CHAPTER 11 — Viruses, Prions, Viroids

Virus Basics

• Acellular

• Require host cells to replicate

• Host range = which species they can infect

• Tropism = which cells/tissues they infect

Structure

• Capsid (protein coat)

• Envelope (lipid membrane)

• Nucleic acid (DNA or RNA)

Classification based on:

• Genome type (DNA/RNA, single vs double)

• Shape

• Envelope vs naked

Lytic Cycle (5 stages)

1. Attachment

2. Entry

3. Synthesis

4. Assembly

5. Release

Lysogenic Cycle

• Viral DNA integrates into host DNA → prophage

• Replicates with host

• Can enter lytic cycle later (induction)

Animal Virus Replication

• Entry by endocytosis or fusion

• Uncoating step required

• Assembly in cytoplasm or nucleus

• Release by budding (enveloped) or lysis (naked)

DNA vs RNA Virus Replication

Be able to compare:

• DNA viruses replicate in nucleus

• +RNA acts like mRNA

• –RNA must bring RNA polymerase

• Retroviruses use reverse transcriptase

Release

• Lysis = bursts host cell

• Budding = steals membrane → becomes enveloped

Viruses & Cancer

Terms:

• Neoplasia – uncontrolled cell growth

• Benign-

  • not cancer, no spread

  vs

• malignant-

  • cancer, invades

• Metastasis-

  • cancer spreading

• Oncogenic viruses (HPV, EBV, Hep B/C cause cancers )

Culturing Viruses

Viruses must be grown inside living cells (they can’t grow on regular media like bacteria).

• Embryonated eggs-

  • Virus injected into a fertilized egg

  Think: Eggs = vaccine factories

  Used for vaccines (flu vaccine)

• Cell culture- Grow viruses inside animal cells in a dish.

• Bacteria (plaque assays) – clear zones show viral infection

Diploid cell lines: limited divisions

Continuous cell lines: immortal (HeLa)

Viroids

• Infect plants

• Naked circular RNA

• No proteins

Prions

• Infectious proteins

• Cause spongiform encephalopathies

• Replicate by converting PrP → misfolded form

• Examples: CJD, BSE (“mad cow”), scrapie