1. Give the classification of viruses based upon capsid and genomes.
Answer: Viruses are classified based on the type of their capsid (protein coat) and the nature of their genome (nucleic acid). This classification is crucial for understanding their structure, replication, and pathogenicity.
Classification based on Capsid:
Helical Capsids: The capsid is a hollow cylinder with the protein subunits arranged in a helical pattern around the nucleic acid. Example: Tobacco Mosaic Virus (TMV). Icosahedral Capsids: The capsid is a regular polyhedron with 20 triangular faces. This is the most efficient way to enclose a nucleic acid with a small number of protein subunits. Example: Adenovirus. Complex Capsids: These capsids have a combination of helical and icosahedral symmetry, or they have additional structures. Example: Bacteriophages (with a head and tail). Enveloped Viruses: These viruses have an outer lipid bilayer envelope derived from the host cell membrane, with viral glycoproteins embedded in it. The capsid is inside the envelope. Example: HIV, Influenza virus.
Classification based on Genome:
DNA Viruses: Contain DNA as their genetic material. They can be single-stranded (ssDNA) or double-stranded (dsDNA). Examples: Herpesvirus (dsDNA), Parvovirus (ssDNA). RNA Viruses: Contain RNA as their genetic material. They can be single-stranded (ssRNA) or double-stranded (dsRNA). Examples: Influenza virus (ssRNA), Rotavirus (dsRNA). Retroviruses: A special class of RNA viruses that use reverse transcriptase to convert their RNA genome into DNA, which then integrates into the host genome. Example: HIV.
The classification based on capsid and genome is essential for viral taxonomy and for developing antiviral drugs and vaccines. It also helps in predicting the replication strategy and pathogenicity of a virus.
2. Describe the general structure of a virus.
Answer: Viruses are acellular, obligate intracellular parasites with a relatively simple structure. They consist of a nucleic acid core surrounded by a protein coat, and sometimes an outer envelope.
1. Nucleic Acid Core (Genome):
The genetic material of a virus can be either DNA or RNA, but not both. The nucleic acid can be single-stranded (ss) or double-stranded (ds). It can be linear or circular. The genome carries the genetic information necessary for viral replication and assembly. It is typically very small, encoding only a few proteins.
2. Capsid (Protein Coat):
The capsid is a protein shell that surrounds and protects the nucleic acid. It is composed of repeating protein subunits called capsomeres. The arrangement of capsomeres determines the shape of the virus (helical, icosahedral, or complex). The capsid also aids in the attachment of the virus to specific receptors on the host cell.
3. Envelope (in some viruses):
Some viruses have an outer lipid bilayer envelope derived from the host cell membrane. The envelope is studded with viral glycoproteins, which are important for host cell recognition and entry. Enveloped viruses are generally more sensitive to environmental factors (e.g., detergents, heat) than non-enveloped viruses.
4. Other Components:
Some viruses carry enzymes within their capsid, such as reverse transcriptase (in retroviruses) or lysozyme (in
bacteriophages). These enzymes assist in the early stages of infection.
In summary, a virus is a simple yet highly effective infectious agent consisting of a nucleic acid genome protected by a protein capsid, and sometimes an envelope. Its structure is optimized for delivery of its genome into a host cell.
3. Describe the structure of bacteriophage with diagram.
Answer: (Diagram Description: A bacteriophage with a hexagonal head, a cylindrical tail, a base plate, and tail fibers.)
A bacteriophage, or simply phage, is a virus that infects bacteria. One of the most well-studied bacteriophages is the T4 phage, which has a complex structure.
1. Head (Capsid):
The head is a hexagonal, icosahedral structure that contains the viral DNA. It is composed of protein subunits and protects the genetic material. The DNA is tightly packed inside the head.
2. Collar and Sheath:
Below the head is a collar, followed by a contractile sheath. The sheath is a cylindrical structure made of protein subunits. It can contract during infection to drive the tail tube through the bacterial cell wall.
3. Tail Tube:
Inside the sheath is a hollow tail tube.
This tube serves as a channel for the viral DNA to pass from the head into the host cell.
4. Base Plate:
At the bottom of the tail is a base plate, a hexagonal structure. The base plate has spikes and tail fibers attached to it.
5. Tail Fibers:
Long, thin tail fibers extend from the base plate. These fibers recognize and bind to specific receptors on the surface of the bacterial host cell.
Function:
The tail fibers attach to the host cell. The base plate anchors the phage. The sheath contracts, driving the tail tube through the cell wall. The viral DNA is injected into the host cell through the tail tube.
The bacteriophage structure is highly adapted for its function: attaching to a host, penetrating the cell wall, and delivering its genetic material. This makes it an efficient killer of bacteria.
