Acellular Life | Best 11th Class Biology Notes | Federal Board
Chapter 5 :
ACELLULAR LIFE
1.1 Viruses: Discovery & Structure
Characteristics
✅ Living:
- Contain genetic material (DNA/RNA)
- Capable of mutation
- Replicate inside a host
- Sensitive to UV radiation
❌ Nonliving:
- No cellular structure
- Can be crystallized
- No independent metabolism
History
- Virus (Latin: "venom" = poison)
- 1892 – Iwanowsky discovered Tobacco Mosaic Disease, proving viruses are filterable
Classification
📌 Baltimore System (I–VII):
| Group | Type | Example |
|---|---|---|
| I | dsDNA | HSV-1 |
| II | ssDNA | Parvovirus |
| III | dsRNA | Rotavirus |
| IV | +ssRNA | Hepatitis C |
| V | -ssRNA | Influenza |
| VI | Retroviruses | HIV |
| VII | RT dsDNA | Hepatitis B |
📌 Host-based:
- Plant Viruses: TMV
- Bacteriophages: Infect bacteria
- Animal Viruses: HIV, Influenza
Structure
- Core: DNA/RNA
- Capsid: Protein coat (made of capsomers)
- Nucleocapsid = Core + Capsid
- Envelope (optional): Lipid membrane from host
- Symmetry:
- Icosahedral (20 faces) – e.g., Adenovirus
- Helical (rod-shaped) – e.g., TMV
Shapes & Sizes
| Shape | Example | Size Range |
|---|---|---|
| Spherical | HIV | 80–120 nm |
| Rod-shaped | TMV | ~300 nm |
| Tadpole | Bacteriophage | ~200 nm |
1.2 Parasitic Nature of Viruses
📌 Obligate Parasites – Cannot replicate outside a host.
- Host specificity: HIV targets CD4+ T-cells.
📌 Life Cycle
1️⃣ Adsorption & Penetration – Virus binds & enters the host.
2️⃣ Uncoating – Genome is released.
3️⃣ Replication – Viral genome hijacks host machinery.
4️⃣ Assembly – New virions form.
5️⃣ Release – By budding (enveloped) or lysis (non-enveloped).
📌 Immune Evasion
- Blocks cytokines
- Suppresses MHC
- Rapid mutations
📌 Survival Strategies
- Crystallization – TMV can be stored as crystals.
- Transmission via fluids – Saliva, blood, semen.
1.3 Bacteriophage Life Cycle
Lytic Cycle
➡ Attachment → DNA injection → Host hijack → Replication → Lysis
Lysogenic Cycle
➡ DNA integrates (prophage) → Host replication → Reactivation → Lysis
📌 Applications
- Used as vectors in genetic engineering
- Phage therapy for antibiotic-resistant bacteria
1.4 HIV Life Cycle
Structure
- Envelope: gp120, gp41
- Core: RNA + Reverse Transcriptase
Infection Stages
1️⃣ CD4 binding → Fusion
2️⃣ Reverse transcription → DNA synthesis
3️⃣ Integration → Provirus formation
4️⃣ Assembly & Release → Budding
📌 AIDS Progression
- Asymptomatic phase
- ARC (AIDS-related complex) – Swollen lymph nodes, fatigue
- Full-blown AIDS – Opportunistic infections (TB, pneumonia)
📌 Treatment & Prevention
- Antiretrovirals (ARTs) – Reverse transcriptase & protease inhibitors
- Prevention – Safe sex, needle hygiene
1.5 Viral Diseases
| Disease | Transmission | Features | Treatment |
|---|---|---|---|
| Hepatitis A | Fecal-oral | Acute liver infection | Vaccine |
| Hepatitis B | Blood, sex | Chronic hepatitis | Antivirals |
| Hepatitis C | Blood | Liver cirrhosis risk | Direct-acting antivirals |
| Herpes (HSV-1) | Oral | Cold sores | Acyclovir |
| Herpes (HSV-2) | Sexual | Genital ulcers | Acyclovir |
| Polio | Fecal-oral | Paralysis risk | Salk (IPV) & Sabin (OPV) vaccines |
| Cotton Leaf Curl | Whitefly | Affects cotton crops | No cure |
1.6 Prions & Viroids
📌 Prions (Protein-only infectious agents)
- Cause spongiform encephalopathies:
- Creutzfeldt-Jakob Disease (CJD)
- Mad Cow Disease (BSE)
- Resistant to heat & UV radiation
📌 Viroids (Circular RNA pathogens)
- Only infect plants (e.g., Potato Spindle Tuber Disease)
