1. What is the structure of a virus? Describe its components.
Answer:
A virus has a simple structure consisting of two main components: the capsid and the genome.
- Capsid: This is a protein coat that surrounds and protects the viral genetic material. The capsid is made up of protein subunits called capsomers and can have different shapes like helical or icosahedral. Some viruses also have a lipid bilayer membrane derived from the host, called the envelope.
- Genome: The genetic material of a virus can be either DNA or RNA (but not both), and it can be single-stranded or double-stranded. The genome carries the instructions for viral replication and protein synthesis. In some viruses, the genome is encapsulated in the capsid; in others, it is enclosed within the envelope.
2. Explain the difference between DNA and RNA viruses in terms of structure and replication.
Answer:
- DNA viruses: These viruses carry DNA as their genetic material. The DNA can be double-stranded (dsDNA) or single-stranded (ssDNA). The replication of the DNA genome occurs in the host cell’s nucleus, where it is transcribed to produce messenger RNA (mRNA) for protein synthesis. Examples of DNA viruses include the herpesvirus and adenovirus.
- RNA viruses: These viruses have RNA as their genetic material, and it can be either single-stranded (ssRNA) or double-stranded (dsRNA). RNA viruses generally replicate in the host cell’s cytoplasm. Some RNA viruses, like retroviruses, use an enzyme called reverse transcriptase to convert their RNA into DNA before integration into the host genome. Examples of RNA viruses include influenza and HIV.
3. What is the function of the viral capsid and the viral envelope?
Answer:
- Capsid: The capsid is a protective protein shell that surrounds the viral genome. It helps protect the viral genome from physical damage and enzymatic degradation. The capsid also facilitates the virus’s attachment to host cells by interacting with specific receptors on the host cell surface.
- Envelope: The viral envelope is a lipid bilayer that surrounds some viruses. It is derived from the host cell’s membrane during viral budding. The envelope contains viral glycoproteins, which are responsible for attaching the virus to host cells. The presence of the envelope makes some viruses more fragile, as they are sensitive to environmental factors like temperature and pH.
4. Describe the life cycle of a virus with reference to the lytic cycle.
Answer:
The lytic cycle is a viral life cycle in which the virus replicates inside the host cell, leading to the destruction (lysis) of the host cell. The stages of the lytic cycle are:
- Attachment: The virus attaches to a specific receptor on the surface of the host cell.
- Penetration: The viral genome enters the host cell, either by fusion with the host membrane (in enveloped viruses) or by direct injection into the cytoplasm (in non-enveloped viruses).
- Replication and Transcription: The viral genome is replicated, and viral RNA is transcribed to produce viral proteins.
- Assembly: New viral genomes and proteins are assembled into new virions.
- Release: The host cell is lysed, releasing newly formed viruses that can infect other cells.
5. What is the lysogenic cycle, and how does it differ from the lytic cycle?
Answer:
The lysogenic cycle is a viral replication cycle in which the viral genome integrates into the host cell’s DNA and remains dormant for an extended period. In contrast to the lytic cycle, where new viruses are rapidly produced and the host cell is destroyed, the lysogenic cycle involves the following stages:
- Attachment and Penetration: Similar to the lytic cycle, the virus attaches to the host cell and injects its genetic material.
- Integration: The viral DNA integrates into the host cell’s genome, forming a provirus.
- Replication: As the host cell divides, it replicates the viral genome as part of its own, passing the provirus to daughter cells.
- Activation: Under certain conditions (like stress or environmental changes), the provirus can be activated and enter the lytic cycle, where the virus starts producing new viral particles.
The key difference is that in the lysogenic cycle, the virus does not immediately harm the host cell.
6. What is a retrovirus, and how does it replicate within a host cell?
Answer:
A retrovirus is a type of RNA virus that replicates its RNA genome by converting it into DNA using an enzyme called reverse transcriptase. The steps involved in retroviral replication are:
- Attachment: The retrovirus attaches to a host cell receptor and enters the host cell.
- Reverse Transcription: The RNA genome is converted into DNA by reverse transcriptase.
