Viruses (A2.3) IB Biology HL

Flashcards on viruses covering structure, diversity, replication cycles, origins, and evolution.

Viruses

Obligate intracellular parasites that must infect cells and reproduce inside them. They are non-living because they are not made of cells, cannot maintain stability, do not grow, cannot replicate themselves, and cannot perform independent metabolism.

Virus Structure

Small, fixed size (typically 20–300 nm), contain nucleic acid (DNA or RNA but never both), have a protein capsid made of capsomeres, no cytoplasm or cell organelles, few or no enzymes, and require a host cell for replication and metabolism.

Viral Diversity

Viruses vary in size, shape, genetic material (DNA or RNA, single-stranded or double-stranded, circular or linear), genome size and type, and may or may not have an envelope.

Virus Envelopes

Some viruses are enveloped with a lipid membrane derived from the host cell (e.g., HIV, coronaviruses), while others are non-enveloped (naked) (e.g., bacteriophages).

Positive-sense RNA viruses

Genome can directly act as mRNA (e.g., coronaviruses).

Negative-sense RNA viruses

Must be transcribed into mRNA before translation (e.g., influenza).

Retroviruses

Contain ssRNA and use reverse transcriptase to make DNA (e.g., HIV).

Bacteriophage Lambda

A dsDNA virus with a complex capsid structure, infecting E. coli bacteria, alternating between lytic and lysogenic cycles.

SARS-CoV 2

A ssRNA(+) virus infecting mammal epithelium cells with an ACE2 receptor protein, causing COVID-19 and has a lytic lifecycle.

HIV

A retrovirus with two copies of ssRNA, infecting primate T-cells with a CD4 receptor protein, causing AIDS, and has a lytic lifecycle.

Lytic Cycle

The main form of viral reproduction where the virus relies on the host cell for energy, nutrition, and protein synthesis, leading to lysis of the host cell and spread of new virus particles.

Lysogenic Cycle

The virus assimilates its genome within the host cell’s genome for replication without killing the host cell. Viral genetic material is integrated into the host cell genome and replicated during mitotic cell division.

Viruses-first hypothesis

Viruses predate or coevolved with their current cellular hosts, NOT supported by current evidence.

Progressive hypothesis

Viruses arose by taking and modifying cell components; supported by current evidence.

Regressive hypothesis

Viruses arose by loss of cellular components; supported by current evidence.

Rapid Evolution of Viruses

Due to high mutation rates, recombination, short generation times, large population sizes, and fast replication rates.

Antigenic Drift

Small changes in surface proteins of viruses (like influenza) that allow the virus to escape immune detection, requiring yearly vaccine updates.