Classification and Replication of RNA and DNA Viruses

Chapter 13: Viruses

Overview of Viral Classification

Viruses are classified based on the type of nucleic acid they contain—either RNA or DNA. This distinction is fundamental in microbiology, as it determines the replication strategies and pathogenic mechanisms of viruses.

• RNA Type Viruses: Viruses whose genetic material is ribonucleic acid (RNA).

• DNA Type Viruses: Viruses whose genetic material is deoxyribonucleic acid (DNA).

RNA Viruses

Types of RNA Viruses and Their Replication Strategies

RNA viruses are further classified based on the sense and structure of their RNA genome. The replication mechanism varies depending on whether the RNA is single-stranded (ss) or double-stranded (ds), and whether it is positive-sense (+) or negative-sense (−).

• Positive-sense RNA (+ssRNA): The viral RNA can act directly as messenger RNA (mRNA) and be translated by host ribosomes to produce viral proteins.

• Negative-sense RNA (−ssRNA): The viral RNA is complementary to mRNA and must be converted to positive-sense RNA by an RNA-dependent RNA polymerase before translation.

• Double-stranded RNA (dsRNA): Contains both a positive and negative strand; replication involves synthesis of mRNA from the negative strand.

• Retroviruses (+ssRNA subgroup): These viruses, such as HIV, use reverse transcriptase to convert their RNA genome into DNA, which integrates into the host genome.

Replication Mechanisms

• +ssRNA Viruses:

  • Example: Polio virus

  • Host ribosomes translate viral RNA directly into proteins.

  • Viral RNA polymerase synthesizes complementary −ssRNA, which serves as a template for more +ssRNA.

• −ssRNA Viruses:

  • Cannot be directly translated by host ribosomes.

  • Carry RNA-dependent RNA polymerase to synthesize +ssRNA from −ssRNA.

  • +ssRNA then serves as mRNA for protein synthesis.

• dsRNA Viruses:

  • Example: Rotavirus

  • Use −ssRNA strand as template to make mRNA.

  • Replication involves synthesis of both strands for new viral genomes.

• Retroviruses (+ssRNA subgroup):

  • Example: HIV

  • Use reverse transcriptase enzyme to make DNA from RNA template.

  • DNA integrates into host genome, serving as template for new viral RNA and proteins.

Key Formula

General equation for RNA virus replication:

$\text{Viral RNA} \xrightarrow{\text{RNA-dependent RNA polymerase}} \text{Complementary RNA} \xrightarrow{\text{Translation}} \text{Viral Proteins}$

Examples and Families of RNA Viruses

RNA viruses are grouped into families based on their genetic and structural characteristics. Below is a summary table of major RNA virus families and representative viruses:

Family	Representative Virus
Picornaviridae	Enterovirus (polio), Hepatovirus (hepatitis A), Rhinovirus (common cold)
Togaviridae	Rubivirus (rubella)
Coronaviridae	Coronavirus (common cold, SARS)
Retroviridae	Human T cell leukemia virus (leukemia), Lentivirus (AIDS)
Orthomyxoviridae	Influenza virus (flu)
Rhabdoviridae	Lyssavirus (rabies)
Reoviridae (dsRNA)	Rotavirus (severe diarrhea)

DNA Viruses

Types and Examples of DNA Viruses

DNA viruses are classified based on the structure of their DNA (single-stranded or double-stranded) and their family. They are responsible for a variety of diseases in humans.

• Poxviridae: Example: Smallpox virus

• Herpesviridae: Includes Herpes simplex virus type 1 (fever blisters), Herpes simplex virus type 2 (genital infections), Varicellovirus (chickenpox), Lymphocytopvirus (Epstein-Barr virus: mononucleosis, Burkitt’s lymphoma), Cytomegalovirus (birth defects)

• Papillomaviridae: Example: Papilloma virus (benign tumors, warts, cervical & penile cancers)

• Hepadnaviridae: Example: Hepatitis B virus (Orthopadnavirus)

Summary Table: DNA Virus Families and Diseases

Family	Representative Virus	Associated Disease
Poxviridae	Smallpox virus	Smallpox
Herpesviridae	Herpes simplex virus type 1 & 2, Varicellovirus, Lymphocytopvirus, Cytomegalovirus	Fever blisters, genital infections, chickenpox, mononucleosis, birth defects
Papillomaviridae	Papilloma virus	Warts, cervical & penile cancers
Hepadnaviridae	Hepatitis B virus	Hepatitis B

Comparative Summary: RNA vs DNA Viruses

Both RNA and DNA viruses are significant in human disease. Their replication strategies and associated diseases differ based on their genetic material.

• RNA Viruses: Often mutate rapidly, leading to frequent changes in antigenicity (e.g., influenza, HIV).

• DNA Viruses: Generally more stable, can establish latent infections (e.g., herpesviruses).

Example Application: Understanding the replication mechanisms of viruses is crucial for developing antiviral drugs and vaccines. For instance, reverse transcriptase inhibitors are used to treat HIV infections.

Additional info: Some virus families may contain both single-stranded and double-stranded genomes, and certain viruses (e.g., hepatitis B) have partially double-stranded DNA.