Definition
Viruses are infectious, obligate intracellular parasites, composed of a nucleic acid core (deoxyribonucleic acid (DNA) or ribonucleic acid (RNA)) surrounded by a protein capsid; at times, viruses are also surrounded by an envelope derived from host cell membranes.
A variety of viruses, including their size and morphology
Image by Lecturio.Structure
Basic structure consists of:
- Inner “machinery” that allows the virus to replicate within host cells
- Outer structural component that allows the virus to survive in the environment and bind to host cells
Inner components
- Viral genome (nucleoid): diverse, composed of either RNA or DNA
- Viral enzymes: required for viral replication within the host cell:
- DNA-dependent RNA polymerase
- RNA-dependent RNA polymerase
- RNA-dependent DNA polymerase (reverse transcriptase)
Outer components
- Capsid:
- Single- or double-layer protein shell that surrounds the nucleoid
- Arranged of subunits, known as capsomers
- 3 structural patterns:
- Icosahedral
- Helical
- Complex
- Envelope:
- May or not be present
- Surrounds nucleocapsid
- Composed of virus-specific proteins, lipids, and carbohydrates (derived from host cell membranes)
Viral capsids can have a (a) helical, (b) polyhedral, or (c) complex shape.
Image: “Figure 6.6”, access for free at https://openstax.org/books/microbiology/pages/1-introduction. License: CC BY 4.0.Classification
Multiple categorization schemes exist for viruses, based on physical characteristics and replication strategies.
| Classification by: | Types |
|---|---|
| Type and structure of nucleoid |
|
| Structure of capsid |
|
| Presence of envelope |
|
| Replication cycle (for bacteriophages) |
|
| Other |
|
Replication
It is important to differentiate between the replication of bacteriophages (viruses that infect bacteria) and eukaryotic viruses (viruses that infect eukaryotic cells).
Replication cycle of bacteriophages
There are 2 pathways of replication once a virus is within the bacterial host cell:
- Lytic cycle:
- Occurs under conditions of DNA damage or other stresses
- Bacteriophage takes over host cell transcription and translation machinery (immediate and rapid phage gene and protein expression).
- Causes lysis of bacterial host cell
- Lysogenic or temperate cycle:
- Bacteriophage genomes incorporated in bacterial host genome
- Phage genome is then replicated in a benign, dormant state: The phage genome may be expressed when the bacteria cell is stressed (e.g., nutrient depletion).
Lytic and lysogenic cycle of bacteriophages and their phases
Image: “Figure 6.8”, access for free at https://openstax.org/books/microbiology/pages/1-introduction. License: CC BY 4.0.
Lytic cycle
Image: “Figure 6.7”, access for free at https://openstax.org/books/microbiology/pages/1-introduction. License: CC BY 4.0.
Replication cycle of eukaryotic viruses
- Viral genome released in infected host cell
- Steps of viral replication:
- Attachment: Specialized viral proteins attach to host cell.
- Penetration: invasion of host cell
- Uncoating: capsid dismantled and genetic material made available
- Biosynthesis: viral nucleic acid and proteins synthesized
- Assembly: virions assembled within the host cell
- Release: via host cell lysis (for naked viruses) or the budding of the cell membrane (for enveloped viruses)
- All DNA viruses replicate in the nucleus (except for the poxvirus).
- All RNA viruses replicate in the cytoplasm (except for influenza and retroviruses).
Influenza virus replication cycle from the infection of a host cell to the release of newly formed viruses
Image: “Figure 6.10”, access for free at https://openstax.org/books/microbiology/pages/1-introduction. License: CC BY 4.0.
Genetic Diversification
Viruses have evolved many processes to increase their genetic diversity:
- Recombination: exchange of genes between 2 chromosomes by crossover at homologous regions
- Reassortment:
- Viruses with segmented genomes exchange genetic material.
- May cause an antigenic shift (e.g., influenza virus)
- Complementation:
- Occurs when the virus has a mutation that results in a non-functional protein
- A non-mutated virus “complements” a mutated virus (when co-infected in a host cell) by making a functional protein that serves both viruses (e.g., the hepatitis D virus (HDV) requires the hepatitis B virus (HBV) to supply HBsAg (a key envelope protein for HDV)).
- Phenotypic mixing:
- Simultaneous infection of host cell with 2 viruses
- Usually involves the mixing of the genome of 1 virus with surface proteins of the 2nd virus
Viruses have evolved diverse strategies for increasing genetic diversity, including recombination (the exchange of genes between 2 chromosomes by crossover at homologous regions), reassortment (exchange of segments of chromosomes), complementation (the exchange of genetic material from a functional virus with a non-functional virus to convert the latter into functional), and phenotypic mixing (mixing of 2 viral genomes and simultaneously infected host cell giving rise to progeny with blended genomes).
Image by Lecturio.Special Cases
Retroviruses
- RNA viruses
- Contain reverse transcriptase (RNA-dependent DNA polymerase)
- Most prominent retrovirus is the human immunodeficiency virus (HIV).
Prions
- Not technically viruses
- Infectious proteins that do not contain a nucleoid
- Cause transmissible spongiform encephalopathies:
- Creutzfeldt-Jakob disease
- Kuru
- Bovine spongiform encephalopathy (“mad cow” disease)
References
- Baron, S. (1996). Medical microbiology. University of Texas Medical Branch at Galveston. https://www.ncbi.nlm.nih.gov/books/NBK7627/
- Parker, N., Schneegurt, M., Thi Tu, A.H., Lister, P., & Forster, B. Microbiology. (2016). Chapter 6.2, The Viral Life Cycle. https://openstax.org/details/books/microbiology
