought into a cell by HIV?

A. a DNA-dependent DNA polymerase
B. RNA polymerase
C. ribosome
D. reverse transcriptase


A positive-strand RNA virus:

A. must first be converted to a mRNA before it can be translated.
B. can be used directly to translate viral proteins.
C. will be degraded by host enzymes.
D. is not recognized by host ribosomes.


What is the name for the transfer of genetic information from one bacterium to another bacterium by a phage?

A. transduction
B. excision
D. translation


Fill in the Blank

An enzyme from HIV that can make a copy of DNA from RNA is called _______________________.

reverse transcriptase

For lytic viruses, _________________ is a phase during a viral growth curve when the virus is not detected.


Short Answer

Briefly explain the difference between the mechanism of entry of a T-even bacteriophage and an animal virus.

Discuss the difference between generalized and specialized transduction.

Differentiate between lytic and lysogenic cycles.

Critical Thinking

Label the five stages of a bacteriophage infection in the figure:

Bacteriophages have lytic and lysogenic cycles. Discuss the advantages and disadvantages for the phage.

How does reverse transcriptase aid a retrovirus in establishing a chronic infection?

Discuss some methods by which plant viruses are transmitted from a diseased plant to a healthy one.

6.3: Isolation, Culture, and Identification of Viruses

Viral cultivation requires the presence of some form of host cell (whole organism, embryo, or cell culture). Viruses can be isolated from samples by filtration. Viral filtrate is a rich source of released virions. Bacteriophages are detected by presence of clear plaques on bacterial lawn. Animal and plant viruses are detected by cytopathic effects, molecular techniques (PCR, RT-PCR), enzyme immunoassays, and serological assays (hemagglutination assay, hemagglutination inhibition assay).

Multiple Choice

Which of the followings cannot be used to culture viruses?

A. tissue culture
B. liquid medium only
C. embryo
D. animal host


Which of the following tests can be used to detect the presence of a specific virus?

D. all of the above


Which of the following is NOT a cytopathic effect?

A. transformation
B. cell fusion
C. mononucleated cell
D. inclusion bodies


Fill in the Blank

Viruses can be diagnosed and observed using a(n) _____________ microscope.


Cell abnormalities resulting from a viral infection are called ____________ _____________.

cytopathic effects

Short Answer

Briefly explain the various methods of culturing viruses.

Critical Thinking

Label the components indicated by arrows.

(credit: modification of work by American Society for Microbiology)

What are some characteristics of the viruses that are similar to a computer virus?

6.4: Viroids, Virusoids, and Prions

Other acellular agents such as viroids, virusoids, and prions also cause diseases. Viroids consist of small, naked ssRNAs that cause diseases in plants. Virusoids are ssRNAs that require other helper viruses to establish an infection. Prions are proteinaceous infectious particles that cause transmissible spongiform encephalopathies. Prions are extremely resistant to chemicals, heat, and radiation.

Multiple Choice

Which of these infectious agents do not have nucleic acid?

A. viroids
B. viruses
C. bacteria
D. prions


Which of the following is true of prions?

A. They can be inactivated by boiling at 100 °C.
B. They contain a capsid.
C. They are a rogue form of protein, PrP.
D. They can be reliably inactivated by an autoclave.


Fill in the Blank

Both viroids and virusoids have a(n) _________ genome, but virusoids require a(n) _________ to reproduce.

RNA, helper virus

Short Answer

Describe the disease symptoms observed in animals infected with prions.

Critical Thinking

Does a prion replicate? Explain.

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21.4 Other Acellular Entities: Prions and Viroids

In this section, you will explore the following questions:

  • What are prions and how do they cause disease?
  • What are viroids and their targets of infection?

Connection for AP ® Courses

The content described in this section is outside the scope for AP ® . However, it’s interesting to note that prions and viroids—pathogens that are far simpler in structure than viruses—can produce deadly diseases, including mad cow disease and Creutzfeldt–Jakob disease. Prions are infectious proteins, whereas viroids are single-stranded RNA pathogens (agents with the ability to cause disease) that infect plants.


Prions, so-called because they are proteinaceous, are infectious particles—smaller than viruses—that contain no nucleic acids (neither DNA nor RNA). Historically, the idea of an infectious agent that did not use nucleic acids was considered impossible, but pioneering work by Nobel Prize-winning biologist Stanley Prusiner has convinced the majority of biologists that such agents do indeed exist.