4. Describe the structure of human immunodeficiency virus with diagram.
Answer: (Diagram Description: A spherical virus with an outer envelope, a conical core, and two copies of RNA.)
The Human Immunodeficiency Virus (HIV) is a retrovirus that causes AIDS. It has a spherical structure with several distinct components.
1. Envelope:
The outermost layer is a lipid bilayer envelope derived from the host cell membrane. Embedded in the envelope are viral glycoproteins: gp120 and gp41. gp120 is the surface glycoprotein that binds to the CD4 receptor on host cells. gp41 is the transmembrane glycoprotein that mediates fusion of the viral envelope with the host cell membrane.
2. Matrix and Capsid:
Inside the envelope is a matrix protein (p17) that lines the inner surface of the envelope. The core of the virus is a cone-shaped capsid made of the protein p24. The capsid protects the viral genome and enzymes.
3. Genome and Enzymes:
Inside the capsid are two identical copies of single-stranded RNA (ssRNA). The virus also carries several enzymes:
Reverse Transcriptase: Converts viral RNA into DNA. Integrase: Integrates the viral DNA into the host genome. Protease: Cleaves viral polyproteins into functional proteins.
Function:
HIV binds to CD4 receptors on helper T cells via gp120. The envelope fuses with the cell membrane, releasing the capsid into the cell. The RNA is reverse-transcribed into DNA, which integrates into the host genome. The virus replicates using the host machinery, eventually killing the host cell.
The structure of HIV is well adapted for its life cycle: binding to specific immune cells, entering them, and integrating its genome for persistent infection.
5. Describe the Lytic and Lysogenic life cycles of a virus.
Answer: Bacteriophages can undergo two types of life cycles: the lytic cycle and the lysogenic cycle.
1. Lytic Cycle:
Attachment: The bacteriophage attaches to specific receptors on the bacterial cell surface. Penetration: The viral DNA is injected into the host cell. Replication: The viral DNA takes over the host's machinery to synthesize viral components. Host DNA is degraded. Assembly: New viral particles are assembled from the synthesized components. Lysis: The host cell bursts (lyses), releasing new bacteriophages to infect other cells. This cycle is rapid and results in the death of the host cell.
2. Lysogenic Cycle:
Attachment and Penetration: Same as the lytic cycle. Integration: The viral DNA integrates into the host chromosome, forming a prophage. Replication: The prophage is replicated along with the host DNA during cell division. The host cell survives and continues to grow and divide, passing the prophage to daughter cells. Induction: Under certain conditions (e.g., stress, UV light), the prophage can excise from the host chromosome and enter the lytic cycle, leading to the production of new viruses and lysis of the host cell.
In summary, the lytic cycle is a virulent cycle that kills the host cell, while the lysogenic cycle is a temperate cycle that allows the host cell to survive and propagate the viral genome. The choice between the two cycles depends on the virus and environmental conditions.
6. Describe the usage of bacteriophage in genetic engineering.
Answer: Bacteriophages have several important applications in genetic engineering and biotechnology.
Cloning Vectors: Bacteriophages, such as lambda phage, are used as cloning vectors to introduce foreign DNA into bacterial cells. The foreign DNA is inserted into the phage genome, which is then packaged into viral particles and used to infect bacteria. This allows for the amplification and study of specific genes. Phage Display: This technique uses bacteriophages to display foreign proteins on their surface. It is used to study protein-protein interactions, identify novel binding partners, and engineer antibodies. Vaccine Development: Phage display can be used to develop vaccines by presenting antigens on the phage surface. Gene Therapy: Modified bacteriophages are being explored as delivery vehicles for gene therapy, although their use in humans is limited by immune responses. Antibacterial Agents: Phage therapy uses bacteriophages to kill antibiotic-resistant bacteria. It is a promising alternative to traditional antibiotics. Diagnostics: Bacteriophages are used in the detection of specific bacteria.
In summary, bacteriophages are versatile tools in genetic engineering, with applications ranging from cloning and protein engineering to therapeutics and diagnostics.
7. Explain the life cycle of HIV.
Answer: The life cycle of HIV involves several key steps:
1. Attachment and Entry:
HIV binds to the CD4 receptor on helper T cells via its gp120 glycoprotein. It also binds to a co-receptor (CCR5 or CXCR4). The viral envelope fuses with the host cell membrane, releasing the viral capsid into the cytoplasm.
2. Reverse Transcription:
Inside the capsid, the enzyme reverse transcriptase converts the viral RNA into complementary DNA (cDNA). The RNA strand is degraded, and a second DNA strand is synthesized to form double-stranded viral DNA.
3. Integration:
The viral DNA enters the nucleus and is integrated into the host cell's DNA by the enzyme integrase. The integrated viral DNA is called a provirus.
