Virology (Chapter 13)

Features of viruses

• Obligatory intracellular parasites

– Require living host cells to multiply

• Contain DNA or RNA

• Contain a protein coat

• Multiply within living host cell, using the host cell’s

synthesizing machinery

• No ribosomes

• No ATP-generating mechanism

Host Range

The spectrum of host cells a virus can infect

-Bacteriophages: viruses that infect bacteria

-Animal viruses: Receptor sites are typically on the plasma membrane

Capsid

protein coat made of capsomeres

(subunits)

Envelope

—lipid, protein, and carbohydrate coating on some viruses

– Composed of lipid, protein, and carbohydrate

– Partially formed from the plasma membrane of the host cell when the virus buds from the cell

Spikes

-projections from outer surface

-Made of carbohydrate and protein

-found on some enveloped viruses

Nucleic Acid

Viral genome is DNA or RNA, never both

• Viral genome may be single-stranded or double-stranded

• Nucleic acid may be linear or circular or segmented

• Total amount of nucleic acid varies from a few thousand nucleotides to as many as 250,000 nucleotides

Helical viruses

-hollow, cylindrical capsid that is helical

– Examples: virus that causes rabies and Ebola virus

Polyhedral viruses

-many-sided

-Most are an icosahedron (20 triangular facets and 12 corners)

-Examples: adenoviruses and poliovirus

Enveloped viruses

Most are roughly spherical

Complex viruses

-complicated structures

-Example: bacteriophage

Baltimore classification system

-based on virus’s nucleic acid and how its mRNA is produced

-Seven groups referred to as “realms”

-Viral species: a group of viruses sharing the same genetic information and ecological niche (host)

For a virus to multiply:

– It must invade a host cell

– It must take over the host’s metabolic machinery

Lytic cycle

– Phage causes lysis and death of the host cell

– Example: Tequatrovirus (T-even bacteriophages) that infect E. coli

Lysogenic cycle

– Phage DNA is incorporated in the host DNA

– Phage conversion

– Specialized transduction

Attachment (Bacteriophage)

-phage attaches by the tail fibers to the host

cell

Penetration (Bacteriophage)

phage lysozyme opens the cell wall; tail sheath contracts to force the tail core and DNA into the cell (similar to a hypodermic syringe)

Biosynthesis (Bacteriophage)

-production of phage DNA and proteins

-Host cell protein synthesis is halted

Maturation (Bacteriophage)

-assembly of phage particles

Release (Bacteriophage)

-phage lysozyme breaks the cell wall

Three potential outcomes of lysogeny…

1. Lysogenic host cells are immune to reinfection by the same phage

2. Phage conversion—the host cell exhibits new properties, encoded by the prophage DNA

Example: production of diphtheria toxin by

Corynebacterium diphtheriae

3. Specialized transduction: Specific bacterial genes transferred to another bacterium via a phage

Changes genetic properties of the recipient

bacteria

Oncolytic viruses

infect and kill tumor cells or cause an

immune response against tumor cells

Prions

-Proteinaceous infectious particles

-Inherited and transmissible by ingestion, transplant, and surgical instruments