- Integration: The newly synthesized DNA is integrated into the host cell’s genome by the enzyme integrase, forming a provirus.
- Replication and Transcription: The host cell’s machinery replicates the proviral DNA and transcribes it into mRNA.
- Assembly and Budding: New viral proteins are synthesized and assembled into new virions, which then bud from the host cell membrane, acquiring an envelope in the process.
Retroviruses, such as HIV, are known for causing chronic infections because of their ability to integrate into the host genome.
7. Explain the concept of viral host specificity.
Answer:
Viral host specificity refers to the ability of a virus to infect certain types of host cells or organisms. This specificity is determined by the interaction between viral proteins (often found in the capsid or envelope) and specific receptors present on the surface of the host cell. The receptors act as entry points for the virus. For example, HIV specifically infects CD4+ T cells in humans, while bacteriophages can only infect bacteria. The host range of a virus can vary widely, from infecting a single species (narrow host range) to infecting a wide variety of species (broad host range).
8. How do viruses interact with the immune system of the host?
Answer:
Viruses interact with the host’s immune system in various ways:
- Immune evasion: Many viruses can evade the immune response by rapidly mutating their surface proteins (e.g., influenza). This helps them escape detection by the host’s immune system.
- Immune suppression: Some viruses, like HIV, can directly suppress the immune system, weakening the host’s defense mechanisms and making them more susceptible to opportunistic infections.
- Host response: When a virus infects a host, the immune system may recognize the virus as foreign and mount an immune response. This involves the activation of T-cells, which kill infected cells, and B-cells, which produce antibodies that neutralize the virus.
9. What is viral latency, and which viruses exhibit this phenomenon?
Answer:
Viral latency refers to a state where a virus remains dormant or inactive within the host for a prolonged period. During latency, the viral genome integrates into the host’s DNA, and the virus does not replicate actively or produce new virions. This state can be reactivated under certain conditions, leading to active viral replication. Herpesviruses (e.g., Herpes simplex virus) and HIV are examples of viruses that can establish latency. These viruses can remain latent in the body for years before being reactivated, often under stress or immunosuppression.
10. Describe the differences between enveloped and non-enveloped viruses.
Answer:
- Enveloped viruses: These viruses have an additional lipid bilayer membrane derived from the host cell during viral replication. The envelope contains viral glycoproteins, which are essential for the virus to attach to and enter host cells. Examples include HIV, influenza virus, and herpesvirus. Enveloped viruses are more sensitive to environmental conditions like heat, drying, and detergents.
- Non-enveloped viruses: These viruses lack an envelope and consist only of the capsid and the viral genome. Non-enveloped viruses are generally more resistant to environmental stressors and disinfectants. Examples include adenovirus and poliovirus.
11. What are the challenges faced in developing antiviral drugs?
Answer:
Developing antiviral drugs is challenging due to:
- Viral diversity: The vast variety of viruses, each with unique characteristics and replication methods, makes it difficult to create broad-spectrum antiviral drugs.
- Mutation rate: Viruses, especially RNA viruses, mutate rapidly, making it hard for drugs to remain effective over time. For instance, influenza and HIV evolve quickly, leading to drug resistance.
- Host-cell dependency: Viruses rely on the host’s cellular machinery to replicate, making it difficult to target the virus without damaging the host’s cells.
- Latent infections: Many viruses can enter a dormant state (e.g., herpesviruses), making them difficult to target during latency.
12. What role do viral proteins play in the virus-host interaction?
Answer:
Viral proteins are critical in the virus-host interaction as they facilitate various stages of the viral life cycle:
- Attachment: Viral surface proteins bind to specific host cell receptors, allowing the virus to enter the cell.
- Replication and transcription: Viral enzymes such as RNA polymerase (in RNA viruses) or reverse transcriptase (in retroviruses) are necessary for genome replication.
- Immune evasion: Some viral proteins can inhibit the host’s immune response, such as preventing the activation of interferons or blocking antigen presentation to immune cells.
- Assembly: Viral proteins help in assembling new virions from the replicated genome and synthesized proteins.
