IB Biology Quiz: Understand Viruses
19 questions · exam conditions
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Understand VirusesQuestion 1 of 19

A bacteriophage experiences a mutation that results in non-functional tail fibres. What would be the most immediate consequence of this mutation for the phage's life cycle?

The phage would be unable to specifically recognize and attach to the surface of a host bacterium.
The phage's DNA would be unable to integrate into the host chromosome to form a prophage.
The phage would successfully inject its genetic material but could not replicate within the host cell.
The components of new virions would be synthesized but would fail to self-assemble correctly.
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IB Biology Quiz

IB Biology Quiz: Understand Viruses

Practice Understand Viruses in IB Biology with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Understand Viruses, giving you a quick way to practice the rules, question types, and explanations that matter most for IB Biology.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

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Question 1

A bacteriophage experiences a mutation that results in non-functional tail fibres. What would be the most immediate consequence of this mutation for the phage's life cycle?

  1. The phage would be unable to specifically recognize and attach to the surface of a host bacterium. (correct answer)
  2. The phage's DNA would be unable to integrate into the host chromosome to form a prophage.
  3. The phage would successfully inject its genetic material but could not replicate within the host cell.
  4. The components of new virions would be synthesized but would fail to self-assemble correctly.
Explanation: Bacteriophage tail fibres are responsible for the specific recognition and binding to receptor sites on the bacterial cell wall. Without functional tail fibres, the phage cannot initiate an infection. Injection of genetic material (C), integration (B), and assembly (D) are all subsequent steps that cannot occur if attachment fails.

Question 2

A scientist isolates a virus that injects its DNA into a host cell, leaving its complex capsid attached to the outer surface. The host cell is observed to have a cell wall primarily composed of peptidoglycan. Which conclusion is most justified?

  1. The virus is a retrovirus infecting a mammalian white blood cell.
  2. The virus is an enveloped virus infecting a eukaryotic plant cell.
  3. The virus is a bacteriophage that infects a prokaryotic cell. (correct answer)
  4. The virus is a non-enveloped virus that infects a fungal cell.
Explanation: The combination of two key features points to a bacteriophage. First, the injection of nucleic acid while the capsid remains outside is the characteristic mechanism of bacteriophages. Second, a cell wall made of peptidoglycan is the defining feature of bacteria (prokaryotes). Animal cells lack cell walls (A), plant cell walls are cellulose (B), and fungal cell walls are chitin (D).

Question 3

A researcher infects a bacterial culture with a temperate bacteriophage. One hour later, the culture is treated with a chemical known to induce the lytic cycle. Surprisingly, no cell lysis is observed in the following hours. Which is the most plausible experimental explanation for this outcome?

  1. The bacteriophage used was actually a virulent phage incapable of entering lysogeny.
  2. The bacterial host cells were already immune to lysis due to a pre-existing prophage.
  3. The initial infection failed, and the phage's genetic material never entered the host cells. (correct answer)
  4. The chemical inducer destroyed the phage's capsid before it could trigger lysis.
Explanation: For induction to occur, a prophage must first be established within the host cell. If no lysis is observed after adding an inducer, the simplest and most direct explanation is that there was no prophage to induce. This would happen if the initial infection step (attachment and/or injection) failed. If the phage were virulent (A), lysis would have occurred without the inducer. Immunity (B) might prevent infection but wouldn't prevent induction if infection did occur. The inducer acts on the host/prophage system, not the external capsid (D).

Question 4

A population of bacteria becomes infected with a temperate phage, and the majority of the phages enter the lysogenic cycle. What is a likely consequence for this bacterial population?

  1. The entire bacterial population will be rapidly destroyed due to widespread cell lysis.
  2. The bacteria will become immune to infection by any other type of virus.
  3. The bacteria will cease to undergo binary fission, leading to a static population size.
  4. The bacteria may express new genes from the prophage, such as those for toxin production. (correct answer)
Explanation: This phenomenon is known as lysogenic conversion. The integrated prophage DNA can contain genes that are expressed by the host bacterium, conferring new properties. A classic example is the gene for diphtheria toxin in Corynebacterium diphtheriae. Rapid lysis (A) is characteristic of the lytic cycle. Immunity is typically conferred only against the same or related phages, not all viruses (B). Bacteria continue to reproduce normally during lysogeny (C).

Question 5

Which statement accurately distinguishes the lysogenic cycle from the lytic cycle in bacteriophages?

