Viruses

Definition and Examples of Viruses

• A virus is a non-cellular infectious agent consisting of nucleic acid enclosed in a protein coat.

Characteristics of Viruses

• Size range: nanometers (1nm = 10^{-9}m).
• Viruses are obligate intracellular parasites, not alive outside living cells.
• Simple structure: protein coat and genetic material (DNA or RNA).
• Multiply/reproduce inside living cells using the host's biosynthetic machinery.
• Lack metabolic enzymes and organelles for protein production.
• Recognition between viruses and host cells is specific.

Structure of Viruses

• Protein coat: capsid, made of capsomeres.
• Genetic material: RNA or DNA.
• Shapes: helical, icosahedral, enveloped, complex.
• Virion: infectious form of virus with complete structure outside of living hosts.

Virus Classification

• Baltimore Classification System: based on method of replication (lytic or lysogenic) and genome type (DNA or RNA).
• International Committee on Taxonomy of Viruses (ICTV): devises rules for naming and classifying viruses with a hierarchy involving order, family, and genus.

Classification Based on Baltimore

• I: dsDNA viruses (e.g., Adenoviruses, Herpesviruses, Poxviruses)
• II: ssDNA viruses (+ strand or "sense") DNA (e.g., Parvoviruses)
• III: dsRNA viruses (e.g., Reoviruses)
• IV: (+)ssRNA viruses (+ strand or sense) RNA (e.g., Picornaviruses, Togaviruses)
• V: (−)ssRNA viruses (− strand or antisense) RNA (e.g., Orthomyxoviruses, Rhabdoviruses)
• VI: ssRNA-RT viruses (+ strand or sense) RNA with DNA intermediate (e.g., Retroviruses)
• VII: dsDNA-RT viruses DNA with RNA intermediate (e.g., Hepadnaviruses)

Bacteriophages

• Viruses that attack only bacteria.
• Complex structures with icosahedral heads enclosing DNA.
• Model viruses for studying viral life cycles.

Mode of Reproduction: Lytic Cycle

1. Attachment: Phage attaches to specific site on bacterium.
2. Penetration: Phage injects DNA into bacterium, hydrolyzing bacterial DNA.
3. Replication: Phage DNA directs synthesis of viral nucleic acids and proteins.
4. Assembly: Viral proteins assemble to form phage heads, tails, and tail fibers; genetic material is packaged inside.
5. Release: Bacterial cell wall is damaged, cell bursts, and new viruses are released..

Mode of Reproduction: Lysogenic Cycle

1. Attachment: Phage attaches to specific site on bacterium.
2. Penetration: Phage injects DNA into bacterium, forming a circle.
3. DNA recombination: Phage DNA integrates into bacterial DNA, becoming a prophage.

• Prophage remains silent and replicates with host DNA during binary fission.
• Environmental signals can activate prophage to initiate lytic cycle.

Variation of Life Cycle in Enveloped Virus at Penetration Step

• Viruses enter host cell through endocytosis.
• Attachment is specific to host protein receptor.
• Plasma membrane forms a vesicle containing the virus.
• Viral capsid and nucleic acid are released inside the cell (e.g., HIV, Semliki Forest Virus).

Variation of Life Cycle in Enveloped Virus at Replication Step

• DNA Virus
  • Viral DNA is replicated and transcribed inside the nucleus.
  • Viral proteins synthesized outside the nucleus (at ribosomes).
  • New virus particles are assembled, glycoprotein spike assembled as they leave the cell.
• RNA Virus
  • Viral genome serves as template for complementary RNA strand synthesis (outside nucleus).
  • Complementary strands become mRNA and are translated.
  • Viral genome RNA is made from complementary strands.
  • Assembly and release steps occur outside of the nucleus.

Replication of Retrovirus (RNA)

• Retrovirus attaches and penetrates via endocytosis.
• Viral capsid is digested (uncoating).
• Reverse transcriptase synthesizes complementary DNA strands from viral RNA.
• DNA integrates into host cell’s DNA as a provirus in the nucleus.
• Viral genes are replicated and transcribed into mRNA.
• mRNA is translated at ribosomes to build viral components.

Viral Disease

• Examples presented: Mosaic disease in tobacco plant (TMV), Banana Bunchy Top Virus, Shingles, Smallpox and symptoms of Microcephaly.

Viruslike Agent - Viroids

• Noncellular infectious agents with a single circular RNA strand and no protein coat
• Do not code for proteins; rely on host cell's proteins for reproduction.
• Affect only plant cells (e.g., coconuts, tomatoes, potatoes).

Viruslike Agent - Prions

• Abnormal PrP protein coded in mammal genes.
• Do not contain genetic material.
• Cause transmissible spongiform encephalopathies (TSEs) in animals.
• Create spongy texture in brain tissue.
• Examples: Scrapie (sheep), mad cow disease (ruminants), Kuru/Creutzfeldt-Jakob (humans).

Comparison Between Viroids & Prions