Unit 6 Viruses

Study Notes

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)

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

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.

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 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

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

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 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

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.

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, (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

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.