IB Biology HL - Unit A2.3: Viruses

Obligate intracellular parasites

AKA Viruses, must infect cells to reproduce.

Non-living Characteristic of Viruses

To lack cellular structure and metabolism.

Fixed size of Viruses

Viruses typically range from 20-300 nm.

Nucleic acid Composition of Viruses

Viruses contain either DNA or RNA.

Protein capsid

Protective outer shell made of protein subunits.

Viruses contain no cytoplasm

Viruses lack cellular organelles and cytoplasm.

Viruses with enzymes

Some viruses carry specific enzymes like reverse transcriptase.

Enveloped viruses

Viruses with a lipid membrane from host cells.

Non-enveloped viruses

Viruses without a lipid membrane, e.g., bacteriophages.

Positive-sense RNA

RNA can directly act as mRNA for translation.

Negative-sense RNA

RNA must be transcribed into mRNA first.

Retroviruses

Contain ssRNA and use reverse transcriptase to make DNA.

Lytic cycle

Viral reproduction involving host cell lysis.

Lysogenic cycle

Viral genome integrates into host genome without killing.

Viral attachment

First step in both lytic and lysogenic cycles.

Viral genetic material entry

Second step where virus injects its genome.

Viral replication

Host cell replicates viral genetic material.

Viral protein synthesis

Host cell synthesizes viral proteins.

Viral assembly

New virus particles are assembled within host.

Host cell lysis

Host cell bursts, releasing new viruses.

Convergent evolution

Unrelated viruses share features due to similar lifestyles.

High mutation rates

RNA viruses mutate rapidly, aiding evolution.

Recombination

Swapping genetic material between co-infecting viruses.

Short generation times

Eclipse periods range from 8 to 72 hours.

Large population sizes

High numbers increase variation and mutation chances.

Antigenic drift

Small changes in virus proteins evade immune detection.

HIV drug resistance

HIV mutates quickly, requiring drug cocktails for treatment.