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
RNA virus identification:
Viruses can be classified in many ways. Most viruses, however, will either have a genome formed by DNA or RNA. Viruses with an RNA genome can be further characterized by whether that RNA is single or double-stranded. If the viruses are covered by a thin coat of cell membrane (usually taken from the host cell), they are called “enveloped” viruses. If that coat is absent, however, the viruses are called “naked” viruses. Viruses with single-stranded genomes are called “positive-sense” viruses if they employ their genome directly as messenger RNA, which is translated into proteins. “Negative-sense” single-stranded viruses employ the viral enzyme RNA-dependent RNA polymerase to transcribe their genome into messenger RNA.
DNA virus identification:
Viruses can be classified in many ways. Most viruses, however, will either have a genome formed by DNA or RNA. Viruses with a DNA genome can be further characterized by whether that DNA is single or double stranded. If the viruses are covered by a thin coat of cell membrane (usually taken from the host cell), they are called “enveloped” viruses. If that coat is absent, however, the viruses are called “naked” viruses. Some of the enveloped viruses translate their DNA into RNA before it is incorporated into the host cell’s genome.
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
