🧬 Classification of Viruses (MDCAT Biology Notes By Syed Ali Academy)

Viruses are obligate intracellular parasites and can be classified based on different criteria:

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a. Classification Based on Host

Host Type	Description	Examples
Phytophage	Infect plants; mostly RNA genome; rod-shaped	TMV (Tobacco Mosaic Virus), CaMV
Zoophage	Infect animals; DNA or RNA genome; spherical	Rhinoviruses, COVID-19
Bacteriophage	Infect bacteria; dsDNA genome; head and tail	T-phages, Lambda phages

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b. Classification Based on Capsid Shape

Capsid Shape	Virus Examples
Helical	TMV
Polyhedral	Adenoviruses
Tadpole-shaped	Bacteriophages
Enveloped	Flu viruses, HIV
Spherical	COVID-19
Circular	Cauliflower Mosaic Virus (CaMV)

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c. Classification Based on Genome Type (Baltimore Classification)

By David Baltimore (1971), modified in 2018–2019.

1. DNA Viruses

• i. Double-Stranded DNA Viruses: Use host enzymes for mRNA synthesis in nucleus.
  🦠 Example: Herpesvirus

• ii. Single-Stranded DNA Viruses: Convert into dsDNA first before transcription.
  🦠 Example: Parvovirus

2. RNA Viruses

• iii. Double-Stranded RNA Viruses: Use host enzymes to produce mRNA from dsRNA.
  🦠 Example: Reoviruses

• iv. Positive Sense Single-Stranded RNA Viruses: RNA acts directly as mRNA.
  🦠 Example: Coronavirus, Dengue, Hepatitis C

• v. Negative Sense Single-Stranded RNA Viruses: RNA is transcribed to mRNA first.
  🦠 Example: Rhabdovirus, Paramyxovirus

3. Reverse Transcribing Viruses

• vi. ssRNA with DNA Intermediate: Uses reverse transcriptase to make DNA.
  🦠 Example: HIV (Retrovirus)

• vii. dsDNA with RNA Intermediate: DNA makes RNA, which is reverse transcribed.
  🦠 Example: Hepatitis B Virus

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d. Classification Based on Diseases

Virus Type	Disease	Examples
Plant Viruses	Mosaic diseases	TMV, CaMV
Animal Viruses	Flu, COVID-19, Hepatitis	Influenza virus, Coronavirus, Hepatitis B & C
Bacteriophages	Bacterial infections	T-phages, Lambda phages
Retroviruses	AIDS	HIV
Oncogenic Viruses	Cancers	HPV, Epstein-Barr Virus
Hepadnaviruses	Liver diseases	Hepatitis B Virus

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🦠 HIV and AIDS

1. Cause

AIDS is caused by HIV (Human Immunodeficiency Virus), a retrovirus that uses reverse transcription to insert its genome into host cells.

2. Structure of HIV

• Contains two single-stranded RNA molecules

• Enveloped virus with lipid bilayer

• Key enzymes: Reverse Transcriptase, Integrase

• Surface glycoproteins (e.g., gp120) help attach to host T-helper cells

3. Symptoms of AIDS

Stage	Symptoms
Initial Infection	Fever, flu-like symptoms, headache (within 6 weeks)
Latent Phase	No major symptoms; swollen lymph nodes
Late Stage (AIDS)	Opportunistic infections (TB, pneumonia, cancers), dementia, weight loss, diarrhea

4. Transmission of HIV

• Blood transfusion with contaminated blood

• Sharing unsterilized needles/syringes

• Sexual contact (via urogenital secretions)

• Mother to child through placenta

5. Control & Prevention

• Avoid sharing needles

• Safe sex practices

• Screening blood before transfusion

• No vaccine or cure yet, but opportunistic infections can be treated

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