Chapter 10 Lecture

Copyright © 2009 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Brock Biology of Microorganisms

Twelfth Edition

Madigan / Martinko Dunlap / Clark

Overview of Viruses and Virology

Chap

ter 1

0

Lectures by Buchan & LeCleir


I. Virus Structure and Growth

10.1 General Properties of Viruses

10.2 Nature of the Virion

10.3 The Virus Host

10.4 Quantification of Viruses



Virus: genetic element that cannot replicate independently of a

living (host) cell

Virology: the study of viruses

Virus particle: extracellular form of a virus; allows virus to exist

outside host and facilitates transmission from one host cell to

another

Virion: the infectious virus particle; the nucleic acid genome

surrounded by a protein coat and, in some cases, other layers

of material



Viral Genomes

Either DNA or RNA genomes

Some circular, but most linear


Viral Genomes

Figure 10.1



Viral Hosts and Taxonomy

Viruses can be classified on the basis of the hosts they

infect

Bacterial viruses (bacteriophages)

Animal viruses

Plant viruses



Most viruses are smaller than prokaryotic cells; range

from 0.02 to 0.3 µm

Most viral genomes are smaller than those of biological

cells



Viral Structure

Capsid: the protein shell that surrounds the genome of a

virus particle

Composed of a number of protein molecules arranged in

a precise and highly repetitive pattern around the nucleic

acid

Capsomer: subunit of the capsid

Smallest morphological unit visible with an electron

microscope



Viral Structure (cont’d)

Nucleocapsid: complete complex of nucleic acid and

protein packaged in the virion

Enveloped virus: virus that contains additional layers

around the nucleocapsid


Comparison of Naked and Enveloped Virus Particles

Figure 10.3



Nucleocapsids of viruses constructed in highly symmetric ways

Helical symmetry: rod-shaped viruses (e.g., tobacco

mosaic virus)

Length of virus determined by length of nucleic acid

Width of virus determined by size and packaging of protein

subunits

Icosahedral symmetry: spherical viruses

Most efficient arrangement of subunits in a closed shell


Icosahedral Symmetry

Figure 10.4


Icosahedral Symmetry

Figure 10.4



Enveloped Viruses

Have membrane surrounding nucleocapsid; lipid bilayer

with embedded proteins

Make initial contact with host cell


Electron Micrographs of Animal and Bacterial Viruses

Figure 10.5



Complex Viruses

Virions composed of several parts, each with

separate shapes and symmetries

Examples of most complex viruses in terms of structure

can be found among bacterial viruses, which contain

icosahedral heads and helical tails



Some virions contain enzymes critical to infection

Lysozyme

Nucleic acid polymerases

Neuraminadases: enzymes that cleave gycosidic bonds;

allows liberation of viruses from host


10.3 The Virus Host

Viruses replicate only in certain types of cells or in whole

organisms

Bacterial viruses are typically easiest to grow; model

systems

Animal viruses (and some plant viruses) can be cultivated

in tissue or cell cultures

Plant viruses typically are most difficult because study

often requires growth of whole plant



Titer: number of infectious units per volume of fluid

Plaque assay: analogous to the bacterial colony; one of

the most accurate ways to measure virus infectivity

Plaques are clear zones that develop on lawns of host

cells

Each plaque results from infection by a single virus

particle


Quantification of Bacterial Virus by Plaque Assay

Figure 10.6


Quantification of Bacterial Virus by Plaque Assay

Figure 10.6



The number of plaque-forming units is almost always

lower than direct counts by microscopy

Inactive virions

Conditions not appropriate for infectivity


II. Viral Replication

10.5 General Features of Virus Replication

10.6 Viral Attachment and Penetration

10.7 Production of Viral Nucleic Acid and Protein



The Phases of Viral Replication

Attachment (adsorption) of the virus to a susceptible

host cell

Entry (penetration) of the virion or its nucleic acid

Synthesis of virus nucleic acid and protein by cell

metabolism as redirected by virus

Assembly of capsids and packaging of viral genomes

into new virions (maturation)