Fatal neurodegenerative diseases, such as kuru in humans and bovine spongiform encephalopathy (BSE) in cattle (commonly known as “mad cow disease”) were shown to be transmitted by prions. The disease was spread by the consumption of meat, nervous tissue, or internal organs between members of the same species. Kuru, native to humans in Papua New Guinea, was spread from human to human via ritualistic cannibalism. BSE, originally detected in the United Kingdom, was spread between cattle by the practice of including cattle nervous tissue in feed for other cattle. Individuals with kuru and BSE show symptoms of loss of motor control and unusual behaviors, such as uncontrolled bursts of laughter with kuru, followed by death. Kuru was controlled by inducing the population to abandon its ritualistic cannibalism.

On the other hand, BSE was initially thought to only affect cattle. Cattle dying of the disease were shown to have developed lesions or “holes” in the brain, causing the brain tissue to resemble a sponge. Later on in the outbreak, however, it was shown that a similar encephalopathy in humans known as variant Creutzfeldt-Jakob disease (CJD) could be acquired from eating beef from animals with BSE, sparking bans by various countries on the importation of British beef and causing considerable economic damage to the British beef industry (Figure 21.18). BSE still exists in various areas, and although a rare disease, individuals that acquire CJD are difficult to treat. The disease can be spread from human to human by blood, so many countries have banned blood donation from regions associated with BSE.

The cause of spongiform encephalopathies, such as kuru and BSE, is an infectious structural variant of a normal cellular protein called PrP (prion protein). It is this variant that constitutes the prion particle. PrP exists in two forms, PrP c , the normal form of the protein, and PrP sc , the infectious form. Once introduced into the body, the PrP sc contained within the prion binds to PrP c and converts it to PrP sc . This leads to an exponential increase of the PrP sc protein, which aggregates. PrP sc is folded abnormally, and the resulting conformation (shape) is directly responsible for the lesions seen in the brains of infected cattle. Thus, although not without some detractors among scientists, the prion seems likely to be an entirely new form of infectious agent, the first one found whose transmission is not reliant upon genes made of DNA or RNA.


Viroids are plant pathogens: small, single-stranded, circular RNA particles that are much simpler than a virus. They do not have a capsid or outer envelope, but like viruses can reproduce only within a host cell. Viroids do not, however, manufacture any proteins, and they only produce a single, specific RNA molecule. Human diseases caused by viroids have yet to be identified.

Viroids are known to infect plants (Figure 21.19) and are responsible for crop failures and the loss of millions of dollars in agricultural revenue each year. Some of the plants they infect include potatoes, cucumbers, tomatoes, chrysanthemums, avocados, and coconut palms.

Career Connection


Virology is the study of viruses, and a virologist is an individual trained in this discipline. Training in virology can lead to many different career paths. Virologists are actively involved in academic research and teaching in colleges and medical schools. Some virologists treat patients or are involved in the generation and production of vaccines. They might participate in epidemiologic studies (Figure 21.20) or become science writers, to name just a few possible careers.

If you think you may be interested in a career in virology, find a mentor in the field. Many large medical centers have departments of virology, and smaller hospitals usually have virology labs within their microbiology departments. Volunteer in a virology lab for a semester or work in one over the summer. Discussing the profession and getting a first-hand look at the work will help you decide whether a career in virology is right for you. The American Society of Virology’s website is a good resource for information regarding training and careers in virology.

Which of the following is not associated with prions?

Which statement is true of viroids?

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?

This prion-based disease is transmitted through human consumption of infected meat.

How are viroids like viruses?

They both replicate in a cell, and they both contain nucleic acid.

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    Science Practice Challenge Questions

    Influenza A virus is the most pathogenic of the human influenza viruses. Its envelope encloses a protein complex (vRNP) and eight, single-stranded, negative RNA (the complement of a positive RNA strand that can be transcribed by a ribosome) segments (vRNA). Each segment encodes one or two proteins that support viral replication. On the outer surface of the envelope are proteins that recognize and bind to host receptors.

    A. Annotate the representation below to briefly describe each process associated with a numbered label.

    B. Describe influenza A viral replication as a process regulated by either positive or negative feedback and justify your selection.

    The human-acquired immunodeficiency syndrome (AIDS) and many cancers are cause by double-stranded RNA retroviruses.

    C. Contrast the processes of viral replication of HIV and influenza A virus.

    D. Explain the difference in the effects of infection by HIV and influenza A virus on host genetic variability.

    E. Measured mutation rates for influenza A virus and HIV are nearly identical (Sanjuan et al., Jour. Virology, 2010). Explain this observation even though host error-checking operates in one of these replication modes.

    Three-dimensional (3D) structures, or folding, of proteins have been shown to contain more information about evolutionary relationships than the sequences of DNA nucleotides that encode the proteins. Amino acid sequences of rabbit skeletal muscle actin (375 amino acids) and bovine ATPase (386 amino acids) have only 39 locations in common. However, the 3D structure of these proteins are nearly identical (Flaherty et al., Proc. Natl. Acad. Sci. USA, 1991). As information about the 3D folding of proteins and the number of sequenced whole genomes has increased, folding has been shown to be an evolutionarily conserved property.