  1. The lytic cycle involves the integration of a prophage, whereas the lysogenic cycle results in immediate cell lysis.
  2. Attachment of the virion to the host cell occurs only in the lytic cycle and not in the lysogenic cycle.
  3. In the lysogenic cycle, the viral DNA is replicated along with the host chromosome, which remains intact. (correct answer)
  4. Host cell ribosomes are used to synthesize viral proteins in the lytic cycle but not in the lysogenic cycle.
Explanation: A key feature of the lysogenic cycle is the integration of the viral DNA (as a prophage) into the host chromosome. This allows the host cell to survive and replicate, copying the prophage with its own DNA. In the lytic cycle, the host chromosome is typically degraded. Distractor A reverses the definitions. Attachment (B) is the first step of both cycles. Host ribosomes (D) are used for protein synthesis in the lytic phase of both cycles, but this step is delayed in the lysogenic cycle until induction occurs.

Question 6

How is the host cell's plasma membrane directly utilized during the life cycle of an enveloped virus like influenza?

  1. It serves as the template for the synthesis of the viral lipid envelope.
  2. It provides the lipids and embedded viral proteins for the envelope as new virions exit by budding. (correct answer)
  3. It is the location where viral ribosomes attach to synthesize all viral proteins.
  4. It is completely dissolved by viral enzymes to provide fatty acids for energy production.
Explanation: Enveloped viruses acquire their envelope during release. Viral proteins are inserted into the host's plasma membrane, and the new virion 'buds' off, taking a piece of the host membrane with it. This piece, now containing viral proteins, becomes the viral envelope. The membrane is not a template (A). Viral proteins are made on host ribosomes in the cytoplasm (C). The membrane is utilized, not dissolved for energy (D).

Question 7

A new antiviral drug is developed that specifically inhibits the action of an enzyme that synthesizes DNA using an RNA template. Which type of virus would this drug be most effective against?

  1. A DNA bacteriophage that exclusively undergoes a lytic cycle.
  2. A retrovirus, such as HIV, that uses reverse transcriptase. (correct answer)
  3. An RNA virus that replicates its genome using an RNA-dependent RNA polymerase.
  4. A double-stranded DNA virus that integrates into the host genome as a provirus.
Explanation: The process of synthesizing DNA from an RNA template is called reverse transcription, and the enzyme responsible is reverse transcriptase. This is the defining characteristic of retroviruses. A DNA bacteriophage (A) and a dsDNA virus (D) replicate their DNA using DNA polymerase. An RNA virus with RNA-dependent RNA polymerase (C) synthesizes RNA from an RNA template, a different process.

Question 8

Viruses are often described as being on the border of living and non-living. Which characteristic provides the strongest argument for classifying viruses as non-living?

  1. They possess genetic material that can be composed of either DNA or RNA.
  2. They are acellular and lack the metabolic machinery for energy production and protein synthesis. (correct answer)
  3. They demonstrate the ability to evolve through mutation and natural selection.
  4. They are typically much smaller in size than the smallest known prokaryotic cells.
Explanation: The core argument for viruses being non-living is their status as obligate intracellular parasites that are metabolically inert. They lack ribosomes, mitochondria (or equivalents), and enzymes for ATP synthesis, making them entirely dependent on a host cell's machinery. Evolution (C) is actually a characteristic of life. Having RNA or DNA (A) and being small (D) are simply descriptive features, not defining characteristics of life vs. non-life.

Question 9

The release of newly assembled virions can occur through cell lysis or budding. Which option correctly pairs a virus type with its release mechanism and a consequence for the host cell?

  1. Bacteriophage – Budding – The bacterial cell wall remains intact during release.
  2. Non-enveloped virus – Lysis – The host cell membrane is used to form new viral envelopes.
  3. Enveloped virus – Budding – The host cell can survive and continue to release virions. (correct answer)
  4. Retrovirus – Lysis – The provirus is excised from the host genome before cell death.
Explanation: Enveloped viruses are released by budding, where they acquire their envelope from the host cell's membrane. This process does not necessarily kill the host cell, which can continue to produce and release new virions over time. Bacteriophages cause lysis, which destroys the cell wall (A). Non-enveloped viruses are released by lysis, and envelopes are not formed in this process (B). Retroviruses are enveloped and are released by budding, not lysis (D).

Question 10

The terms 'prophage' and 'provirus' both describe a viral genome integrated into a host chromosome. What is the fundamental distinction between these two terms?

