5 Most SAQ’s of Viruses Chapter in Inter 2nd Year Botany (TS/AP)

4 Marks

SAQ-1 : What is ICTV? How are viruses named?

Introduction

Understanding the classification and naming of viruses is crucial in virology studies. The International Committee on Taxonomy of Viruses (ICTV) plays a key role in this process, providing a structured system for the nomenclature of these infectious agents.

1. About ICTV: The International Committee on Taxonomy of Viruses (ICTV) oversees the classification and naming of viruses. It uses a hierarchical system composed of three levels: family, genus, and species.
2. Naming Conventions: In the ICTV’s system, family names of viruses end with the suffix “-Viridae”, while genus names end with “-virus”. Species names are often common English terms reflecting the virus’s characteristics. For example, the virus causing Acquired Immunodeficiency Syndrome (AIDS) in humans is classified as: Family – Retroviridae, Genus – Lentivirus, Species – Human immune deficiency virus (HIV).
3. Naming Based on Disease: Sometimes, viruses are named after the diseases they cause, such as Poliovirus, named after the disease it causes, Polio.

Summary

The International Committee on Taxonomy of Viruses (ICTV) is instrumental in classifying and naming viruses. This systematic approach aids in scientific research and global communication about viruses. It is important for students in virology, health professionals, and researchers to be familiar with these naming conventions and classification systems.

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SAQ-2 : Explain the chemical structure of viruses.

Introduction

Viruses, as tiny infectious agents, exhibit a diversity in shape and size, yet they uniformly comprise two primary components: a core and a capsid. Understanding these components is key in virology.

1. The Core and Genome: The core of a virus consists of nucleic acid, forming its genome or genetic material. The type of nucleic acid varies among viruses, including single-stranded DNA (ssDNA), double-stranded DNA (dsDNA), single-stranded RNA (ssRNA), or double-stranded RNA (dsRNA).
2. The Capsid and Capsomeres: The core is enveloped by a protein shell called the capsid, providing shape and protection. Composed of smaller protein units called capsomeres, the number of capsomeres varies, distinguishing different virus types.
3. Viral Nucleic Acid: The nucleic acid within a virus can be either circular or linear. The nucleic acid type also differs; for instance, many plant viruses have ssRNA, while many animal viruses contain dsDNA.
4. Multiplicity of Nucleic Acid Molecules: Most viruses contain a single molecule of nucleic acid, but some have multiple. An example is Human Immunodeficiency Virus (HIV), which has two identical RNA molecules.

Summary

In conclusion, the chemical structure of viruses is fundamental in virology, characterized by their core and capsid. Understanding the type and number of nucleic acid molecules, along with the capsid structure, simplifies the complex nature of viruses.

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SAQ-3 : Explain the structure of TMV.

Introduction

The Tobacco Mosaic Virus (TMV) is a well-known virus infecting tobacco plants, characterized by a unique ‘mosaic’-like pattern on tobacco leaves and a distinctive rod-like structure. We will explore its structural components in detail.

1. TMV: A ssRNA Virus: TMV is notable for containing single-stranded RNA (ssRNA), spirally coiled within the virus, comprising about 6,500 nucleotides—the basic units of RNA.
2. The Size and Shape of TMV: TMV has an elongated, rod-shaped form, measuring approximately 300 nanometers (nm) in length and 18 nm in diameter. Its molecular weight is around 39 million Daltons.
3. The Capsid and Capsomeres: TMV possesses a protective protein coat, the capsid, composed of 2,130 protein subunits known as capsomeres. These capsomeres form a helical pattern around a central hollow core that is 4 nm in diameter.
4. Amino Acids in Capsomeres: Each capsomere consists of 158 amino acids, the fundamental units of proteins, constructing the protective capsid of TMV.

Summary

In summary, the Tobacco Mosaic Virus is characterized by its rod-like shape, single-stranded RNA content, and the complex arrangement of its protein coat. Understanding these features is vital in studying the TMV’s impact on tobacco plants and its broader significance in virology.

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SAQ-4 : Explain the structure of T-even bacteriophages.

Introduction

Bacteriophages, specifically the T-even bacteriophages, are a unique class of viruses that infect bacteria. Their distinct structure makes them a subject of interest in scientific research.

What are Bacteriophages?

Bacteriophages, or ‘phages’, are viruses targeting bacteria. They have a tadpole shape with various parts, each with a specific function.

1. Head of T-even Bacteriophages: The most prominent feature of T-even bacteriophages is their hexagonal head, capped with a smaller hexagonal pyramid. This part contains the genetic material of the virus.
2. Tail Structure: The bacteriophage’s head extends into a tail structure, which is vital for injecting viral DNA into the host cell. The tail includes a tail sheath, base plate, pins, and tail fibers.
3. The Collar: The collar connects the head to the tail, acting as a bridge between these two major components.
4. Tail Plate and Tail Fibers: At the end of the tail lies a hexagonal tail plate, equipped with six tail pins and tail fibers. The tail fibers are crucial as they enable the virus to attach to host cells, initiating infection.

Summary

In summary, T-even bacteriophages exhibit a complex and distinctive structure, characterized by a hexagonal head, a multi-component tail, and a collar connecting these parts. Each element plays a specific role in the virus’s life cycle, from attaching to host cells to delivering viral DNA. Understanding this structure sheds light on their functioning and significance in bacterial infections.

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SAQ-5 : Explain the lytic cycle with reference to certain viruses.

Introduction

The lytic cycle is a significant method by which viruses reproduce and multiply, leading to the destruction of the host cell and the release of new viruses. This cycle is particularly associated with virulent phages or viruses.

What is the Lytic Cycle?

The lytic cycle is a primary viral reproduction cycle where viruses destroy the host cell to produce and release new virus particles.

1. Attachment Stage: The cycle begins with the virus attaching to a host cell. The virus’s tail fibers adhere to specific receptor sites on the host cell’s wall, a pivotal step for initiating infection.
2. Penetration Stage: Here, the virus injects its genetic material (DNA or RNA) into the host cell. The virus acts like a syringe, inserting its tail core through the host’s cell wall and plasma membrane. The empty virus shell outside is termed a ‘ghost.’
3. Biosynthesis Stage: Inside the host cell, viral DNA leads to the production of viral DNA, enzymes, and protein shells (capsids), utilizing the host’s cellular machinery. At this stage, complete virus particles are not yet formed within the host cell.
4. Maturation Stage: During maturation, newly synthesized viral components assemble into complete virus particles or virions. The ‘eclipse period’ refers to the time between virus infection and the appearance of mature virus particles inside the host cell.
5. Release Stage: The final stage involves the destruction (lysis) of the host cell. The host’s cell wall is broken down by an enzyme called lysozyme, allowing the new virus particles to escape and infect other cells.

Summary

The lytic cycle is a critical viral replication process followed by virulent phages, encompassing stages of attachment, penetration, biosynthesis, maturation, and release. Each stage is essential in the virus’s life cycle and its spread to other cells. Understanding this cycle is vital for comprehending viral multiplication and informing the development of antiviral treatments and therapies.