Virus Structure and Capsid Functions

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Questions and Answers

What term describes a complete, fully developed, infectious viral particle?

Virion (correct)

Capsomere

Nucleocapsid

Envelope

Which component is present in the viral structure that surrounds the nucleic acid?

Viral glycoprotein

Host membrane

Capsid (correct)

Cytoplasm

What is the primary function of viral glycoproteins found in the envelope?

To replicate viral RNA

To provide energy for viral reproduction

To promote virion segregation

To recognize and bind to host cell receptors (correct)

What type of viruses are primarily naked or nonenveloped?

Bacterial and plant viruses Signup and view all the answers

In the context of viral structure, what is a nucleocapsid?

The combination of nucleic acid and the capsid Signup and view all the answers

What is one of the primary functions of the capsid in a virus?

To protect and package the nucleic acid genome Signup and view all the answers

Which method is NOT used by nonenveloped viruses to deliver their genome into a host cell?

Lipid infusion Signup and view all the answers

How do helical capsids differ in their packaging compared to other types?

Their length is determined by the length of the nucleic acid Signup and view all the answers

Which statement is true regarding the structure of icosahedral viruses?

They possess many-sided symmetrical structures Signup and view all the answers

What is the process called when a host cell engulfs an entire virus for genome delivery?

Endocytosis Signup and view all the answers

Why can helical capsids only package ssRNA viruses?

The rigidity of double-stranded nucleic acids Signup and view all the answers

What characterizes complex viruses compared to other capsid structures?

They possess complicated structures Signup and view all the answers

What is the primary structural feature of icosahedral capsid viruses?

They possess a closed shell enclosing the nucleic acid. Signup and view all the answers

Which arrangement of protein subunits is considered the most efficient for constructing larger icosahedral capsids?

Utilizing both pentamers and hexamers. Signup and view all the answers

Which type of viruses commonly exhibit icosahedral capsids?

Most animal viruses. Signup and view all the answers

What characterizes complex viruses like bacteriophages?

They feature a polyhedral capsid with a helical tail sheath. Signup and view all the answers

How many pentamers are present in the construction of icosahedral capsids?

The number is variable. Signup and view all the answers

What shape is described for poxviruses?

Brick-shaped. Signup and view all the answers

Which of the following statements accurately describes the assembly of proteins in icosahedral capsids?

Proteins can assemble as either pentamers or hexamers. Signup and view all the answers

What defines the capsid symmetry of complex viruses?

It is neither icosahedral nor helical. Signup and view all the answers

Which of the following statements about icosahedral capsids is incorrect?

They are typically composed of a single type of protein. Signup and view all the answers

What characteristic makes enveloped viruses more susceptible to environmental changes compared to naked viruses?

Their lipid bilayer structure Signup and view all the answers

Which method of viral entry into host cells involves the engulfing of the virus?

Endocytosis Signup and view all the answers

Which of the following statements about enveloped virions is true?

They are derived from the host cell during viral release. Signup and view all the answers

What is the first step in the process of delivering the viral genome into a host cell?

Attachment of the virus to the host cell membrane Signup and view all the answers

In the context of viral structure, how is metastability defined?

Rapid disassembly upon reaching the host cell. Signup and view all the answers

What is the key factor that does NOT determine whether a virus has an envelope?

Structure of the viral genome Signup and view all the answers

Which of the following enveloped viruses enters host cells through membrane fusion?

Influenza virus Signup and view all the answers

What advantage does the envelope provide to enveloped viruses?

Protection from physical and chemical agents Signup and view all the answers

What happens during the uncoating process of a virus?

The viral genome is released from the capsid. Signup and view all the answers

Study Notes

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Virus Structure

A virion is a complete, infectious viral particle.
A capsid is a protein coat surrounding the nucleic acid, composed of protein subunits (capsomeres).
The nucleocapsid (core) is the nucleic acid enclosed by the capsid.
Envelopes are made of lipids and proteins (host-derived) and may contain glycoproteins (spikes) for host receptor recognition.
Most animal viruses are enveloped, while most bacterial and plant viruses are naked.