  1. A prophage refers to integrated phage DNA in a prokaryote; a provirus refers to integrated viral DNA in a eukaryote. (correct answer)
  2. A prophage is formed during the lytic cycle, while a provirus is formed during the lysogenic cycle.
  3. A prophage consists of RNA, while a provirus consists of DNA that has been reverse transcribed.
  4. A prophage can exit the chromosome to initiate lysis, whereas a provirus is always a permanent integration.
Explanation: The primary distinction is the type of host cell. A prophage is the genome of a bacteriophage integrated into a bacterial (prokaryotic) chromosome. A provirus is the genome of a virus (often a retrovirus) integrated into a eukaryotic chromosome. Both are associated with a lysogenic-like state (A). While proviruses are often formed from reverse transcription, not all are, and the host type is the defining difference (C). Permanence is not the key distinction, as some proviruses can also become active (D).

Question 11

Analysis of a viral genome reveals the following base composition: Adenine 18%, Guanine 35%, Cytosine 24%, Uracil 23%. What can be correctly deduced about this viral genome?

  1. It is a double-stranded DNA genome.
  2. It is a double-stranded RNA genome.
  3. It is a single-stranded DNA genome.
  4. It is a single-stranded RNA genome. (correct answer)
Explanation: First, the presence of Uracil (U) instead of Thymine (T) indicates the nucleic acid is RNA. This eliminates options A and C. Second, for a nucleic acid to be double-stranded, the percentage of complementary bases must be equal (A=U and G=C). In this sample, A (18%) is not equal to U (23%), and G (35%) is not equal to C (24%). Therefore, the genome must be single-stranded. The correct conclusion is that it is a single-stranded RNA (ssRNA) genome.

Question 12

The Baltimore classification system is a fundamental tool in virology. What is the primary criterion used by this system to classify viruses?

  1. The shape of the protein capsid, such as helical or icosahedral.
  2. The type of host organism the virus infects, such as bacteria, plants, or animals.
  3. The nature of the viral genome and its pathway to synthesizing messenger RNA (mRNA). (correct answer)
  4. The presence or absence of a lipid envelope derived from the host cell membrane.
Explanation: The Baltimore classification system groups viruses into seven classes based on their type of nucleic acid (dsDNA, ssDNA, dsRNA, ssRNA, etc.) and, crucially, the mechanism they use to produce mRNA, which is essential for synthesizing viral proteins. While capsid shape (A), host type (B), and presence of an envelope (D) are all important viral characteristics, the genome and its replication/transcription strategy form the basis of this fundamental classification scheme.

Question 13

Which molecular process is fundamental to the replication cycles of both a lytic bacteriophage and a eukaryotic retrovirus?

  1. The utilization of host cell ribosomes and tRNA to translate viral mRNA into proteins. (correct answer)
  2. The integration of viral nucleic acid into the host chromosome to form a stable provirus.
  3. The assembly of new virions around a double-stranded DNA genome.
  4. The destruction of the host cell by lysis to facilitate the release of new virions.
Explanation: Despite their vast differences, all viruses are obligate parasites that must hijack the host cell's protein synthesis machinery. Both a lytic phage and a retrovirus must produce viral proteins (e.g., capsid proteins, enzymes). To do this, they both rely on the host cell's ribosomes, tRNA, and amino acids to translate their mRNA. Integration (A) only happens in the retrovirus (and lysogenic phages). A retrovirus's genome is RNA, and it reverse-transcribes it to DNA, while a lytic phage may have a DNA or RNA genome (C). Lysis (D) is characteristic of the lytic phage, while the retrovirus is released by budding.

Question 14

A temperate bacteriophage is integrated as a prophage into the chromosome of its host bacterium. Which event is most likely to induce the transition from the lysogenic to the lytic cycle?

  1. The host bacterium undergoing binary fission to produce two identical daughter cells.
  2. A significant increase in the availability of nutrients for the host bacterium.
  3. Exposure of the host bacterium to UV radiation, causing damage to the host's DNA. (correct answer)
  4. The infection of the same host cell by a second, unrelated type of virus.
Explanation: The switch from the lysogenic to the lytic cycle, known as induction, is often triggered by host cell stress or DNA damage. UV radiation is a classic example of an inducer that activates the host's SOS repair system, which in turn can lead to the excision of the prophage and initiation of the lytic cycle. Binary fission (A) is the normal mode of reproduction during the lysogenic cycle. Increased nutrients (B) would likely favor continued lysogeny as the host is thriving. A second infection (D) is not a standard trigger for induction.

Question 15

What process occurs during the maturation stage of a viral lytic cycle?

