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Introduction
1. CARS
2. Psych/soc
3. Bio/biochem
3.1 Structure and function of proteins and their constituent amino acids
3.2 Transmission of genetic information from the gene to the protein
3.3 Heredity and genetic diversity
3.4 Principles of bioenergetics and fuel molecule metabolism
3.5 Assemblies of molecules, cells, groups of cells
3.6 Structure and physiology of prokaryotes and viruses
3.6.1 Cell Theory, prokaryote structure and physiology
3.6.2 Virus structure and life cycle
3.7 Processes of cell division, differentiation, and specialization
3.8 Structure and functions of nervous and endocrine systems
3.9 Structure and functions of main organ systems
4. Chem/phys
Wrapping up
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3.6.2 Virus structure and life cycle
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3. Bio/biochem
3.6. Structure and physiology of prokaryotes and viruses

Virus structure and life cycle

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Virus structure

Viruses are extremely small - often about 100 times smaller than bacteria and up to 1000 times smaller than eukaryotic cells. Their defining feature is a protein coat that surrounds genetic material. This genetic material can be DNA or RNA, and it may be single- or double-stranded.

Some viruses also have a lipid envelope taken from the host cell’s membrane. These are called enveloped viruses, and they typically leave the host by budding from the host membrane. In contrast, nonenveloped viruses usually exit when the host cell bursts (lysis), releasing newly made viral particles.

Rabies virus structure with ssRNA core, envelope, and helical capsid
Rabies virus structure with ssRNA core, envelope, and helical capsid

Genetic material

Viruses contain genetic material, but they lack organelles and a nucleus. Their nucleic acid is packaged directly inside the protective protein coat.

Certain bacteriophages show this structure clearly:

  • A head (capsid) stores genetic material
  • A sheath forms a channel that helps inject that material into bacterial hosts
  • Tail fibers attach to the host’s surface.

Viral life cycle

Viruses vary in the type of genome they carry. Some store RNA, and viruses that convert RNA into DNA after infection are called retroviruses. Retroviruses carry the enzyme reverse transcriptase, which makes DNA from the viral RNA. That DNA can then be handled by the host cell’s replication machinery.

HIV, a retrovirus, infects helper T cells by attaching to specific receptors on the cell surface, fusing with the plasma membrane, and releasing its genetic material and proteins into the host cell.

HIV virus binding to CD4 receptor on immune cell during viral attachment
HIV virus binding to CD4 receptor on immune cell during viral attachment

Replication

  • All viruses depend on a host cell for ribosomes, ATP, and molecular building blocks. A viral particle attaches to the host, penetrates the membrane or wall, delivers its genetic material, and then uses the host’s systems to synthesize viral components.
  • These components self-assemble into new virions, which exit by lysis or budding.
  • A special phenomenon called transduction occurs when a lysogenic virus accidentally packages fragments of a previous host’s DNA. When the virus infects a new cell, it injects that leftover DNA, which can integrate into the new host’s chromosome by recombination. This type of horizontal gene transfer contributes to genetic diversity, along with other virus-host interactions that shape evolutionary dynamics.

Prions and viroids: subviral particles

  • Viroids are extremely small, circular, single-stranded RNA molecules with no protein coat. They infect plants and cause various diseases. In contrast, prions are misfolded proteins that act as infectious particles. They cause transmissible and inherited neurodegenerative diseases through protein misfolding. Classic prion diseases include Creutzfeldt-Jakob disease, Gerstmann-Straussler syndrome, and mad cow disease. Emerging research suggests that prion-like mechanisms may also contribute to Alzheimer’s disease, Huntington’s disease, Parkinson’s disease, amyotrophic lateral sclerosis, and certain cancers.

Virus structure

  • Protein coat (capsid) surrounds genetic material (DNA or RNA)
  • May have lipid envelope (enveloped viruses) or lack it (nonenveloped viruses)
    • Enveloped: exit by budding; nonenveloped: exit by lysis
  • Much smaller than bacteria and eukaryotic cells

Genetic material

  • No organelles or nucleus; nucleic acid inside capsid
  • Bacteriophage structure:
    • Head (capsid) stores genetic material
    • Sheath injects genetic material into host
    • Tail fibers attach to host cell

Viral life cycle

  • Genome can be DNA or RNA; retroviruses convert RNA to DNA (reverse transcriptase)
  • HIV: retrovirus infecting helper T cells via specific receptors
  • All viruses require host cell machinery for replication

