viruses

Causes of Diseases in Humans

  • All five kingdoms of living organisms can cause diseases in humans:

    • Monera (prokaryotes):

    • Example: Typhoid disease caused by Salmonella typhi.

    • Protista:

    • Example: Malaria caused by Plasmodium parasite.

    • Fungi:

    • Example: Athlete's foot caused by Trichophyton fungus.

    • Animalia:

    • Example: Scabies caused by mites.

    • Plantae:

    • Example: Allergic reactions to plants such as poison ivy.

Non-Living Causes of Diseases

  • Some non-living agents can also cause diseases in humans:

    • Prions:

    • Definition: A prion is a type of protein that triggers normal proteins in the brain to behave abnormally.

    • Example: Diseases caused by prions include Creutzfeldt-Jakob disease (CJD).

    • Viruses:

    • Definition: A virus is a submicroscopic infectious agent that can only replicate within the living cells of a host organism.

    • Example: Diseases caused by viruses include AIDS.

General Characteristics of Viruses

  • Obligate Intracellular Parasites:

    • Viruses cannot replicate independently; they require a host.

  • Induction of Host Cell Activities:

    • Once inside a living cell, viruses trigger the host cell to synthesize new virus particles.

  • Genetic Material:

    • The viral genome can be either DNA or RNA and can exist in single or double-stranded forms.

  • Energy Production:

    • Viruses do not possess the machinery to produce ATP.

  • Size Range:

    • Viruses vary in size from 25 to 270 nanometers.

  • Viral Tropism:

    • Refers to the spectrum of tissues or cells that a virus can infect.

Viral Structure

  1. Virion: The complete viral particle.

  2. Capsid: The protein coat surrounding the genetic material of the virus.

  3. Capsomer: The protein subunit that constitutes the capsid.

  4. Nucleocapsid: The combination of the capsid and viral genetic material.

  5. Envelope:

    • Definition: An outer layer comprising a phospholipid bilayer embedded with viral glycoproteins.

    • Derived from the host cell's membrane (can be from plasma membrane or other membranes).

    • Naked Nucleocapsid: Viruses without an envelope.

Types of Viruses

  • Helical Viruses:

    • Example: Tobacco Mosaic Virus

  • Polyhedral Viruses:

    • Example: Adenovirus

  • Spherical Viruses:

    • Example: Influenza virus

  • Complex Viruses:

    • Example: Bacteriophage

Viral Classification and Replication

Classification Based on Genome Structure

  • DNA Viruses:

    • Linear Genome: E.g., Adenoviruses

    • Circular Genome: E.g., Papillomaviruses

  • RNA Viruses:

    • Single-stranded Positive Sense RNA: E.g., Picornaviruses

    • Single-stranded Negative Sense RNA: E.g., Orthomyxoviruses

    • Retroviruses: E.g., HIV

  • Double-stranded DNA: E.g., Poxviruses

  • Double-stranded RNA: E.g., Reoviruses

Viral Life Cycle

  • Life Cycle Steps:

  1. Attachment: Virus binds to the host cell.

  2. Penetration: Virus enters the host cell.

  3. Biosynthesis: Host cell synthesizes viral components.

  4. Assembly: New viral particles are assembled.

  5. Release: New virions are released from the host cell.

Lytic vs. Lysogenic Cycle

Lytic Cycle:

  • Process includes the binding of bacteriophage to bacterium, penetration of phage DNA, transcription, translation of phage DNA, assembly, and lysis of the cell releasing new phage.

Lysogenic Cycle:

  • Process involves integration of phage DNA as a prophage in the bacterial chromosome, which can replicate across cell divisions and may later induce lytic cycle.

Characteristics of Retroviruses

  • Definition: Retroviruses are RNA viruses that replicate in a host cell through reverse transcription.

  • Structure:

    • Enveloped virion with a diameter of 100 nm.

    • Contains linear positive single-stranded RNA genome, usually packaged as two identical genomes (7-10 kb).

    • Contains reverse transcriptase enzyme for converting RNA to DNA.

  • Associated Diseases:

    • They cause diseases such as AIDS, leukemia, and some types of cancer.

Example Retroviruses and Their Diseases

  • HIV-1 and HIV-2:

    • Affiliation: Lentivirus (causes AIDS).

  • HTLV-1:

    • Affiliation: Deltaretrovirus (causes Adult T-cell leukemia/lymphoma).

  • Hepatitis viruses (B and C):

    • Potentially cause liver cancers.

Neoplasm / Tumor

  • Definition: An abnormal tissue mass from excessive cell growth or failure to die; can be benign (non-cancerous) or malignant (cancerous).

Cell Cycle Regulation and Cancer

  • Regulatory Mechanisms:

    • Tumor suppressor genes, proto-oncogenes, and DNA repair genes regulate the cell cycle.

    • Tumor Suppressor Genes:

    • Function: Stop cell cycle progression and promote apoptosis.

    • Proto-oncogenes:

    • Function: Stimulate cell growth and division; when mutated, can become oncogenes leading to cancer.

  • Mutation Examples:

    • Burkitt lymphoma due to MYC gene translocation, leading to overexpression of cyclins.

    • Philadelphia chromosome involving fusion gene BCR-ABL which causes uncontrolled cell growth in chronic myeloid leukemia.

Oncogenic Viruses

  • Definition: Viruses that can induce cancer.

  • Notable Oncogenic Viruses:

    • Human Papillomavirus (HPV): Causes cervical carcinoma.

    • Epstein-Barr Virus (EBV): Associated with Hodgkin lymphomas.

    • Human T-lymphotropic Virus (HTLV): Causes Adult T-cell leukemia.

    • Kaposi's Sarcoma-associated Herpesvirus (KSHV): Causes Kaposi's sarcoma.

    • Merkel Cell Polyomavirus: Associated with Merkel cell carcinoma.

    • Hepatitis B and C Viruses: Linked to hepatocellular carcinoma.