Virion Morphology & Classification

# Virion Morphology - **Genetic material:** - **DNA or RNA** never found simultaneously - **Number of strands:** - Linear - Circular - Segmented - **Structure:** - Monocatenary - Bicatenary - **Capsid:** - **Proteins:** - Structu...

# Virion Morphology - **Genetic material:** - **DNA or RNA** never found simultaneously - **Number of strands:** - Linear - Circular - Segmented - **Structure:** - Monocatenary - Bicatenary - **Capsid:** - **Proteins:** - Structural (50-90% of the virus): Provides shape and structure to the capsid - Non-structural: - May modify the capsid structure - May be involved in viral replication - We can find: - **Attachment proteins:** - Allow the virus to bind to the host cell - May trigger the process of identifying a host cell - **Glycoproteins:** - Found in enveloped viruses - Act as antigens - **Enzymes:** - Stimulate replication by helping with the synthesis of macromolecules and continuing replication - **Essential proteins:** End the virus's replication - **Matrix proteins (Protein M):** - Located between the nucleocapsid and the envelope in enveloped viruses - May or may not be present - Involved in the assembly of the virus - **Envelope:** - **Double lipid layer:** - May or may not be present - Protects the virus - **Enveloped Viruses:** More sensitive to: - Detergents - Capsid - Form pleomorphic viruses - Finalizes the replication process - **Viroplasms:** - Particles that provide extra protection for the virus within an infected cell - Extra protection for the genome inside the organism - **Viral Symmetry:** - **Helical:** capsomers assemble around the genetic material to form a hollow tube - **Icosahedral:** Different numbers of faces, including pentameres, hexameres - **Complex:** - *e.g:* Rabies virus, Ebola virus - Head (genetic material) - Tail (tube for entering the host cell) - Base (fibriae) - *e.g:* Bacteriophage - **Features of Capsid-less Viruses:** - Allows them to be found in: - Objects - The host cell - Living in extremely dry environments - Surviving desiccation (dehydration) - Inanimate objects - **Enveloped Viruses:** - Leave the cell membrane - Lipids - Glycoproteins - Proteins - **Routes of transmission**: - **Inhalation:** - Dust - Droplets - **Vectors:** - *e.g:* Mosquitoes - **Ingestion:** - *e.g:* Water - *e.g*: Food - **Fomites** - **Blood transfusion** - **Organ transplantation** # Viral Replication and Mechanisms of Pathogenicity - **Replication steps** are the same in all viruses - **Macromolecule synthesis:** - Followed by assembly - **The cell functions as a factory that helps with replication** - **Two phases**: - **Early phase:** - Starts with the identification of a host cell - Introduction - Loss of the capsid - Release of the genome - **Late phase:** - Starts with the synthesis of new viral components - Exit from the cell by: - Budding: - No symptoms - Eclipse period: No infection - Latent period: No symptoms - Lysis: Causes cell death - *e.g*: Period of latency includes the eclipse period and ends with the release of the virus - **Antiviral drugs:** - **Interferon:** can prevent the spread of virus infection - **Nucleoside analogs:** - *e.g*: Ribavirin - Inhibit the synthesis of nucleotides and cause mutations - **Thymidine kinase inhibitors:** - *e.g:* Acyclovir - *e.g:* Penciclovir - Inhibit the activation of drugs - **Neuraminidase inhibitors:** - *e.g:* Zanamavir - *e.g:* Oseltamivir - Inhibition of viral neuraminidase, an enzyme that helps viruses escape from infected cells - **Other important targets:** - Transcription - Replication - Protein synthesis - **Most viral infections are treated symptomatically. - **A single infected cell can produce** 100,000 viral particles. - Only 1%-10% of viral particles are infectious. - The rest mutate or die. # Genetic of Viruses - **DNA Viruses:** - Not transient - Not labile - Can be latent - Can cause chronic or persistent infections - Generally, replicate in the nucleus - *e.g:* P53 gene: - Monitors the integrity of the cell genome - When damaged, it induces apoptosis - **RNA Viruses:** - Replication in the cytoplasm - **Acute infections** - More prone to mutate because they are highly labile - **All RNA viruses have envelopes.** - Conditional: The envelope is required for viral replication. - **Types of mutations:** - Lethal: Loss of a gene essential for survival in a given host environment - Deletion: complete loss of a segment of the genome - **Essential gene mutations:** - Virus dies - **Structural gene mutations:** - Change in shape - Modification in resistance - Allows survival - **Interactions:** - **Recombination:** exchange of genetic material between viruses of the same species - **Reassortment:** Exchange of genetic material between viruses of different species - **Complementation:** - *e.g*: Defective viral mutant can be rescued by a wild type virus or a cell line harboring the missing gene. - **Viral substitution:** - **Genetically manipulated viruses can be excellent systems for gene administration.** - **Provides therapeutic gene substitution approaches.**