Viruses

Viruses are Nonliving

  • Viruses are not considered living because:

    • They are not made out of cells (A2.2.1)

    • They cannot maintain a stable state

    • They do not grow

    • They cannot replicate independently

    • They cannot perform independent metabolism

Structure of Viruses

Components of Viruses

  • Spike (S) Proteins

  • Nucleocapsid (N)

  • Membrane (M)

  • Envelope (E)

  • Genetic Material

    • RNA viral genome

Types of Viral Genetic Material

  • RNA

    • Single-stranded RNA (ssRNA)

    • Double-stranded RNA (dsRNA)

  • DNA

    • Double-stranded DNA (dsDNA)

    • Single-stranded DNA (ssDNA)

Classification of Viruses by Structure

Enveloped Viruses:

  • Poxviridae (Chordopoxvirinae)

Nonenveloped Viruses:

  • Adenoviridae

  • Papovaviridae

Examples of ssRNA and dsRNA Viruses:

  • ssRNA: Coronaviridae, Bunyaviridae, Picornaviridae

  • dsRNA: Reoviridae

Viruses as Obligate Intracellular Parasites

  • Viruses must infect host cells to reproduce.

  • The infective cycle involves the following steps:

    1. Attachment: Virus attaches to the cell.

    2. Penetration: Virus injects nucleic acid into the cell.

    3. Replication: The viral genome replicates using the host's machinery.

    4. Assembly: New viral nucleic acids are packaged into particles.

    5. Release: Viruses released from the host cell may cause cell destruction (lysis) or can remain dormant (lysogenic cycle).

Types of Viral Life Cycles

  • Lytic Cycle (A2.3.3*): Virus replicates quickly and destroys the host cell.

  • Lysogenic Cycle (A2.3.4*): Viral genome integrates into the host's genome, replicates with the host cell.

Diversity of Viruses

  • Viruses exhibit wide diversity in:

    • Size

    • Shape

    • Types of genetic material

    • Genes present

    • Presence of envelopes

Size Range of Viruses

  • Most viruses range from 0.02 to 0.2 μm; some exceed 1 μm.

  • Bacteria (2-3 μm) and eukaryotic cells (10-30 μm) are larger than most viruses.

Structures Common to All Viruses

  • All viruses contain nucleic acids as genetic material (Either DNA or RNA).

  • They consist of protein capsids.

  • Lack cytoplasm and cellular membranes.

Viral Capsids

  • Capsids can be:

    • Helical

    • Icosahedral

    • Complex Structure

Virus Envelopes

  • Enveloped Viruses: Acquire lipid bilayer from host cell during budding, more sensitive to environmental factors.

  • Non-Enveloped Viruses: More resistant, usually released by cell lysis.

Examples

  • Enveloped Viruses: Influenza, HIV, Coronaviruses.

  • Non-Enveloped Viruses: Adenovirus, Rhinovirus.

Example Viruses

Bacteriophage Lambda

  • Host: E. coli

  • Structure: Double-stranded linear DNA, complex capsid.

  • Lifecycle: Alternates between lytic and lysogenic cycles.

SARS-CoV-2

  • Host: Mammal epithelium cells with ACE2 receptor.

  • Structure: Single-stranded RNA, enveloped.

Human Immunodeficiency Virus (HIV)

  • Host: Primate T-cells with CD4 receptor.

  • Structure: Two copies of single-stranded RNA, icosehedral capsid with an envelope.

  • Lifecycle: Retrovirus, uses reverse transcriptase.

Origin of Viruses

Hypotheses on Virus Origin

  1. Virus-First Hypothesis: Proposes that viruses predate cellular life.

  2. Progressive Hypothesis: Viruses arose from cell components that gained independence.

  3. Regressive Hypothesis: Viruses originated from more complex cellular organisms losing functions.

Evolution and Characteristics of Viruses

  • Viruses evolve rapidly due to:

    • High mutation rates in RNA viruses.

    • Short generation times.

    • Large population sizes leading to strong selection pressures for favorable traits.

Mechanisms of Rapid Evolution

  • Antigenic drift and shift in viruses like Influenza lead to new variants and necessitate updated vaccines annually.

Implications

  • Rapid evolution allows viruses to adapt, leading to challenges in treatment and vaccine efficacy.

Conclusions on Viral Biology

  • Understanding viral structure and lifecycle is crucial for tackling viral infections, vaccine design, and improving treatment methods.

  • Continued research is necessary to fully understand viruses' impacts and interactions with host organisms.