chapter 13-Viruses, Viroids, Prions

Viruses

Not alive — can’t reproduce without a host cell.

What are viruses made of?

• Capsid: protein coat.

• Envelope: extra outer layer (like a jacket) made from host cell membrane.

• Contain either DNA or RNA (not both).

Lytic Cycle (Virus Reproduction)

1. Attach to host cell

2. Enter cell

3. Use host to make more virus parts

4. Assemble new viruses

5. Burst (lyse) the cell and release them

Direct penetration

injects its genetic material.

Membrane fusion

merges with host membrane (HIV)

Endocytosis

tricked cell “eats” it

HIV Life Cycle

• Attaches to CD4 receptors on immune cells.

• Enters via fusion.

• Uses reverse transcription to turn its RNA into DNA.

• Inserts DNA into host genome and makes new viruses.

• Drugs target steps in this cycle (entry, reverse transcriptase, protease, etc.).

Proto-oncogenes

normal genes that help cells grow.

Tumor suppressor genes

stop cells from growing too much.

Metastasis

when cancer spreads.

Whole organisms

realistic but complex.

Embryonated eggs

cheap, used for vaccines.

Cell cultures

easy to control, used in labs.

Prions

• Misfolded proteins (no DNA/RNA).

• Cause brain diseases (like mad cow).

• Conversion: normal protein (c-PrP) → bad prion (p-PrP).

• Can be inactivated by heat or strong chemicals.