Release of mature virions from host cell


The Replication Cycle of a Bacterial Virus

Figure 10.8



Virus replication typically characterized by a one-step

growth curve

Latent period: eclipse + maturation

Burst size: number of virions released


The One-Step Growth Curve of Virus Replication

Figure 10.9



Attachment of virion to host cell is highly specific

Requires complementary receptors on the surface of a

susceptible host and its infecting virus

Receptors on host cell carry out normal functions for cell

(e.g., uptake proteins)

Receptors include proteins, carbohydrates,

glycoproteins, lipids, lipoproteins, or complexes



The attachment of a virus to its host cell results in

changes to both virus and cell surface that facilitate

penetration

Permissive cell: host cell that allows the complete

replication cycle of a virus to occur



Bacteriophage T4: virus of E. coli; example of one of the

most complex penetration mechanisms known

Virions attach to cells via tail fibers that interact specifically

with polysaccharides on E. coli cell envelope

Tail fibers retract and tail core makes contact with E. coli

cell wall

Lysozyme-like enzyme forms small pore in peptidoglycan

Tail sheath contracts and viral DNA passes into cytoplasm


Attachment of Bacteriophage T4 to the Cell Wall of E. coli

Figure 10.10



Many eukaryotes possess mechanisms to diminish viral

infections

E.g., immune defense mechanisms, RNA interference



Many bacteria employ restriction-modification systems

to evade viral infection

DNA destruction system; only effective against double-

stranded DNA viruses

Restriction enzymes (restriction endonucleases) cleave

DNA at specific sequences

Modification of host’s own DNA at restriction enzyme

recognition sites prevent cleavage of own DNA



Viral mechanisms to evade bacterial restriction systems

Chemical modification of viral DNA (glycosylation or

methylation)

Production of proteins that inhibit host cell restriction

system



David Baltimore, Howard Temin, and Renato Dulbecco

discovered retroviruses and reverse transcriptase

Shared 1975 Nobel Prize for Physiology or Medicine

Baltimore developed classification scheme for viruses

based on relationship of viral genome to its mRNA


The Baltimore Classification System of Viruses



Once a host has been infected, new copies of the viral

genome must be made and virus-specific proteins

synthesized in order for the virus to replicate

Generation of messenger RNA (mRNA) occurs first

Typically viral genome serves as template for viral mRNA

In some RNA viruses, viral RNA itself is the mRNA

In some cases essential transcriptional enzymes are

contained in the virion



Retroviruses: animal viruses responsible for causing

certain types of cancers and acquired

immunodeficiency syndrome (AIDS)

Class VI and VII viruses

Require reverse transcriptase



Viral Proteins

Production follows synthesis of viral mRNA

Early proteins

synthesized soon after infection

necessary for replication of virus nucleic acid

typically act catalytically

synthesized in smaller amounts



Production of Viral Proteins (cont’d)