    A. Analyze these data to refine the following model: The evolutionary history of life on Earth can be inferred from variations over time of the nucleotide sequence of a gene.

    By applying a classification scheme based on protein folding, Nasir and Caetano-Anollés (Sci. Adv. 2015) have determined the number of folding families that viruses share with the three domains. Approximately 60% of the folding patterns found in viruses were common to all three domains, as shown below. Fewer than 10% were unique to viruses.

    Viruses are acellular, and, consequently, they lie outside of the three domains of cellular life. However, their exclusion is increasingly challenged. Since 2012, several very large viruses have been discovered, each a double-stranded DNA virus with more than one million bases, with some encoding nucleotides and amino acids. However, none encode ribosomes, so these viruses are still dependent on a marine bacteriovore (amoeba or flagellate) host for replication.

    Hypotheses regarding the origin of life on Earth need to account for the relationship between proteins and genetic information. Proteins are required to read and write genetic information, but genetic information is required to synthesize proteins. Which of these systems evolved first, and if neither came first, how could they evolve simultaneously? The RNA-first model is based on the idea that ribosomal RNA both encodes and synthesizes proteins.

    B. Describe a hypothesis for the origin of life on Earth that combines the dual functionality of RNA and the function of retroviral reverse transcriptase to propose a mechanism leading to an ancient, acellular lineage of very large, double-stranded DNA viruses and a first DNA-based cellular life form.

    C. Like viruses, the nucleus of a eukaryote uses the machinery of the cell to transcribe DNA and synthesize proteins. Evaluate the possibility of the origin of Eukarya by specialization of a very large double-stranded DNA virus.

    Viruses evolve but leave no fossil evidence that can be used to construct phylogenies. However, viral DNA, especially that of retroviruses, is commonly found in the host genome. By comparing sequences from the same virus integrated at different points in time, the evolutionary history of the virus can be constructed. The viral genomes are typically found incomplete, in segments, and interrupted by stop codons. In jawed vertebrates, retroviral sequences or sequences that have been derived from them are a significant fraction of the whole genome.

    A. Explain why retroviral DNA rather than the genomes of single-stranded or double-stranded DNA or single-stranded RNA viruses are found in host DNA.

    Exaptation occurs when gene expression provides a function that is independent of the selection pressures that have acted on the gene. For example, a pigment that provided selective advantage by reducing damage from solar radiation becomes an element of mating behavior. Feathers that evolved under selection to prevent heat loss become a means of flight.

    In a study of viral evolution within host genomes of primates, Katzuorakis and Gifford (PLOS Genetics, 2010) found that viral genomes within the host were surprisingly stable with computer simulation, they estimated the probability of such constancy at 1 in 100,000.

    B. Explain in terms of selection how viral genetic information that no longer replicates the virus is maintained by the host.

    Distemper is an incurable disease of cats, dogs, and their sister lineages caused by a parvovirus. The virus exploits the host’s transferrin, a membrane-bound protein used for iron transport, to attach to the cell. The phylogeny of the Parvoviridae family has been constructed (J. Kaebler, PLOS Pathogens, 2012). That study revealed the evolution of both the virus and the host protein through selection to resist infection. About 54 million years ago when the lineage of cats (Feliformia) diverged from that of dogs (Caniformia), the parvovirus envelope diverged as well, conforming to changes in the host’s transferrin. In 1978, a worldwide disease in dogs due to a parvovirus suddenly appeared.

    C. Explain how this pandemic could have originated in the cat population.

    A simple calculation of the rate of spread of a pox virus (virion) led researchers at Imperial College London to a new insight. Virions communicate with other virions. The researchers observed that the radius of an approximately circular plaque of infected cells grew to 1.45 mm in just 3 days. They measured the distance between adjacent cells to be 0.037 mm to obtain the apparent time for the lytic cycle (from infection to lysis). They compared this time to the actual rate at which new virions are formed: 5 to 6 hours.

    A. Predict the radius of infection if the infection process involved a sequence of entry, replication, lysis, and infection of an adjacent cell.

    To account for this discrepancy between observed and predicted growth rates, the researchers examined the viral entry process and discovered that the actin protein on the host cell’s surface that provided the viral receptor was modified by attachment. They then found a mutant virus that did not modify the cell surface protein. The dependence of the growth of plaque radius on time for the wild type and mutant are shown in the graph.

    B. Analyze these data and compare the infection rates calculated with those predicted in part A.

    C. Use the results of this experiment to support the claim that responses to information and communication of information affect natural selection.

    Describe how viral replication introduces genetic variation in the viral population.