Functions of the Capsid

The capsid protects the viral genome, packaging the nucleic acid.
Non-enveloped viruses deliver genomes through direct penetration (genome alone enters the cell) or endocytosis (whole virus engulfed and capsid uncoated).
For enveloped viruses, the membrane fuses with the host cell membrane, releasing the capsid into the cell cytoplasm.

Capsid Morphology

Helical viruses have hollow, cylindrical capsids.
Icosahedral viruses have many-sided capsids.
Complex viruses have complicated structures.

Helical Capsid Viruses

Helical capsids have spiral or rod shapes.
Length is determined by the nucleic acid length, and the width by the capsomere size and packaging.
Helical capsids typically contain single-stranded RNA (ssRNA).

Icosahedral Capsid Viruses

Icosahedral capsids are spherical and rigid.
They enclose the nucleic acid inside an identical triangular faces and vertices.
They are efficient and use the fewest capsomeres for assembly.
Many animal viruses have icosahedral capsids (e.g., adenovirus, poliovirus, HPV).

Complex Viruses

Complex viruses don't have a simple icosahedral or helical symmetry.
Bacteriophages are complex, including a polyhedral capsid and a helical tail sheath.
Poxviruses are brick-shaped.

Enveloped Virions

Enveloped virions are acquired via budding from the host cell, during viral replication.
Envelope proteins, as well as glycoproteins often play significant roles in host recognition.
Envelopes protect viruses from the immune system.
Enveloped viruses are more fragile than naked viruses.

Delivery of Genome from Enveloped Virions

Enveloped virions enter host cells via endocytosis (engulfing the virus into the host cell) or membrane fusion (viral envelope and host membrane fuse).
Uncoating of the capsid releases the genome.

DNA and RNA Viruses

Viruses can be enveloped or non-enveloped, irrespective of their nucleic acid type (DNA or RNA).
There's no correlation between virus genome structure and its envelope status.

Virus Particles (Virions)

Virions are metastable, meaning they're stable during protection of the genome, but become unstable during infection, leading to structural changes for genome release.
Viral metastability is achieved via symmetrical arrangement of identical proteins for maximal contact.

RNA Viruses

RNA viruses encode or carry RNA-dependent RNA polymerase (RdRP) for replication, as mammalian cells lack this enzyme.

Viral Genomes

Viral genomes can be DNA or RNA, single-stranded or double-stranded.
Some genomes are linear, others circular or segmented.
Viral genomes typically contain a limited number of genes (4 to hundreds).

(+) vs. (-) Sense RNA and DNA

mRNA (ribosome ready) is always the positive (+) strand.
RNA and DNA complements of (+) strands are the negative (-) strands.
Negative-sense RNA must be converted to (+) sense RNA before translation.

Eukaryotic DNA and RNA Viruses

Eukaryotic DNA viruses typically replicate in the nucleus, or cytoplasm (e.g. poxviruses).
Eukaryotic RNA viruses usually replicate in the cytoplasm.

RNA Viruses Challenge Central Dogma

RNA viruses carry or encode the enzymes necessary for replication and transcription (RNA-dependent RNA polymerase).
Retroviruses are an exception to this because they use reverse transcription (RNA to DNA).

Polymerases

DNA polymerases synthesize DNA using DNA templates (DNA-dependent DNA polymerase). RNA templates can sometimes be used (RNA-dependent DNA polymerase).
RNA polymerases synthesize RNA using DNA templates (DNA-dependent RNA polymerase). RNA templates can sometimes be used (RNA-dependent RNA polymerase).

Genome Replication and Expression of RNA Viruses

RNA viruses replicate their genomes and synthesize viral mRNAs usually via an RNA-dependent RNA polymerase.
Exceptions include retroviruses.

Viral Classification

Viruses are classified based on various attributes (e.g., nucleic acid type, symmetry, presence/absence of an envelope, host range).
The International Committee on Taxonomy of Viruses (ICTV) provides a standardized classification system.
The Baltimore classification system is based on genome composition, replication, and mRNA production, for better understanding of DNA/RNA virus groups.

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Description

This quiz covers the structure of viruses, focusing on the components such as the virion, capsid, and nucleic acids. It also explores the functions of the capsid and its role in protecting viral particles. Enhance your understanding of virology with this interactive quiz!