Introduction to Viruses

Introduction to Viruses

• Viruses are infectious, obligate intracellular parasites.
• They consist of genetic material (DNA or RNA) enclosed in a protein coat (capsid).
• Some viruses have an additional lipid envelope derived from host cells.

Learning Objectives

• Compare and contrast viruses with other cell forms.
• Describe properties of viruses, including their molecular structure, composition, and replication strategies.
• Recognize the differences between DNA and RNA viruses.

Virus Structure

• Viruses are simple structures.
• They consist of a capsid surrounding genetic material.
• A nucleocapsid is composed of nucleic acid genome and capsid proteins.
• Some viruses are enveloped, having a lipid bilayer membrane with viral proteins.

Properties of Viruses

• Viruses lack cellular machinery to synthesize proteins.
• Viruses are obligate intracellular parasites.
• Viruses don't have energy source like ATP.
• Viruses do not have ribosomes or organelles.
• Viruses cannot perform metabolic processes independently.

Virus Size

• Viruses vary in size.
• DNA viruses, RNA viruses and bacteriophages are different sizes.
• Viruses are much smaller than bacteria or human cells.
• Their dimensions are measured in nanometers (nm).

Virus Shape

• Capsid: The protein coat surrounding the virus genome; composed of repeating subunits called capsomeres
• Nucleocapsid: The structure composed of the nucleic acid genome and the capsid proteins.
• Icosahedral: Capsomers arranged in triangles, forming a spherical shape; can be enveloped or naked
• Helical: Capsomers arranged in a hollow coil, forming a rod-shaped structure; always enveloped

Viral Envelopes

• Derived from host cell membranes.
• Contain glycoproteins that attach to host receptors.
• Make viruses more sensitive to heat, drying, detergents, and lipid solvents
• Viruses lose their stability when their envelopes are removed

Virus Composition

• Nucleic acid: DNA or RNA; can be single- or double-stranded, linear or circular, or segmented.
• Proteins: Provide structure and functions like protecting the genome, facilitating transfer, attachment, and acting as antigens.
• Genetic material is either DNA or RNA, single or double stranded, linear or circular or segmented

RNA Viruses

• Genetic material is RNA.
• Single-stranded, either (+) or (-)
• Classification is by the type and function of the RNA.
• Replication occurs in the cytoplasm.
• Most are enveloped

RNA Virus Polarity

• Positive (+) stranded RNA viruses: act directly as mRNA in the process of translation.
• Negative (-) stranded RNA viruses: not directly translatable, need to be converted to (+) stranded RNA.
• Most RNA viruses do have RNA polymerase in their capsid.

DNA Viruses

• Genetic material is DNA.
• Double-stranded.
• Replication occurs in the nucleus
• Most have negative (-) and positive (+) strand
• Must be transcribed into mRNA; then translated into structural proteins and enzymes

DNA vs. RNA Viruses

• RNA viruses are single-stranded and have helical symmetry (may or may not have envelopes), replicating in the cytoplasm.
• DNA viruses are double-stranded and have icosahedral symmetry (may or may not have envelopes), replicating in the nucleus.
• DNA viruses tend to be genetically complex compared to RNA viruses.

Viral Transmission and Replication

• Direct transmission: Person-to-person contact (no fomites)
• Indirect transmission: Inanimate objects (fomites)
• Animal-to-animal: with humans as an accidental host
• Arthropod vector transmission

Bacteriophages

• Viruses that infect bacteria.
• Frequently used for phage therapy targeting specific bacteria.

Specific Outbreaks (e.g. SARS-CoV, MERS-CoV, COVID-19, Monkeypox, RSV)

• Specific characteristics, origins, transmission, clinical findings.
• COVID-19 pandemic, including clinical findings, long-COVID, and treatments, vaccination.
• Monkeypox outbreak, including symptoms, transmission, diagnosis, treatments and vaccination.
• RSV epidemiology. Symptoms and treatment options.

Virus Taxonomy

• The scientific classification of viruses follows a hierarchical system, including family, subfamily, genus and species names, using suffixes.

Coronavirus

• Relatively large viral particles
• Non-segmented, single-stranded RNA
• Enveloped with helical nucleocapsid
• Prominent spike proteins

COVID-19 Clinical Findings

• Varying clinical features; Incubation periods, symptoms.

COVID-19 Risk Factors

• Older adults (especially over 70)
• Co-morbidities (compromised immunity, diabetes, chronic heart, kidney, or respiratory disease.)
• Obese patients (especially BMI over 30)
• Smoking or vaping history
• Gender considerations and factors relating to infection rates.

COVID-19 Outpatient Treatments

• Recommended medications and clinical indications for certain use cases.
• Hospitalization and outpatient treatment preferences.

COVID-19: "Long COVID"

• Persistence of symptoms after initial infection
• Variety of symptoms, including cognitive difficulties, fatigue, dyspnea, insomnia, and impaired daily functioning; disproportionality affecting women

COVID-19 Vaccine Schedules

• mRNA vaccines are preferred, with bivalent boosters becoming available for certain timeframes.
• Vaccinations may be given to different age groups, and at different timeframes from initial infection or other vaccination.

MPOX

• Orthopoxvirus, endemic to Africa.
• Most common in Men who have sex with Men (MSM).
• Direct contact with rash and scabs; contact with saliva and upper respiratory secretions.
• Not strictly sexually transmitted

RSV

• Respiratory syncytial virus, affecting respiratory tracts; resulting in various levels of severity.
• Epidemiology information, seasonal pattern, prevalence.

Virus Diagnosis

• PCR testing for viral RNA or antigen testing.
• Serum antibody tests to detect immunity level.

Explore the fascinating world of viruses, their structures, properties, and replication strategies. This quiz will help you compare viruses with other cell forms and distinguish between DNA and RNA viruses. Test your knowledge about these obligate intracellular parasites and their complex nature.