Viruses, Viroids, and Prions Chapter 13, part A Viruses, Viroids, and Prions

Viruses Are very small Viruses contain DNA or RNA And a protein coat Capsid with Capsomeres Some are enclosed by an envelope Acellular Obligate intracellular parasites No ATP generating system No Ribosomes or means of Protein Synthesis Some viruses have spikes Most viruses infect only specific types of cells in one host

Viruses: only 1 nucleic acid DNA or RNA (But never both) ssDNA ds DNA ss RNA ds RNA

Viruses are small Figure 13.1

Shapes of Viruses: Helical Figure 13.4a, b

Shapes: Polyhedral icosahedral Figure 13.2a, b

Shapes: Complex Viruses Figure 13.5a

Envelope Membrane may be around capsid It may contain spikes

Viral Taxonomy 1. Nucleic Acid 2. Morphology 3. Strategy for replication 4. Symptoms Viral species: A group of viruses sharing the same genetic information and ecological niche (host). Common names are used for species Subspecies are designated by a number e.g. HHVII

Growing Viruses Viruses must be grown in living cells. The are never grown in culture media Bacteriophages form plaques on a lawn of bacteria. Plaques on E. coli Ø

Growing Viruses Animal viruses may be grown in living animals or in embryonated eggs. Vaccine virus in eggs Figure 13.7

Viral cultures 1. Primary Cell Lines die out after a few generations 2. Diploid Cell Lines derived from human embryos maintained for up to 100 generations 3. Continuous Cell Lines Transformed Cells (Cancerous Cells) may be maintained indefinitely HeLa Cells Henrietta Lacks 1951 (Cervical Cancer)

Growing Viruses Animal and plants viruses may be grown in cell culture. Continuous cell lines may be maintained indefinitely. Figure 13.8

Virus Identification Cytopathic effects CPE - Negri bodies rabies Serological tests Detect antibodies against viruses in a patient Use antibodies to identify viruses in neutralization tests, viral hemagglutination, and Western blot Nucleic acids RFLPs PCR

Cytopathic effects CPE Virus Identification Cytopathic effects CPE Figure 13.9

Multiplication of Bacteriophages (Lytic Cycle) APBMR - Bacteriophage Lytic multiplication cycle Attachment Phage attaches by tail fibers to host cell Penetration Phage lysozyme opens cell wall, tail sheath contracts to force tail core and DNA into cell Biosynthesis Production of phage DNA and proteins Maturation Assembly of phage particles Release Phage lysozyme breaks cell wall

1 2 3 Bacterial cell wall Bacterial chromosome Capsid DNA Capsid Sheath Tail fiber 1 Attachment: Phage attaches to host cell. Tail Base plate Pin Cell wall Plasma membrane 2 Penetration: Phage penetrates host cell and injects its DNA. Sheath contracted Tail core 3 Biosynthesis: DNA is copied and capsomeres are produced Figure 13.10.1

4 5 Tail DNA Maturation: Viral components are assembled into virions. Capsid 5 Release: Host cell lyses and new virions are released. Tail fibers Figure 13.10.2

Burst size and Burst time during Eclipse Period One-step Growth Curve Burst size and Burst time during Eclipse Period Eclipse period no whole virions Figure 13.11

Lytic cycle versus Lysogenic Lytic cycle Phage causes lysis and death of host cell Lysogenic cycle Prophage DNA incorporated in host DNA

The Lysogenic Cycle Figure 13.12

Specialized Transduction gal gene Prophage Bacterial DNA 1 Prophage exists in galactose-using host (containing the gal gene). Galactose-positive donor cell gal gene 2 Phage genome excises, carrying with it the adjacent gal gene from the host. 3 Phage matures and cell lyses, releasing phage carrying gal gene. gal gene 4 Phage infects a cell that cannot utilize galactose (lacking gal gene). Galactose-negative recipient cell 5 Along with the prophage, the bacterial gal gene becomes integrated into the new host’s DNA. 6 Lysogenic cell can now metabolize galactose. Galactose-positive recombinant cell Figure 13.13

Multiplication of Animal viruses Same as bacteriophage except must uncoat Attachment Viruses attaches to cell membrane Penetration By endocytosis or fusion Uncoating By viral or host enzymes Biosynthesis Production of nucleic acid and proteins Maturation Nucleic acid and capsid proteins assemble Release By budding (enveloped viruses) or rupture