Viruses: Structure, Replication, and Detection PDF

Viruses are **submicroscopic, infectious agents** Virology is **the study of viruses** Viruses are… (3): – Extremely small (e.g., 20–400 nm) – Acellular – Obligate intracellular pathogens Viruses can infect: – Bacteriophages (or phages)—viruses that infect bacteria – Animal viruses—viruses that i...

Viruses are **submicroscopic, infectious agents** Virology is **the study of viruses** Viruses are… (3): – Extremely small (e.g., 20–400 nm) – Acellular – Obligate intracellular pathogens Viruses can infect: – Bacteriophages (or phages)—viruses that infect bacteria – Animal viruses—viruses that infect animals and humans – Plant viruses—viruses that infect plants Virion: – Single, infectious virus particle – Have an exterior protective protein capsid – Contain genetic material (DNA or RNA) Capsid (3): – Protein shell that packages and protects the genome – Accounts for the bulk of a virion’s mass – Made of capsomere subunits Enveloped viruses: – Have a lipid-based envelope that surrounds the capsid – Arise from budding off the host cell (take a portion of the cell membrane with them) Naked (or nonenveloped): – Viruses lack an envelope – Arise from lysing (bursting) the host cell Viral Spikes (Peplomers): – Protrude from the viral capsid or envelope – Glycoprotein extensions that help viruses attach and gain entry to host cells – Only bind to specific factors on a given host cell Viral genes encode: – Capsomere proteins – Enzymes needed for viral replication – Structural factors Viral genomes can be either: – RNA or DNA – Single or double-stranded – Single or segmented sections – Circular or linear RNA viruses have different modes of making mRNA that the host cell will translate (2): – Single-stranded positive RNA (ssRNA +): ssRNA genome functions as an mRNA, directly translated by host cell ribosomes – Single-stranded negative RNA (ssRNA -): RNA genome is complementary to mRNA, transcribed into mRNA by RNA-dependent RNA polymerases (RdRPs) Single-stranded retroviruses: – RNA genome is made into DNA by reverse transcriptase – DNA is usually inserted into the host DNA – DNA is then transcribed into mRNA Double-stranded RNA genome (dsRNA): – Transcribed to make mRNA – Requires RNA-dependent RNA polymerases Viruses exhibit a faster rate of genomic change than do living infectious agents because: – Quick replication time – Large quantity of virions are produced – RNA genomes mutate more than DNA ▪ DNA polymerases have proofreading capabilities ▪ RNA polymerases lack proofreading Genetic changes that limit infectivity of a virus lead to **attenuated strains, which are used in vaccines** Beneficial mutations may allow the virus to: – Escape host immune system detection – Broaden host range – Expand tropism (the type of cells or tissues the virus infects) – Increase infectivity Host range refers to **a collection of species that a virus can infect** Tropism: – Refers to the tissues or cell specificity – Due to viral surface factors Some viruses can infect a wide range of host cells or tissues: **broad tropism** Other viruses can infect only one type of host cell or tissue: **narrow tropism** Viruses hijack host cell machinery to **multiply** Generalized Bacteriophage Replication (5): 1. Attachment (adsorption): Phage binds to bacterial cell 2. Penetration (entry): Phage injects genetic material into the cell 3. Replication (synthesis): Phage commandeers host cell factors to transcribe and translate viral genes 4. Assembly (maturation): Genome packed into capsid and phage structures assembled 5. Release: Bacterial cell lyses, and new phages are released Penetration (Entry) (2nd step in Generalized Animal Virus Replication): – Enveloped viruses enter through endocytosis or membrane fusion, while naked viruses enter by endocytosis Release (6th step in Generalized Animal Virus Replication): – Enveloped viruses are released by budding – Naked viruses rupture the host cell during release Acute infections: **viruses infect a host cell, and new virions are made immediately** Persistent infections: **viruses have replication strategies that allow them to avoid immune system clearance (chronic or latent)** Some viruses (e.g., HIV) form a **provirus**: integration of the viral genome into the host cell Oncogenic viruses (oncoviruses): – Viruses that can cause cancer – Cause approximately 10–15% of cancers Viruses aren’t viewable via standard light microscopy, so most detection techniques use molecular methods (3): – Identify viral genetic material – Viral proteins – Antibodies that a patient may have against viral proteins Some virus detection methods involve searching for **viral proteins** in a sample. It utilizes **purified antibodies** to bind viral antigens Agglutination tests: – Purified antibodies linked to tiny latex beads – Mixed with the sample – Antibodies bind the viral antigen – Beads agglutinate Enzyme-linked immunosorbent assays (ELISA): – Can be adapted to detect either antigens or antibodies in a sample – Target adheres to a surface – Change of color indicates binding Limitations of ELISA and Agglutination Assays: – Takes time to build up detectable antibodies ▪ Seroconversion window Detecting **viral nucleic acids** is a growing trend in diagnostics Very specific segments of viral nucleic acid are detected by: – Fluorescent-labeled probes – Sequencing – PCR (polymerase chain reaction) In most cases, antiviral drugs only **limit infections rather than cure them** Vaccines **train the immune system to recognize viruses and are an effective means to limit infection** Nucleoside analogs: **block nucleic acid replication** Interferons: **naturally occurring substances released by cells in response to viral infections** Oseltamivir (Tamiflu) and zanamivir (Relenza): **prevent influenza A and influenza B virions from budding off the host cell surface** Prions (4): – Infectious proteins; no genetic material – Do not replicate – Protein in a misfolded form triggers specific host proteins to aggregate – Cause transmissible spongiform encephalopathies (TSEs) Spongiform encephalopathies can be **inherited** or **acquired**