Late proteins

Synthesized later

Include proteins of virus coat

Typically structural components

Synthesized in larger amounts


III. Viral Diversity

10.8 Overview of Bacterial Viruses

10.9 Virulent Bacteriophages and T4

10.10 Temperate Bacteriophages, Lambda, and P1

10.11 Overview of Animal Viruses

10.12 Retroviruses



Bacteriophages are very diverse

Best-studied bacteriophages infect enteric bacteria

E.g., E. coli, Salmonella enterica

Most contain dsDNA genomes

Most are naked, but some possess lipid envelopes

They are structurally complex, containing heads, tails

and other components



Viral Life Cycles

Virulent mode: viruses lyse host cells after infection

Temperate mode: viruses replicate their genomes in

tandem with host genome and without killing host



Temperate viruses: can undergo a different life cycle resulting

in a stable genetic relationship within the host

But can also kill cells through lytic cycle

Lysogeny: state where most virus genes not expressed and

virus genome (prophage) is replicated in synchrony with host

chromosome

Lysogen: a bacterium containing a prophage

Under certain conditions lysogenic viruses may revert to the

lytic pathway and begin to produce virions


The Consequences of Infection by a Temperate Phage

Figure 10.16



Bacteriophage Lambda

Linear, dsDNA genome

Complementary, single-stranded regions 12 nucleotides

long at the 5′-terminus of each strand

Upon penetration, DNA ends base-pair forming the cos

site, DNA ligates and forms double-stranded circle

When lysogenic, integrates into E. coli chromosome at the

lambda attachment site (att


Bacteriophage Lambda

Figure 10.17


Integration of Lambda DNA into the Host

Figure 10.18



Unlike prokaryotes, entire virion enters the animal cell

Eukaryotic cells contain a nucleus, the site of replication

for many animal viruses

Animal viruses contain all known modes of viral genome

replication

Many more kinds of enveloped animal viruses than

bacterial viruses exist

As animal viruses leave host cell, they can remove part of host

cell’s lipid bilayer for their envelope


Diversity of Animal Viruses: DNA Viruses

Figure 10.21a


Diversity of Animal Viruses: RNA Viruses

Figure 10.21b



Consequences of Virus Infection in Animal Cells

Persistent infections: release of virions from host cell

does not result in cell lysis

Infected cell remains alive and continues to produce virus

indefinitely

Latent infections: delay between infection by the virus

and lytic events

Transformation: conversion of normal cell into tumor cell


10.12 Retroviruses

Retroviruses: RNA viruses that replicate through a DNA

intermediate

Enveloped viruses

Contain a reverse transcriptase (copies information from

its RNA genome into DNA), integrase, and protease

Virion contains specific tRNA molecules


Retrovirus Structure and Function

Figure 10.23a


10.12 Retroviruses

Retroviruses have a unique genome

Two identical ssRNA molecules of the plus (+)

orientation

Contain specific genes

gag: encode structural proteins

pol: encode reverse transcriptase and integrase

env: encode envelope proteins


10.12 Retroviruses

Process of Replication of a Retrovirus

Entrance into the cell

Removal of virion envelope at the membrane

Reverse transcription of one of the two RNA genomes

Integration of retroviral DNA into host genome

Transcription of retroviral DNA

Assembly and packaging of genomic RNA

Budding of enveloped virions; release from cell


Replication Process of a Retrovirus

Figure 10.24


IV. Subviral Entities

10.13 Defective Viruses

10.14 Viroids

10.15 Prions


10.13 Defective Viruses

Helper viruses (defective viruses): viruses that are

parasitic on other viruses

Satellite viruses: defective viruses for which no intact

version exists; rely on unrelated viruses as helpers


10.14 Viroids

Viroids: infectious RNA molecules that lack a protein

coat

Small, circular, ssRNA molecules

Smallest known pathogens (246–399 bp)

Cause a number of important plant diseases

Do not encode proteins; completely dependent on host-

encoded enzymes


Viroids and Plant Disease: Healthy vs. PSTV-Infected

Figure 10.25


10.15 Prions

Prions: infectious proteins whose extracellular form contains no nucleic acid

Known to cause disease in animals (transmissible

spongiform encephalopathies)

Host cell contains gene (PrnP) that encodes native form of

prion protein that is found in healthy animals

Prion misfolding results in neurological symptoms of

disease (e.g., resistance to proteases, insolubilty, and

aggregation)


Mechanisms of Prion Misfolding

Figure 10.28


10.15 Prions

Prion disease occurs by three distinct mechanisms

Infectious prion disease: pathogenic form of prion

protein is transmitted between animals or humans

Sporadic prion disease: random misfolding of a normal,

healthy prion protein in an uninfected individual

Inherited prion disease: mutation in prion gene yields a

protein that changes more often into disease-causing

form

Chapter 10 Lecture

Automotive

viral nucleic

nucleic acid

viral genome

viral genomes

6 viral attachment

prion protein

rna viruses

animal viruses