HL Viruses

Viruses are not considered living organisms since they do not perform all functions of life and are not made of cells.

They likely evolved independently multiple times, unlike living organisms which share a common ancestor.

Structure and Composition

  • All viruses are very small (<300 nm).

  • They consist of nucleic acid (DNA or RNA) and a protein capsid.

  • They lack cytoplasm and most enzymes, relying on host cells for replication.

  • Some may also have a lipid envelope.

Viruses infect host cells by injecting their genetic material (DNA or RNA). This can initiate either the lytic or lysogenic cycle.

Lytic Cycle

  • Virus attaches to host → injects genetic material → replicates DNA → transcribes and translates proteins → assembles new viruses → bursts the host cell (lysis).

  • Leads to host cell death and spreads to nearby cells.

  • Balance of virulence is key: too high and it kills all hosts; too low and it's detected by the immune system.

Lysogenic Cycle

  • Virus injects DNA, which integrates into host DNA.

  • As the host cell divides, it replicates viral DNA silently.

  • No immediate cell death; virus is undetectable until activated.

  • Certain stimuli can trigger a switch to the lytic cycle

Evolution of Viruses

  • Viruses may have evolved through convergent evolution, developing similar traits from different origins.

  • The universal use of genetic code suggests a functional convergence.

  • Two hypotheses:

    • Progressive: evolved from mobile genetic elements.

    • Regressive: evolved from simplified cells.

Rapid Evolution and Mutation

  • Viruses mutate rapidly due to short generation times and lack of proofreading, especially RNA viruses.

  • Mutations create variation, and natural selection favors variants that evade immune detection

    Influenza Virus Case Study

    • Uses RNA (8 segments), allowing for frequent mutations and new strains.

    • Antigens on viral surface mutate, making previous immunity or vaccines ineffective.

    • Annual flu shots target specific strains but may not cover new mutations.

    HIV Case Study

    • HIV is an RNA virus that reverse transcribes RNA into DNA, increasing mutation risk.

    • Multiple strains can infect the same person and recombine, complicating treatment.

    • Rapid mutation makes it highly evasive and drug-resistant.

Viruses' structure, infection mechanisms, and evolutionary strategies illustrate diversity and adaptability. Their ability to mutate rapidly makes controlling them with vaccines and drugs difficult.