13. How do viruses adapt to new host environments?
Answer:
Viruses adapt to new host environments by:
- Mutations: Rapid mutations allow viruses to alter their surface proteins, making them less recognizable to the host immune system.
- Recombination and reassortment: Some viruses, particularly RNA viruses, undergo genetic reassortment when infecting a host with multiple strains, resulting in new viral variants.
- Host shift: Viruses can jump to new species (host shift), often through changes in the virus’s ability to recognize new host receptors. This is a key factor in the emergence of new viral diseases (e.g., SARS-CoV-2).
14. What are viral vectors, and how are they used in gene therapy?
Answer:
Viral vectors are modified viruses used in gene therapy to deliver therapeutic genes to a patient’s cells. The viruses are engineered to be non-pathogenic, and they transport the desired genetic material into target cells, which can then express the gene for therapeutic purposes. Common viral vectors include adenoviruses, lentiviruses (a type of retrovirus), and adeno-associated viruses (AAVs). These vectors have been used in gene therapy for conditions like cystic fibrosis and hemophilia.
15. Explain the concept of viral recombination and reassortment.
Answer:
- Viral recombination: This occurs when two genetically distinct viral genomes exchange genetic material during co-infection of a host cell. This can result in new viral strains with genetic characteristics of both parent viruses.
- Viral reassortment: This occurs specifically in segmented RNA viruses (e.g., influenza), where different viral strains exchange entire segments of RNA, leading to new virus strains. Reassortment can cause significant changes in a virus’s ability to infect new species or evade immune responses.
16. What are prions, and how are they different from viruses?
Answer:
Prions are infectious agents composed entirely of protein, lacking nucleic acids (DNA or RNA). Unlike viruses, prions do not have a genome and replicate by inducing normal proteins in the host to adopt abnormal, disease-causing conformations. Prions cause neurodegenerative diseases like mad cow disease and Creutzfeldt-Jakob disease. Viruses, in contrast, contain genetic material (either DNA or RNA) and reproduce by hijacking the host’s cellular machinery.
17. How do viruses contribute to the evolution of their hosts?
Answer:
Viruses can influence the evolution of their hosts through genetic transfer. Viruses can introduce new genes into the host genome during infection, a process known as horizontal gene transfer. This genetic variation can provide the host with new traits, such as resistance to viruses or other pathogens. Viruses also drive selective pressure on host populations, encouraging the evolution of immune defenses. For example, the frequent mutations in HIV challenge the human immune system, leading to the evolution of better immune responses.
18. Describe the role of viral infections in cancer development.
Answer:
Certain viruses, known as oncoviruses, are linked to the development of cancer. These viruses can alter host cell DNA, leading to uncontrolled cell growth. They do this by integrating their viral genome into the host cell’s DNA and disrupting the regulation of cell cycle genes. Examples of oncogenic viruses include Human Papillomavirus (HPV), which is linked to cervical cancer, and Hepatitis B and C viruses, which are associated with liver cancer.
19. What are the ethical considerations related to viral research?
Answer:
Ethical considerations in viral research include:
- Biological safety: Handling dangerous viruses, such as those that cause pandemics, requires strict protocols to prevent accidental releases.
- Gene editing: Modifying viruses for use in gene therapy or vaccines raises concerns about potential misuse or unintended consequences.
- Vaccination: The development of vaccines using viral vectors can raise concerns about long-term safety and efficacy, especially in vulnerable populations.
- Privacy: The collection of genetic data related to viral infections can lead to privacy concerns regarding personal health information.
20. How do viruses cause disease, and what factors influence the severity of viral infections?
Answer:
Viruses cause disease by invading and damaging host cells, disrupting normal cellular functions. The severity of the infection depends on factors such as:
- Viral factors: The type of virus, its replication rate, and the presence of virulence factors (e.g., toxins or immune evasion proteins).
- Host factors: The age, immune status, and genetic predisposition of the host can influence the severity of the disease. For example, individuals with weakened immune systems are more susceptible to severe infections.
- Environmental factors: Conditions such as stress, malnutrition, or co-infection with other pathogens can exacerbate viral diseases.
