Viruses: Structure, Replication, and Disease

Viruses

Are Viruses Living?

Viruses occupy a unique position at the boundary between living and non-living entities. They possess some characteristics of life but lack others, leading to ongoing debate about their classification.

• Obligate intracellular parasites: Viruses can only replicate within a host cell; they cannot reproduce or carry out metabolic activities independently.

• Genetic material: Viruses contain either DNA or RNA as their genetic material, but never both.

• Structure: Composed of nucleic acid (DNA or RNA) surrounded by a protein coat (capsid).

• Absence of cellular components: Viruses lack a plasma membrane, ribosomes, organelles, and cytoskeleton.

• Lipid envelope: Some viruses possess an external lipid envelope derived from the host cell membrane, often with glycoprotein spikes.

Conclusion: Viruses are not considered living organisms because they cannot carry out essential life processes independently.

Viral Morphology

Viruses exhibit diverse shapes and sizes, but share common structural features.

• Size: Viruses are much smaller than bacteria, typically in the nanometer (10-9 meter) range.

• Genome: The central core contains the viral genome, which may be single- or double-stranded DNA or RNA. Some viral genomes are segmented.

• Capsid: The genome is enclosed by a protein coat called the capsid, made up of protein subunits called capsomeres.

• Envelope: Some viruses have an additional lipid envelope with glycoprotein spikes, which aids in host cell recognition and entry.

Proviruses and Retroviruses

Certain viruses can integrate their genetic material into the host genome, affecting the host cell's function and enabling persistent infection.

• Provirus: A viral genome integrated into the host's DNA (DNA → DNA integration). The host's RNA polymerase transcribes proviral DNA into mRNA, leading to the production of new viruses.

• Retroviruses: A class of viruses (e.g., HIV) with RNA genomes that are reverse transcribed into DNA by the viral enzyme reverse transcriptase (RNA → DNA). The resulting DNA integrates into the host genome as a provirus.

Example: HIV is a retrovirus that integrates into human DNA, establishing lifelong infection.

Coronaviruses

Coronaviruses are a family of enveloped RNA viruses that cause respiratory infections in humans and other animals.

• Structure: Enveloped viruses with glycoprotein spikes on their surface, giving a crown-like ("corona") appearance.

• Genome: Single-stranded RNA (~30 kilobases).

• Diseases: Responsible for about 15% of common colds; notable zoonotic pathogens.

• Recombination: High rate of genetic recombination, contributing to the emergence of new strains.

• SARS-CoV-2: The virus responsible for the COVID-19 pandemic.

• Terminology: The virus (SARS-CoV-2) is transmissible; the disease (COVID-19) is the result of infection and is not itself transmissible.

Influenza

Influenza viruses are enveloped RNA viruses that cause seasonal flu epidemics and occasional pandemics.

• Surface antigens: Hemagglutinin (HA) and neuraminidase (NA) are key proteins on the viral surface targeted by the immune system.

• Immunity: Most people have antibodies to various influenza strains, which can provide protection or reduce symptom severity.

• Antigenic variation: Changes in surface antigens can alter immune recognition.

• Antigenic drift: Gradual accumulation of mutations in HA and NA genes, leading to new strains.

• Antigenic shift: Sudden, major changes due to reassortment of gene segments, potentially resulting in pandemics.

Prions

Prions are infectious agents composed solely of misfolded proteins, with no nucleic acid component.

• Nature: Prions are abnormal, misfolded forms of normal proteins that can induce misfolding in other proteins.

• Diseases: Cause neurodegenerative diseases such as Mad Cow Disease (Bovine Spongiform Encephalopathy), Chronic Wasting Disease, Scrapie, Creutzfeldt-Jakob Disease, and Kuru.

• Properties: Prions are highly resistant to heat and common disinfectants, making them difficult to destroy.

• Pathology: Prion aggregation in the brain leads to tissue damage and characteristic symptoms.