4. Replication and Transcription:
The provirus is transcribed into viral RNA by the host's RNA polymerase. Some of this RNA serves as the genome for new viruses, while other parts are spliced and translated into viral proteins.
5. Assembly and Budding:
Viral proteins and RNA assemble near the host cell membrane. New viral particles bud off from the host cell, acquiring an envelope from the host membrane.
6. Maturation:
The viral protease cleaves viral polyproteins into functional proteins, making the virus mature and infectious.
HIV primarily infects and destroys helper T cells, leading to immunodeficiency. The life cycle provides several targets for antiretroviral drugs.
8. Describe the treatments available for AIDS.
Answer: There is no cure for AIDS, but Antiretroviral Therapy (ART) can effectively manage the disease and prolong life. ART involves a
combination of drugs that target different stages of the HIV life cycle.
Classes of Antiretroviral Drugs:
Nucleoside Reverse Transcriptase Inhibitors (NRTIs): These drugs mimic normal nucleotides and block reverse transcription (e.g., zidovudine, lamivudine). Non-Nucleoside Reverse Transcriptase Inhibitors (NNRTIs): These drugs bind directly to reverse transcriptase and inhibit its activity (e.g., efavirenz, nevirapine). Protease Inhibitors (PIs): These drugs block the viral protease, preventing the maturation of new viral particles (e.g., ritonavir, lopinavir). Integrase Inhibitors: These drugs block the integration of viral DNA into the host genome (e.g., raltegravir, dolutegravir). Fusion Inhibitors: These drugs prevent the fusion of the viral envelope with the host cell membrane (e.g., enfuvirtide). Entry Inhibitors: These drugs block the binding of HIV to co-receptors (e.g., maraviroc).
Principles of ART:
ART is typically a combination of three or more drugs from different classes (Highly Active Antiretroviral Therapy, HAART). This combination prevents the virus from developing resistance. ART must be taken consistently and for life. ART reduces the viral load to undetectable levels, preserving immune function and preventing transmission.
In addition to ART, patients receive prophylaxis for opportunistic infections and supportive care. Research into new drugs and vaccines is ongoing.
9. Describe the causative agent, symptoms, transmission, treatment and prevention of the following diseases: (a) Hepatitis (b) Herpes
(c) Polio (d) Cotton leaf curl disease
Answer:
(a) Hepatitis:
Causative Agent: Hepatitis viruses (A, B, C, D, E). Hepatitis A and E are RNA viruses; Hepatitis B is a DNA virus; Hepatitis C is an RNA virus; Hepatitis D is a defective RNA virus. Symptoms: Jaundice (yellowing of skin and eyes), fatigue, nausea, vomiting, abdominal pain, dark urine, and loss of appetite. Transmission: Hepatitis A and E are transmitted via the fecal-oral route (contaminated food/water). Hepatitis B, C, and D are transmitted via blood and bodily fluids (needles, sex, childbirth). Treatment: Antiviral drugs for chronic Hepatitis B and C. Supportive care for acute infections. Hepatitis A and B can be prevented by vaccines. Prevention: Vaccination (Hepatitis A and B), safe sex, avoiding sharing needles, screening blood products, and good hygiene.
(b) Herpes:
Causative Agent: Herpes simplex virus (HSV-1 and HSV-2). Symptoms: Painful blisters or sores on the mouth, lips, genitals, or skin. Fever, swollen lymph nodes, and muscle aches may occur during initial infection. Transmission: Direct contact with infected skin or bodily fluids (kissing, sexual contact). Treatment: Antiviral medications (e.g., acyclovir, valacyclovir) can reduce symptoms and frequency of outbreaks but cannot cure the infection. Prevention: Avoiding contact with sores, using condoms, and avoiding sharing utensils or towels during outbreaks.
(c) Polio:
Causative Agent: Poliovirus (an RNA virus). Symptoms: Most infections are asymptomatic. Mild cases cause fever, sore throat, headache, and nausea. Severe cases cause meningitis and paralysis. Transmission: Fecal-oral route (contaminated water and food). Treatment: No cure. Treatment is supportive (pain relief, physical therapy). Prevention: Vaccination (IPV and OPV) is highly effective. Global vaccination campaigns have nearly eradicated polio.
(d) Cotton Leaf Curl Disease (CLCuD):
Causative Agent: Cotton leaf curl virus (a begomovirus, transmitted by whiteflies). Symptoms: Upward curling of leaves, thickening of veins, formation of leaf-like enations on the underside of leaves, stunted growth, and reduced cotton yield. Transmission: By whiteflies (Bemisia tabaci). Treatment: No direct cure for the viral infection. Management focuses on controlling the whitefly vector with insecticides and developing resistant cotton varieties. Prevention: Use of resistant varieties, vector control, and removal of infected plants.
