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Epidemics and medicine >> Miscellaneous >> Virus infection

Sections on virus infection

• Discovery of virus



• General structure of virus



• Classification of virus



• Viral reproductive cycle
  
  
  • DNA-based
  
  
  
  • RNA-based
    

Discovery of virus

Virus was formally recognized as microscopic bacteria. When German scientist Adolf Mayer attempted to determine the cause of tobacco mosaic disease in 1883, he observed that the disease could be transmitted to other plants by spraying sap from infected leaves. He later claimed that the disease was caused by small bacteria that could not be seen under the microscope.

Figure 1: Tobacco mosaic virus (TMV). Discovery of TMV was a scientific breakthrough to modern microbiology, uncovering identities of "unseen" living forms.

Figure 2: Wendell Stanely. Stanely won the 1946 Nobel prize in chemistry for his crucial research on chemical structures of viruses.

A decade after Mayer's finding, Dimitri Ivanowsky sought to remove the pathogenic bacteria by using the filter. The disease, however, continued to infect plants in vicinity. Ivanowsky's demonstration confirmed the existence of a living form smaller than bacteria. Furthermore, Martinus Beijernick in 1897 showed that the agent of the disease was able to reproduce despite the filtering. Finally, Wendell Stanley proved the existence of virus when he successfully crystallized the infectious particle.

General structure of virus

Genome: Viruses may have different types of genes to propagate themselves. Viral genomes include double-stranded DNA, singled-stranded DNA, double-stranded RNA, and singled-stranded RNA. Protective proteins: Viruses may have a protective protein shell, capsid, and a membranous envelope. Some viruses contain few functional enzymes in their capsids. Different types of viruses have different shape of the capsid. Diagram on the left, for instance, is an example of the adenovirus capsid. Glycoprotein: Glycoprotein is a protein bonded to a chain of carbohydrates. It may function as a tool used for cell-to-cell recognition.

Courtesy of Biology, 5th edition

Classification of virus

Virus classification	Description	Diseases
Hepatovirus	Uses single-stranded RNA as mRNA	Hepatitis A
Poxvirus	Uses double-stranded DNA for replication	Smallpox, vaccinia
Filovirus	Uses single-stranded RNA for replication	Ebola, marburg virus
Retrovirus	Uses single-stranded RNA for reverse transcription	AIDS, HIV, leukemia
Coronavirus	Uses single-stranded RNA for replication	SARS

For more detailed classification, please visit Wikipedia virus classification.

Viral reproductive cycle (DNA-based)

In step (1), a virus enters a host cell and injects its DNA strands and capsids into the cell. In step (2a), the virus uses the host cell's DNA polymerase to replicate its viral DNA strands. On the other hand, in steps (2b) and (3), the virual DNA produces capsids via RNA transcription and translation. After assembly of vital parts, the newly formed DNA and capsids fuse to form a new virus in step (4).

Viral reproductive cycle (RNA-based)

In step (1), a virus enters a host cell and injects its RNA strand and capsids into the cell. In step (2a), the virus uses the host cell's resources to generate a new RNA by replicating a template RNA. In steps (2b), mRNA generates new capsids through RNA translation. In step (3), newly formed RNAs and capsids fuse together to form a new RNA virus.