48. In viruses, capsids are made up of:
(A) Proteins
(B) Nucleic acids
(C) Lipids
(D) Sterols
Viral Capsids: Structure, Composition, Functions, and Importance in Virology
Introduction
Viruses are acellular infectious agents that exist at the boundary between living and non-living organisms. Unlike bacteria, fungi, or protozoa, viruses do not possess cellular organelles, cytoplasm, or metabolic machinery. Instead, they consist primarily of genetic material enclosed within a protective protein shell known as the capsid. Some viruses possess an additional lipid envelope surrounding the capsid, whereas others are non-enveloped and consist only of the nucleic acid and capsid.
The viral capsid performs several essential biological functions. It protects the viral genome from environmental damage, facilitates attachment to host cells, assists in delivering the viral genome into host cells, and determines the characteristic shape of the virus. Capsids are constructed from repeating protein subunits called capsomeres, which self-assemble into highly organized structures. Depending on the virus, the capsid may exhibit icosahedral, helical, or complex symmetry.
Correct Answer
Correct Option: (A) Proteins
Detailed Explanation
A capsid is the protein coat that surrounds and protects the viral nucleic acid. It is composed entirely of repeating protein subunits called capsomeres. These proteins assemble spontaneously into highly symmetrical structures, allowing the virus to efficiently package its genetic material while using a relatively small number of genes.
The capsid serves multiple functions. It protects viral DNA or RNA from physical damage, chemical degradation, and enzymatic digestion by host nucleases. In non-enveloped viruses, capsid proteins also interact directly with receptors on host cells to initiate infection. Furthermore, the antigenic properties of capsid proteins often stimulate immune responses, making them important targets for vaccines and diagnostic tests.
Although some viruses possess a lipid envelope derived from the host cell membrane, this envelope lies outside the capsid. The capsid itself remains exclusively proteinaceous.
Explanation of Each Option
Option (A): Proteins
This option is correct. Viral capsids are composed of structural proteins called capsomeres. These proteins form a rigid protective shell around the viral genome and determine the morphology of the virus.
Option (B): Nucleic Acids
This option is incorrect. Nucleic acids constitute the viral genome, which may be DNA or RNA. They are enclosed within the capsid but do not form the capsid itself.
Option (C): Lipids
This option is incorrect. Lipids are present only in the envelopes of enveloped viruses. They are absent from the capsid and are acquired from the host cell membrane during viral budding.
Option (D): Sterols
This option is incorrect. Sterols such as cholesterol may be present within the lipid envelope of certain enveloped viruses because they originate from host cell membranes. However, sterols are not structural components of the viral capsid.
Why Option (A) is Correct
The capsid is a highly organized protein shell made of repeating capsomere proteins. Its primary role is to protect the viral genome and facilitate infection of host cells. Therefore, proteins are the fundamental structural components of viral capsids.
Comparison of All Options
| Option | Component | Forms the Viral Capsid? |
|---|---|---|
| A | Proteins | Yes |
| B | Nucleic acids | No |
| C | Lipids | No |
| D | Sterols | No |
Major Components of a Virus
| Component | Composition | Function |
|---|---|---|
| Genome | DNA or RNA | Stores genetic information |
| Capsid | Proteins | Protects the genome |
| Capsomeres | Protein subunits | Build the capsid |
| Envelope (if present) | Lipids and glycoproteins | Host recognition and membrane fusion |
Types of Viral Capsid Symmetry
| Capsid Type | Characteristics | Example |
|---|---|---|
| Icosahedral | 20 triangular faces and 12 vertices | Adenovirus, Poliovirus |
| Helical | Protein subunits arranged around nucleic acid | Tobacco Mosaic Virus, Rabies Virus |
| Complex | Neither purely helical nor icosahedral | Bacteriophage T4, Poxvirus |
Functions of the Viral Capsid
| Function | Importance |
|---|---|
| Protection | Shields viral genome from damage |
| Attachment | Helps viruses bind to host cell receptors |
| Genome Delivery | Assists entry of viral nucleic acid into host cells |
| Structural Support | Maintains viral shape and stability |
| Antigenicity | Acts as target for immune responses and vaccines |
Biological Significance
The capsid is essential for viral survival because it protects the genetic material outside the host cell and enables successful transmission between hosts. Capsid proteins determine viral morphology, host specificity, tissue tropism, and immune recognition. They are widely used in vaccine development, diagnostic assays, and antiviral research because they contain highly immunogenic epitopes recognized by the immune system.
Final Answer
The viral capsid is a protective protein shell composed of repeating structural proteins known as capsomeres. It encloses and protects the viral nucleic acid.
Correct Option: (A) Proteins
