HL Viruses

Viruses

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

Size of Viruses

All viruses are very small, typically less than 300 nm in size.

Virus Composition

Viruses consist of nucleic acid (DNA or RNA) and a protein capsid, and they may also have a lipid envelope.

Replication of Viruses

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

Genetic Material Injection

Viruses infect host cells by injecting their genetic material (DNA or RNA).

Lytic Cycle Steps

The cycle includes virus attachment, genetic material injection, DNA replication, protein assembly, and host cell lysis.

outcome of Lytic Cycle

The lytic cycle leads to host cell death and spreads the virus to nearby cells.

Virulence Balance

The balance of virulence is critical for viruses; too high virulence kills all hosts, while too low virulence makes them detectable by the immune system.

Lysogenic Cycle Function

In the cycle, the virus integrates its DNA into the host DNA and replicates silently during host cell division.

Cell Death in Lysogenic Cycle

The cycle does not cause immediate cell death; the virus remains undetectable until activated.

Triggers for Lytic Cycle

Certain stimuli can trigger a switch from the lysogenic cycle to the lytic cycle.

Convergent Evolution of Viruses

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

Universal Genetic Code

The universal use of genetic code among viruses suggests functional convergence.

Progressive Hypothesis

The progressive hypothesis suggests that viruses evolved from mobile genetic elements.

Regressive Hypothesis

The regressive hypothesis suggests that viruses evolved from simplified cells.

Mutation Rate in Viruses

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

Impact of Mutations

Mutations create variation and natural selection favors variants that can evade immune detection.

Influenza Virus Characteristics

Influenza virus uses RNA with 8 segments, allowing for frequent mutations and new strains.

Antigen Mutation in Influenza

Antigens on the influenza viral surface mutate, making previous immunity ineffective.

HIV Overview

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

Strain Variation in HIV

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

HIV's Drug Resistance

Rapid mutation of HIV makes it highly evasive and often resistant to treatment.

Vaccine and Drug Control in Viruses

The rapid mutation of viruses makes controlling them with vaccines and drugs difficult.

Adaptability of Viruses

Viruses demonstrate diversity and adaptability through their structure, infection mechanisms, and evolutionary strategies.