Unit 6 Viruses

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Questions and Answers

What is the primary reason a virus must infect a host cell?

To gain nutrients for survival

To replicate and make copies of itself (correct)

To activate its immune response

To establish a symbiotic relationship

Which component is NOT a common feature of viral structure?

Mitochondrial membrane (correct)

RNA or DNA genome

Capsid of proteins

Surface receptors

What mechanism do enveloped viruses typically use to enter a host cell?

Fusion with the host membrane (correct)

Active transport into the cell

Endocytosis of the viral genome only

Direct penetration

During the viral replication cycle, what happens during the uncoating phase?

The capsid is removed and genome is exposed Signup and view all the answers

What is a key difference between RNA viruses and DNA viruses regarding replication?

RNA viruses replicate faster and with fewer mistakes Signup and view all the answers

Which option describes how a virus might be released from a host cell without killing it?

Budding from the plasma membrane Signup and view all the answers

What role do hemagglutinin spikes play in the influenza virus?

They help the virus attach to and enter host cells Signup and view all the answers

What is the term for the process by which new viral particles are assembled inside a host cell?

Assembly Signup and view all the answers

What is the role of hemagglutinin in the influenza virus?

It binds the virus to host cells with sialic acid. Signup and view all the answers

How does neuraminidase contribute to the influenza virus lifecycle?

By helping the virus penetrate mucus and facilitate viral shedding. Signup and view all the answers

What is antigenic shift in relation to influenza virus?

Genetic re-assortment between different viral strains. Signup and view all the answers

What is the main consequence of antigenic drift in influenza viruses?

It causes localized flu epidemics due to new strains not recognized by host antibodies. Signup and view all the answers

What type of cells do bacteriophages specifically infect?

Bacterial cells. Signup and view all the answers

Which of the following is a characteristic of the bacteriophage Lambda?

It was isolated from E. coli cells. Signup and view all the answers

What does antigenic drift primarily result from?

Point mutations in genes coding for hemagglutinin and neuraminidase. Signup and view all the answers

What major public health event can result from antigenic shift?

Worldwide pandemics. Signup and view all the answers

Study Notes

What is a Virus?

A virus is an acellular infectious agent that cannot self-replicate
Viruses must infect a cell in order to reproduce
Examples of viruses include Influenza, HIV, and Ebola

Viral Structure

There is variation in viral structure, but some common features include an RNA or DNA genome, a capsid of proteins, an envelope lipid bi-layer, and surface receptors (spikes of protein or glycoprotein)

Viral Replication Cycle

The replicative cycle of a virus depends upon its genome
For example, the replicative cycle of a ssRNA virus is different than that of a dsDNA virus

Viral Replication Cycle Steps

Attachment: Proteins on the viral envelope or capsid recognize and bind with target host cell receptors
Penetration: Entry is by one of three possible mechanisms:
- Direct penetration (naked viruses) – Only the viral genome enters the host (typical of bacteriophages)
- Fusion – Viral envelope fuses with the host membrane, capsid containing the genome enters the cell (e.g., HIV)
- Receptor-mediated endocytosis – Attachment stimulates endocytosis of the entire virus (e.g., Influenza)
Uncoating: The virus removes its capsid (uncoats), and the nucleic acid of its genome is exposed to the cytoplasm
Transport to Nucleus: The viral genome enters the nucleus
Synthesis (Transcription & Translation): Reproduction of the viral genome (transcription in the nucleus) and viral proteins (translation in the cytoplasm)
Assembly: Assembly of new virions (complete viruses). The genome and capsid are put together to form new viruses
Release: Release of the virus from the host cell (viral shedding) by one of three mechanisms.

Viral Shedding Mechanisms

Apoptosis: The host cell lyses and releases mature viral particles (naked viruses). The host cell dies.
Budding: Through the nuclear or plasma membrane, creating an envelope. This does not kill the host.
Exocytosis: Viruses leave the host cell using vesicles, but does not kill the host.

Influenza Virus

Influenza virus is an enveloped RNA virus
RNA viruses make more “mistakes” than DNA viruses
RNA viruses readily adapt to environmental changes due to high mutation rates
The envelope is covered with two proteins (Antigens) required for the infection process:
- Hemagglutinin (H) spike
- Neuraminidase (N) spike

Hemagglutinin

Glycoprotein on the surface of the influenza virus binds the virus to cells with sialic acid on host cell membranes, such as cells in the upper respiratory tract or erythrocytes. This helps the virus enter the cell
Sialic Acid is a receptor found on most vertebrate cells

Neuraminidase

Enzyme helps the virus to penetrate the mucus of the respiratory tract, and also aids in viral shedding of influenza (budding)
It does this by cutting sialic acid from host glycoproteins as the virus is being released

Antigenic Variation

Antigenic variation occurs in two ways with the flu virus:
- Antigenic Drift: Results from point mutation of genes coding for hemagglutinin and neuraminidase
- Antigenic Shift: Caused by reassortment of viral genes

Antigenic Drift

Mutations in genes that code for hemagglutinin and neuraminidase
These two proteins are the antigens that cause the formation of host antibodies
Produces new strains of flu virus that host antibodies won’t recognize
If a sufficient amount of antigenic drift occurs, this will cause a new flu epidemic (localized flu)
Antigenic drift causes new formulations of the flu vaccine every year.

Antigenic Shift

Results from gene reassortment from two different viruses that infect the same cell
Viruses exchange a large part of their genome
The virus that emerges is antigenically different from either of the two viruses
Antibodies formed from the two viruses are ineffective against the new combined virus genome
Can lead to pandemics

Bacteriophages

Bacteriophages, (or just phages) are viruses that only infect bacterial cells
Phages (Viruses) cannot be grown on agar plates like bacteria; they have to be grown using host cells, in this case, bacteria
For animal viruses, hens eggs, or live animals can be used

Bacteriophage Lambda

One of the earliest model systems for studying the physical nature of DNA and genes
Isolated from an E. coli cells in 1951
Has contributed enormously to our understanding of molecular biology and protein synthesis mechanisms.

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Description

Explore the fundamental concepts of virology, including the definition of viruses, their structure, and the replication cycle. This quiz covers various types of viruses, including examples like Influenza and HIV, and details how they invade host cells.