Replication

  • Virus attaches to host, penetrates membrane/wall, delivers genetic material
  • Uses host resources to synthesize viral components
  • New virions self-assemble; exit by lysis or budding
  • Transduction: lysogenic virus transfers host DNA to new cell, increasing genetic diversity

Prions and viroids: subviral particles

  • Viroids: small, circular, single-stranded RNA; infect plants; no protein coat
  • Prions: misfolded infectious proteins; cause neurodegenerative diseases via protein misfolding
    • Examples: Creutzfeldt-Jakob disease, mad cow disease, possible role in Alzheimer’s and other disorders

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Virus structure and life cycle

Virus structure

Viruses are extremely small - often about 100 times smaller than bacteria and up to 1000 times smaller than eukaryotic cells. Their defining feature is a protein coat that surrounds genetic material. This genetic material can be DNA or RNA, and it may be single- or double-stranded.

Some viruses also have a lipid envelope taken from the host cell’s membrane. These are called enveloped viruses, and they typically leave the host by budding from the host membrane. In contrast, nonenveloped viruses usually exit when the host cell bursts (lysis), releasing newly made viral particles.

Genetic material

Viruses contain genetic material, but they lack organelles and a nucleus. Their nucleic acid is packaged directly inside the protective protein coat.

Certain bacteriophages show this structure clearly:

  • A head (capsid) stores genetic material
  • A sheath forms a channel that helps inject that material into bacterial hosts
  • Tail fibers attach to the host’s surface.

Viral life cycle

Viruses vary in the type of genome they carry. Some store RNA, and viruses that convert RNA into DNA after infection are called retroviruses. Retroviruses carry the enzyme reverse transcriptase, which makes DNA from the viral RNA. That DNA can then be handled by the host cell’s replication machinery.

HIV, a retrovirus, infects helper T cells by attaching to specific receptors on the cell surface, fusing with the plasma membrane, and releasing its genetic material and proteins into the host cell.

Replication

  • All viruses depend on a host cell for ribosomes, ATP, and molecular building blocks. A viral particle attaches to the host, penetrates the membrane or wall, delivers its genetic material, and then uses the host’s systems to synthesize viral components.
  • These components self-assemble into new virions, which exit by lysis or budding.
  • A special phenomenon called transduction occurs when a lysogenic virus accidentally packages fragments of a previous host’s DNA. When the virus infects a new cell, it injects that leftover DNA, which can integrate into the new host’s chromosome by recombination. This type of horizontal gene transfer contributes to genetic diversity, along with other virus-host interactions that shape evolutionary dynamics.

Prions and viroids: subviral particles

  • Viroids are extremely small, circular, single-stranded RNA molecules with no protein coat. They infect plants and cause various diseases. In contrast, prions are misfolded proteins that act as infectious particles. They cause transmissible and inherited neurodegenerative diseases through protein misfolding. Classic prion diseases include Creutzfeldt-Jakob disease, Gerstmann-Straussler syndrome, and mad cow disease. Emerging research suggests that prion-like mechanisms may also contribute to Alzheimer’s disease, Huntington’s disease, Parkinson’s disease, amyotrophic lateral sclerosis, and certain cancers.
Key points

Virus structure

  • Protein coat (capsid) surrounds genetic material (DNA or RNA)
  • May have lipid envelope (enveloped viruses) or lack it (nonenveloped viruses)
    • Enveloped: exit by budding; nonenveloped: exit by lysis
  • Much smaller than bacteria and eukaryotic cells

Genetic material

  • No organelles or nucleus; nucleic acid inside capsid
  • Bacteriophage structure:
    • Head (capsid) stores genetic material
    • Sheath injects genetic material into host
    • Tail fibers attach to host cell

Viral life cycle

  • Genome can be DNA or RNA; retroviruses convert RNA to DNA (reverse transcriptase)
  • HIV: retrovirus infecting helper T cells via specific receptors
  • All viruses require host cell machinery for replication

Replication

  • Virus attaches to host, penetrates membrane/wall, delivers genetic material
  • Uses host resources to synthesize viral components
  • New virions self-assemble; exit by lysis or budding
  • Transduction: lysogenic virus transfers host DNA to new cell, increasing genetic diversity

Prions and viroids: subviral particles

  • Viroids: small, circular, single-stranded RNA; infect plants; no protein coat
  • Prions: misfolded infectious proteins; cause neurodegenerative diseases via protein misfolding
    • Examples: Creutzfeldt-Jakob disease, mad cow disease, possible role in Alzheimer’s and other disorders