5.1.3 General Structure of A Virus

Introduction

Viruses have a very simple structure compared to cells. A complete viral particle is called a virion. It is made of a core and a coat. The core contains the genetic material, while the coat protects the core and helps the virus attach to host cells. Despite their simplicity, viruses are highly efficient at infecting host cells and causing disease.

Definition

Virion: A complete viral particle is called a virion.

In simple words: A virion is a fully assembled, infectious virus particle.

Core Concept

• A complete viral particle is called a virion.
• Primarily, it can be divided into two parts:
• The core consists of viral genome and various proteins (enzymes).
• The coat consists of capsid and envelope.
• The capsid is the protective coat of protein surrounding the core.
• The capsid is composed of identical repeating subunits called capsomeres.
• A few viruses have an additional lipoprotein envelope around the capsid.

Detailed Explanation

General Structure of a Virus

Diagrammatic Representation:

``` Virus Particle ├── Coat │ ├── Capsid protein │ └── Envelope (not found in all viruses) └── Core ├── DNA or RNA └── Various proteins (enzymes) ```

1. Core

Definition: The core is the inner part of the virion which consists of viral genome and various proteins (enzymes).

Key Points:

• Genome:
• The genetic material of the virus.
• It is either DNA or RNA.
• It may be single-stranded or double-stranded.
• It may be linear or circular.
• It may be segmented or non-segmented.

• Core Proteins (Enzymes):
• Include one or more enzymes that facilitate the virus in its mode of action within the host body.
• Examples:
• Reverse transcriptase: Converts single-stranded RNA genome into double-stranded DNA genome.
• RNA-dependent RNA polymerase: Replicates RNA genome.
• Integrase: Integrates viral DNA into host genome.
• Protease: Cleaves viral proteins.

• All single-stranded RNA viruses have the enzyme to convert single-stranded RNA genome into double-stranded RNA genome.
• Retroviruses and hepatitis B virus contain the enzyme reverse transcriptase to convert single-stranded RNA genome into double-stranded DNA genome.

2. Coat

Definition: The coat is the outer covering of the viral particle which consists of capsid and envelope.

Key Points:

• Capsid
• Envelope (not found in all viruses)

2.1 Capsid

Definition: The capsid is the protective coat of protein surrounding the core.

Key Points:

• Composed of identical repeating subunits called capsomeres.
• The number of capsomeres is specific to a particular kind of virus.
• Examples:
• Herpes virus has 162 capsomeres in its capsid.
• Adenovirus that causes the common cold contains 252 capsomeres in its capsid.

Forms of Symmetry in Virus Capsid:

Functions of Capsid:

• Protects the viral genome from nucleases and other degradative enzymes.
• Helps in attachment to host cell receptors.
• Determines the shape of the virus.
• Provides specificity for host range.

2.2 Envelope

Definition: A few viruses have an additional lipoprotein envelope around the capsid.

Key Points:

• Not found in all viruses.
• Derived from the cell surface membrane of the host.
• Contains virally encoded proteins.
• The viral envelope is often covered with glycoprotein spikes that help them to recognize the host cell.

Functions of Envelope:

• Protects the capsid.
• Helps in attachment to host cell receptors.
• Helps in entry into host cells.
• Helps in evasion of the host immune system.

Examples of Enveloped Viruses:

Summary Table of Viral Structure

Diagrammatic Summary

Intermediate Level

Core in Detail

Capsid in Detail

Envelope in Detail

Comparison of Naked and Enveloped Viruses

Examples of Viruses and Their Structures

Important Points

• 1. A complete viral particle is called a virion.
• 2. Primarily, it can be divided into two parts:
• - Core
• - Coat
• 3. The core is the inner part of the virion which consists of viral genome and various proteins (enzymes).
• 4. Genome is the genetic material, which is either DNA or RNA, which may be single-stranded or double-stranded.
• 5. Core proteins include one or more enzymes that facilitate the virus in its mode of action within the host body.
• 6. For example, all single-stranded RNA viruses have the enzyme to convert single-stranded RNA genome into double-stranded RNA genome.
• 7. Retroviruses and hepatitis B virus contain the enzyme reverse transcriptase to convert single-stranded RNA genome into double-stranded DNA genome.
• 8. The coat is the outer covering of the viral particle which consists of capsid and envelope.
• 9. The capsid is the protective coat of protein surrounding the core.
• 10. Capsid is composed of identical repeating subunits called capsomeres.
• 11. The number of capsomeres is specific to a particular kind of virus. For example: Herpes virus has 162 capsomeres in its capsid while adenovirus that causes the common cold contains 252 capsomeres in its capsid.