  1. The viral nucleic acid is injected through the host cell membrane into the cytoplasm.
  2. The host cell undergoes lysis, releasing the completed virus particles to the environment.
  3. The viral genome is replicated many times using the host's enzymes and free nucleotides.
  4. Newly synthesized viral genomes and capsid proteins are assembled into new virions. (correct answer)
Explanation: The lytic cycle proceeds in a specific order: attachment, penetration, biosynthesis, maturation, and release. The maturation stage (also called assembly) is when the 'parts' of the virus—the nucleic acid genomes and protein capsids synthesized during biosynthesis—are put together to form complete, infectious virions. Injection (A) is penetration. Lysis (B) is release. Genome replication (C) is part of biosynthesis.

Question 16

Bacteriophages are being investigated as a potential therapy against bacterial infections. What is a primary advantage of phage therapy compared to traditional antibiotics?

  1. Phages can enter a lysogenic cycle, killing bacteria more gently than antibiotics.
  2. Phages have high host specificity, targeting pathogenic bacteria while sparing beneficial microbiota. (correct answer)
  3. Bacteria are incapable of evolving resistance to bacteriophages due to the phage's simple structure.
  4. Phages destroy bacteria by inhibiting their protein synthesis, a novel mechanism of action.
Explanation: A key advantage of phage therapy is the narrow host range of most phages. They are highly specific, often to a particular species or even strain of bacteria. This allows for targeted killing of a pathogen without disrupting the body's normal, beneficial bacteria, unlike broad-spectrum antibiotics. The therapeutic goal is the lytic cycle, not lysogeny (A). Bacteria can and do evolve resistance to phages (C). Phages kill by lysis, not by inhibiting protein synthesis (D).

Question 17

From an evolutionary standpoint, what is the main advantage for a temperate phage to maintain a lysogenic cycle?

  1. It allows the viral genome to be replicated passively within a healthy, dividing host population. (correct answer)
  2. It ensures a much more rapid production of new virions compared to the lytic cycle.
  3. It guarantees that the viral DNA is protected from all of the host cell's defense mechanisms.
  4. It facilitates the horizontal gene transfer of antibiotic resistance to other bacteria.
Explanation: The lysogenic cycle is an effective viral strategy when host cells are healthy and abundant. By integrating into the host chromosome, the virus ensures its own replication every time the host cell divides, without harming the host. This allows the virus population to increase exponentially along with its host. The lytic cycle is faster for virion production but destroys the host (B). Integration does not offer complete protection (C). While horizontal gene transfer can occur (D), the primary advantage for the virus is its own propagation.

Question 18

Some viruses possess a lipid envelope derived from a host cell membrane, while others are non-enveloped. What is a key functional difference that results from the presence of this envelope?

  1. The envelope contains the viral genetic material, while the inner capsid provides structural support.
  2. Enveloped viruses enter host cells via membrane fusion, whereas non-enveloped viruses often use direct penetration. (correct answer)
  3. The envelope enables the virus to perform metabolic processes independently of a host cell.
  4. Non-enveloped viruses are restricted to infecting prokaryotic cells due to the lack of a membrane.
Explanation: The lipid envelope is structurally similar to the host's plasma membrane, allowing it to fuse with the host membrane to release the capsid into the cytoplasm. This is a common entry mechanism for enveloped viruses. Non-enveloped viruses cannot fuse and must use other methods. The genetic material is inside the capsid (A). All viruses are metabolically inert without a host (C). Many non-enveloped viruses, such as adenoviruses, infect eukaryotes (D).

Question 19

During the biosynthesis stage of a lytic bacteriophage's life cycle, what is the primary role of the host cell's machinery?

  1. To integrate the viral DNA into the host chromosome to form a stable prophage.
  2. To synthesize viral proteins and replicate viral nucleic acid using host ribosomes and enzymes. (correct answer)
  3. To selectively replicate the host's own DNA while degrading the injected viral DNA as a defense.
  4. To package newly assembled virions into vesicles for release via exocytosis.
Explanation: The biosynthesis stage involves the hijacking of the host cell's metabolic machinery. The host's ribosomes are used for translation of viral mRNA into viral proteins (like capsid proteins and enzymes), and the host's enzymes (like DNA polymerase) and nucleotides are used to replicate the viral genome. Integration (A) is part of the lysogenic cycle. The host machinery is forced to replicate viral DNA, not destroy it (C). Packaging is part of maturation/assembly, and bacteriophages are released by lysis, not exocytosis (D).