- Which statement accurately distinguishes the lysogenic cycle from the lytic cycle in bacteriophages?
Answer: In the lysogenic cycle, the viral DNA is replicated along with the host chromosome, which remains intact.
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.
- During the biosynthesis stage of a lytic bacteriophage's life cycle, what is the primary role of the host cell's machinery?
Answer: To synthesize viral proteins and replicate viral nucleic acid using host ribosomes and enzymes.
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).
- The Baltimore classification system is a fundamental tool in virology. What is the primary criterion used by this system to classify viruses?
Answer: The nature of the viral genome and its pathway to synthesizing messenger RNA (mRNA).
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.
- 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?
Answer: The bacteria may express new genes from the prophage, such as those for toxin production.
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).
- 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?
Answer: Enveloped viruses enter host cells via membrane fusion, whereas non-enveloped viruses often use direct penetration.
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).
- How is the host cell's plasma membrane directly utilized during the life cycle of an enveloped virus like influenza?
Answer: It provides the lipids and embedded viral proteins for the envelope as new virions exit by budding.
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).
- 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?
Answer: They are acellular and lack the metabolic machinery for energy production and protein synthesis.
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.
- 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?
Answer: Exposure of the host bacterium to UV radiation, causing damage to the host's DNA.
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.
- 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?
Answer: A retrovirus, such as HIV, that uses reverse transcriptase.
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.
- 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?
Answer: The phage would be unable to specifically recognize and attach to the surface of a host bacterium.
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.
- The terms 'prophage' and 'provirus' both describe a viral genome integrated into a host chromosome. What is the fundamental distinction between these two terms?
Answer: A prophage refers to integrated phage DNA in a prokaryote; a provirus refers to integrated viral DNA in a eukaryote.
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).
- What process occurs during the maturation stage of a viral lytic cycle?
Answer: Newly synthesized viral genomes and capsid proteins are assembled into new virions.
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.
- Which molecular process is fundamental to the replication cycles of both a lytic bacteriophage and a eukaryotic retrovirus?
Answer: The utilization of host cell ribosomes and tRNA to translate viral mRNA into proteins.
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.
- 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?
Answer: It is a single-stranded RNA genome.
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.
- From an evolutionary standpoint, what is the main advantage for a temperate phage to maintain a lysogenic cycle?
Answer: It allows the viral genome to be replicated passively within a healthy, dividing host population.
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.
- 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?
Answer: The initial infection failed, and the phage's genetic material never entered the host cells.
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).
- 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?
Answer: The virus is a bacteriophage that infects a prokaryotic cell.
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).
- Bacteriophages are being investigated as a potential therapy against bacterial infections. What is a primary advantage of phage therapy compared to traditional antibiotics?
Answer: Phages have high host specificity, targeting pathogenic bacteria while sparing beneficial microbiota.
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).
- 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?
Answer: Enveloped virus – Budding – The host cell can survive and continue to release virions.
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).
