L17-18 Viruses and Bacteriophages

MIC230 Exam 3

virus

acellular genetic element that cannot replicate independently of a living host cell

virus particle (virion)

extracellular form of a virus

why can viruses not reproduce independently?

they lack ribosomes

• have everything for duplication

do viruses process energy (metabolism)

no, but they can influence host metabolism

do viruses go through growth and development?

no

• living things are not identical from “birth” to “death”

viral size

significantly smaller than prokaryotic cells

• 0.02 - 0.3 µm

• viruses discovered because they passed through filters that caught bacteria (Ivanovsky)

viral diversity

most viruses highly specific to host cells

• at least as diverse as host cells

• ALL domains of life have associated viruses

viral structure

viral genome + protein capsid

• nucleocapsid → capsid + nucleic acid

some viruses contain envelope that surrounds nucleocapsid

• derived from host cell, mainly in animal viruses

tegument proteins

proteins in space between envelope and capsid

• what is the advantage?

viral genome

• nucleic acids

• most are linear

• DNA, RNA, ds, or ss

phases of viral replication

1. attachment (adsorption)

2. penetration (injection)

3. synthesis (replication)

4. assembly and packaging

5. release (lysis)

viral attachment

virus binds to cell surface protein

• requires complementary receptors on surface of host and virus → highly specific

permissive cell

host cell that allows complete replication cycle of a virus to occur

virus replication characterized by ___ growth curve

one step

• virus particles released almost simultaneously

• latent period occurs inside cell, so no observed virus outside the cell

• burst → many virions released all at once

plaque assays

used for quantification of viruses

• dilute virus sample mixed with cells in cell culture → look for areas of clearing called plaques

• plaque forming unit = 1 viable viral particle

bacteriophages

viruses that infect bacteria

bacteriophage structure

can be helical or icosahedral

• some are complex and composed of several parts

• may have icosahedral heads and helical tails

virulent bacteriophage

phage replication followed by host cell lysis

• always kills, typically by lysis

• lytic cycle only

temperate phage

can integrate its DNA into host DNA or kill the host by lysis

• chooses between lysogenic and lytic pathway upon infection

lytic cycle

phage genes expressed

• host killed

lysogenic cycle

phage genome integrates into host genome and can be excised later

• dormant phase → prophage

• replicated when host genome replicates

transduction

transfer of bacterial DNA from one cell to another by a phage

generalized transduction

any region of the donor DNA can be transferred

specialized transduction

only certain genes near the insertion site of a lysogenic phage can be transferred

Joshua Lederberg nobel prize 1958

discovery of

• random mutations

• conjugation

• transduction

viroids

infectious RNA

• small, circular, infectious ssRNA

• lack a protein coat → no capsid

• no genes

prions

infections proteins

• survive outside host

• very heat resistant

• normal protein misfolds, abnormal protein promotes more misfolding

• infection with prion causes misfolding in new host