Viruses: Acellular Life Forms

Viruses: Acellular Life Forms

• Viruses are acellular entities, lacking cellular structures like a nucleus or cytoplasm.
• They are considered non-living outside a host cell as they cannot carry out metabolic activities or reproduce independently.
• Inside a host cell, they replicate using the host's machinery, making them obligate intracellular parasites.

Structure of Viruses

• Genetic Material: Contain either DNA or RNA, never both. Can be single-stranded or double-stranded.
• Capsid: A protein coat surrounding the genetic material, composed of protein subunits called capsomeres.
• Envelope (some viruses): A lipid membrane derived from the host cell, containing glycoprotein spikes for attachment to host cells.
• Shape and Symmetry: Helical, icosahedral, or complex (e.g., bacteriophages).

Classification of Viruses

• Based on Genetic Material: DNA Viruses (e.g., Adenoviruses, Herpesviruses), RNA Viruses (e.g., Retroviruses (HIV), Influenza virus).
• Based on Host Range: Bacteriophages (infect bacteria), Animal Viruses (infect animals and humans), Plant Viruses (infect plants).
• Based on Replication Mechanism: Lytic Cycle (leads to destruction of the host cell), Lysogenic Cycle (viral DNA integrates into the host genome).

Viral Diseases

• AIDS (Acquired Immunodeficiency Syndrome): Caused by HIV (a retrovirus).

  • Symptoms: Severe weakening of the immune system, increased susceptibility to opportunistic infections and certain cancers.
  • Transmission: Bodily fluids (blood, semen, vaginal secretions, breast milk), not through casual contact.
  • Mechanism: HIV targets CD4+ T-helper cells, impairing immune function.
  • Prevention: Safe sex practices, screening blood products, avoiding needle sharing.

• Other Viral Diseases:

  • Influenza: Respiratory illness caused by influenza viruses.
  • Hepatitis B and C: Affect the liver, transmitted through blood and bodily fluids.
  • Polio: Affects the nervous system, can cause paralysis.
  • Measles, Mumps, Rubella: Highly contagious, preventable by MMR vaccine.
  • COVID-19 (if within the syllabus): Caused by SARS-CoV-2 coronavirus.

Discovery of Viruses

• Late 19th century: Discovery of agents smaller than bacteria.
• Tobacco Mosaic Virus (TMV): First virus discovered.
• Contributions by scientists like Dmitri Ivanovsky and Martinus Beijerinck.

Key Concepts for MDCAT

• Differences Between Viruses and Bacteria: Viruses are acellular, bacteria are unicellular prokaryotes. Antibiotics are ineffective against viruses.
• Viral Replication Cycles:
  • Lytic Cycle: Attachment → Penetration → Biosynthesis → Maturation → Release. Results in the lysis (destruction) of the host cell.
  • Lysogenic Cycle: Viral DNA integrates into the host genome as a prophage. Replicates along with host DNA until triggered to enter the lytic cycle.
  • Retroviruses: Use reverse transcriptase to convert RNA into DNA, integrate into the host genome (e.g., HIV).

General Biology Concepts

• Biodiversity: Understanding the variety of life forms, including acellular entities like viruses.
• Role of Viruses in Biotechnology: Used as vectors in genetic engineering, gene therapy applications.

Important Terminology

• Capsid: Protein shell of a virus.
• Capsomeres: Protein subunits that make up the capsid.
• Envelope: Lipid layer surrounding some viruses.
• Bacteriophage: Virus that infects bacteria.
• Prophage: Viral DNA integrated into bacterial DNA.
• Host Range: Spectrum of hosts a virus can infect.
• Virion: Complete virus particle.

Preparation Tips for MDCAT

• Understand Core Concepts: Focus on the fundamental properties of viruses, grasp the differences between acellular and cellular organisms.
• Memorize Key Facts: Types of genetic material in viruses, steps of viral replication cycles.
• Study Viral Diseases: Know the causative agents, transmission modes, symptoms, and prevention.
• Practice MCQs: Solve past MDCAT questions related to virology.
• Revise Regularly: Keep revisiting the key points to reinforce memory.

Explore the fascinating world of viruses, the acellular entities that lack cellular structures. Learn about their unique characteristics, including their genetic material, structure, and classification. This quiz will enhance your understanding of how viruses function as obligate intracellular parasites.