The study of viruses encompasses various aspects including their morphology, classification, reproduction/replication, and cultivation.
Morphology of Viruses
Structure:
1. Capsid:
The protein coat surrounding the viral genome, composed of protein subunits called capsomeres.
The capsid provides protection and aids in the attachment to host cells.
Helical: Capsids with a rod-like appearance (e.g., tobacco mosaic virus).
Icosahedral: Capsids with a roughly spherical shape (e.g., adenovirus).
Complex: Capsids with intricate structures (e.g., bacteriophage).
2. Envelope:
Some viruses have an outer lipid membrane derived from the host cell membrane, containing viral glycoproteins (e.g., influenza virus, HIV).
3. Genome:
The genetic material of viruses, which can be DNA or RNA, single-stranded or double-stranded, linear or circular.
DNA Viruses: Have DNA as their genetic material (e.g., herpesvirus).
RNA Viruses: Have RNA as their genetic material (e.g., influenza virus).
Additional Structures:
1. Tegument:
A layer of proteins found between the envelope and capsid in some viruses (e.g., herpesviruses).
2. Tail Fibers and Spikes:
Structures used for attachment to host cells, commonly found in bacteriophages and some animal viruses.
Classification of Viruses
Viruses are classified based on several criteria including the type of nucleic acid, replication strategy, morphology, and host range.
The main classification systems include:
1. Based on Genetic Material:
I. DNA Viruses:
Contain DNA as their genetic material.
Examples: Adenoviruses, Herpesviruses.
II. RNA Viruses:
Contain RNA as their genetic material.
Examples: Influenza viruses, Coronaviruses.
2. Based on Replication Method:
I. Positive-Sense RNA Viruses:
RNA serves directly as mRNA.
Examples: Poliovirus, Hepatitis C virus.
II. Negative-Sense RNA Viruses:
RNA must be transcribed to mRNA.
Examples: Rabies virus, Ebola virus.
III. Reverse Transcribing Viruses:
RNA viruses that reverse transcribe into DNA.
Examples: HIV (retroviruses), Hepatitis B virus (hepadnaviruses).
3. Based on Morphology:
I. Icosahedral Viruses:
Capsid with icosahedral symmetry.
Examples: Adenoviruses, Polioviruses.
II. Helical Viruses:
Capsid with helical symmetry.
Examples: Influenza viruses, Tobacco mosaic virus.
III. Complex Viruses:
Combination of icosahedral and helical structures or other complex shapes.
Examples: Bacteriophages, Poxviruses.
4. Based on Host Range:
I. Animal Viruses:
Infect animals.
Examples: Rabies virus, SARS-CoV-2.
II. Plant Viruses:
Infect plants.
Examples: Tobacco mosaic virus, Potato virus Y.
III. Bacteriophages:
Infect bacteria.
Examples: T4 phage, Lambda phage.
Each of these groups can be further divided based on more specific characteristics and genetic differences.
Reproduction/Replication of Viruses
The replication cycle of viruses varies between different types, but generally includes the following stages:
1. Attachment:
The virus binds to specific receptors on the surface of the host cell.
2. Entry:
The viral genome enters the host cell through mechanisms such as fusion with the host membrane, endocytosis, or direct penetration.
3. Uncoating:
The viral capsid is removed, releasing the viral genome into the host cell.
4. Replication:
DNA Viruses: Replicate in the nucleus using the host's DNA polymerase (e.g., herpesvirus).
RNA Viruses: Replicate in the cytoplasm using viral RNA polymerase.
Positive-sense RNA Viruses: Can be directly translated into proteins.
Negative-sense RNA Viruses: Must be transcribed into positive-sense RNA by RNA-dependent RNA polymerase.
5. Transcription and Translation:
Viral mRNA is transcribed (if necessary) and translated into viral proteins using the host's ribosomes.
6. Assembly:
New viral particles are assembled from the replicated genome and synthesized proteins.
7. Release:
Newly formed viruses are released from the host cell through lysis (breaking open the cell) or budding (enveloped viruses).
Replication Cycles:
1. Lytic Cycle:
Virus replicates rapidly and lyses the host cell to release progeny viruses (e.g., bacteriophage T4).
2. Lysogenic Cycle:
Viral DNA integrates into the host genome and replicates along with it without causing immediate lysis.
It can later enter the lytic cycle (e.g., bacteriophage λ).
Cultivation of Viruses
Host Systems:
Cell Cultures: Viruses are grown in monolayers of cultured cells. Cell lines can be primary (directly taken from tissues) or continuous (immortalized).
Cytopathic Effect (CPE): Observable damage to host cells due to viral infection.
Embryonated Eggs: Used for the cultivation of certain viruses (e.g., influenza virus). Different parts of the egg (chorioallantoic membrane, amniotic fluid, yolk sac) are used for different viruses.
Laboratory Animals: Some viruses require a whole organism for propagation.
Detection and Quantification:
Plaque Assay: Measures the number of virus particles by counting plaques (clear zones) formed on a layer of host cells.
Hemagglutination Assay: Measures the ability of viruses to agglutinate red blood cells.
ELISA (Enzyme-Linked Immunosorbent Assay): Detects viral antigens or antibodies against viruses.
PCR (Polymerase Chain Reaction): Detects and quantifies viral DNA or RNA.
Safety and Ethical Considerations:
Proper biosafety protocols must be followed when working with viruses, especially pathogenic ones.
Ethical considerations are important when using animals for virus cultivation and research.