• 12. There are two forms of symmetry in virus capsid:
• - Icosahedral (polyhedral or spherical): When the capsomeres are arranged in 20 triangles.
• - Helical: When the capsomeres are arranged in a hollow coil that appears rod-shaped.
• 13. A few viruses have an additional lipoprotein envelope around the capsid which is derived from the cell surface membrane of the host and also contain virally encoded proteins.
• 14. The viral envelope is often covered with glycoprotein spikes that help them to recognize the host cell.

Remember

• - Virion = complete viral particle
• - Core = genome + enzymes
• - **Genome** = DNA or RNA
• - Enzymes = reverse transcriptase, integrase, protease, RNA polymerase
• - Coat = capsid + envelope
• - Capsid = protein coat; made of capsomeres
• - Capsomeres = identical repeating subunits
• - Symmetry = icosahedral, helical, complex
• - Envelope = lipoprotein; derived from host
• - Glycoprotein spikes = help recognize host
• - Naked virus = no envelope (e.g., adenovirus)
• - Enveloped virus = has envelope (e.g., influenza virus)

Common Mistakes

• 1. Confusing the core and coat – Core = genome + enzymes; Coat = capsid + envelope.
• 2. Confusing capsid and capsomeres – Capsid = protein coat; Capsomeres = subunits of capsid.
• 3. Thinking all viruses have an envelope – Only some viruses have an envelope.
• 4. Confusing icosahedral and helical symmetry – Icosahedral = 20 triangles; Helical = rod-shaped.
• 5. Thinking the number of capsomeres is the same for all viruses – The number is specific to each virus.
• 6. Forgetting the role of glycoprotein spikes – They help in host recognition.
• 7. Thinking the envelope is derived from the virus – The envelope is derived from the host cell membrane.
• 8. Confusing naked and enveloped viruses – Naked = no envelope; Enveloped = has envelope.
• 9. Thinking the core contains only DNA – The core contains either DNA or RNA.
• 10. Forgetting the enzymes in the core – The core contains enzymes like reverse transcriptase.
• 11. Thinking the capsid is always icosahedral – Capsids can be helical, icosahedral, or complex.
• 12. Forgetting the function of the capsid – The capsid protects the genome and helps in attachment.

Quick Revision

• Viruses have a very simple structure.
• A complete viral particle is called virion.
• Primarily, it can be divided into two parts i.e., core and coat.
• The core is inner part of virion which consists of viral genome and various proteins (enzymes).
• Genome is the genetic material, which is either DNA or RNA, which may be single stranded or double stranded.
• Core proteins include one or more enzymes that facilitate the virus in its mode of action within host body.
• For example; all single stranded RNA viruses have the enzyme to convert single stranded RNA genome into double stranded RNA genome.
• Retroviruses and hepatitis B virus contain the enzyme reverse transcriptase to convert single stranded RNA genome into double stranded DNA genome.
• The coat is the outer covering of viral particle which consists of capsid and envelope.
• The capsid is the protective coat of protein surrounding the core.
• Capsid is composed of identical repeating subunits called capsomeres.
• The number of capsomeres is specific to a particular kind of virus.
• For example: Herpes virus has 162 capsomeres in its capsid while adenovirus that causes common cold contains 252 capsomeres in its capsid.
• There are two forms of symmetry in virus capsid. When the capsomeres are arranged in 20 triangles, it is called icosahedral (polyhedral or spherical). When the capsomeres are arranged in a hollow coil that appears rod shaped, it is called helical.
• A few viruses have an additional lipoprotein envelope around the capsid which is derived from the cell surface membrane of the host and also contain virally encoded proteins.
• The viral envelope is often covered with glycoprotein spikes that help them to recognize the host cell.

60-Second Revision

• Virion = complete viral particle
• Core = genome + enzymes
• **Genome** = DNA or RNA
• Enzymes = reverse transcriptase, integrase, protease, RNA polymerase
• Coat = capsid + envelope
• Capsid = protein coat; made of capsomeres
• Capsomeres = identical repeating subunits
• Symmetry = icosahedral, helical, complex
• Envelope = lipoprotein; derived from host
• Glycoprotein spikes = help recognize host
• Naked virus = no envelope (e.g., adenovirus)
• Enveloped virus = has envelope (e.g., influenza virus)
• Warning: The core contains the genome and enzymes. The coat contains the capsid and envelope. The capsid is made of capsomeres. Not all viruses have an envelope. The envelope is derived from the host cell membrane. Glycoprotein spikes help in host recognition. The number of capsomeres is specific to each virus. Capsids can be icosahedral, helical, or complex. Naked viruses have no envelope; enveloped viruses have an envelope.