WHAT DOES IT MEAN TO BE

ALIVE?

Are the tiny viruses infecting this E. coli cell alive?

0.5 mm

“I’D LIKE TO INFECT YOU WITH MY DNA” (VIRAL REPRODUCTION)

;)

VIRAL VIDEO LINKS

NPR podcast : how viruses invade your body

SciShow: Top 5 Deadliest Disease!!

LECTURE PRESENTATIONSFor CAMPBELL BIOLOGY, NINTH EDITION

Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson

© 2011 Pearson Education, Inc.

Lectures byErin Barley

Kathleen Fitzpatrick

Viruses

Chapter 19

Concept 19.1: A virus consists of a nucleic acid surrounded by a protein coat

• Viruses were detected indirectly long before they were actually seen

© 2011 Pearson Education, Inc.

The Discovery of Viruses: Scientific Inquiry

• Tobacco mosaic disease stunts growth of tobacco plants and gives their leaves a mosaic coloration

• In the late 1800s, researchers hypothesized that a particle smaller than bacteria caused the disease

• In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV)

© 2011 Pearson Education, Inc.

Figure 19.2

Extracted sapfrom tobaccoplant withtobacco mosaicdisease

RESULTS

Passed sapthrough aporcelain filterknown to trapbacteria

Healthy plantsbecame infected

Rubbed filteredsap on healthytobacco plants

1 2 3

4

Structure of Viruses

• Viruses are not cells, they are infectious particles

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Capsids and Envelopes

• A capsid is the protein shell that encloses the viral genome

• Capsids are built from protein subunits called capsomeres

• A capsid can have various structures

© 2011 Pearson Education, Inc.

Examples of viral structure

Capsomereof capsid

RNA CapsomereDNA

Glycoprotein Glycoproteins

Membranousenvelope RNA

CapsidHead

DNA

Tailsheath

Tailfiber

18 250 nm 80 225 nm70–90 nm (diameter) 80–200 nm (diameter)

20 nm 50 nm 50 nm 50 nm(a)Tobacco

mosaic virus(b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

Concept 19.2: Viruses replicate only in host cells

• Viruses are obligate intracellular parasites, which means they can replicate only within a host cell using the cell’s parts, much like…

© 2011 Pearson Education, Inc.

General Features of Viral Replicative Cycles

• Once a viral genome has entered a cell, the cell begins to manufacture viral proteins

• The virus makes use of host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules

• Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses

© 2011 Pearson Education, Inc.

© 2011 Pearson Education, Inc.

Animation: Simplified Viral Reproductive Cycle

Right-click slide / select “Play”

VIRUS

2

1

3

4

Entry anduncoating

Replication

Transcriptionand manufacture ofcapsid proteins

Self-assembly ofnew virus particlesand their exit fromthe cell

DNA

Capsid

HOSTCELL

Viral DNA

ViralDNA

mRNA

Capsidproteins

A simplified viral replicative cycle.

Replicative Cycles of Phages

• Phages are the best understood of all viruses• Phages have two reproductive mechanisms: the

lytic cycle and the lysogenic cycle (see animations that follow)

© 2011 Pearson Education, Inc.

© 2011 Pearson Education, Inc.

Animation: Phage T4 Lytic Cycle

Right-click slide / select “Play”

Figure 19.5-1

Attachment1

Figure 19.5-2

Attachment

2

1

Entry of phageDNA anddegradation of host DNA

Figure 19.5-3

Attachment

2

1

3

Entry of phageDNA anddegradation of host DNA

Synthesis ofviral genomesand proteins

Figure 19.5-4

Attachment

2

1

43

Entry of phageDNA anddegradation of host DNA

Synthesis ofviral genomesand proteins

Assembly

Phage assembly

Head Tail Tailfibers

Figure 19.5-5

Attachment

2

1

5

43

Entry of phageDNA anddegradation of host DNA

Release

Synthesis ofviral genomesand proteins

Assembly

Phage assembly

Head Tail Tailfibers

The Lysogenic Cycle

• The lysogenic cycle replicates the phage genome without destroying the host

• The viral DNA molecule is incorporated into the host cell’s chromosome

• This integrated viral DNA is known as a prophage• Every time the host divides, it copies the phage

DNA and passes the copies to daughter cells

© 2011 Pearson Education, Inc.

© 2011 Pearson Education, Inc.

Animation: Phage Lambda Lysogenic and Lytic Cycles

Right-click slide / select “Play”

• An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode

• Phages that use both the lytic and lysogenic cycles are called temperate phages

© 2011 Pearson Education, Inc.

Figure 19.6

New phage DNA and proteinsare synthesized and assembledinto phages.

The cell lyses, releasing phages.

Phage

PhageDNA

The phageinjects its DNA.

Bacterialchromosome

Lytic cycle

lytic cycleis induced

or

Phage DNAcircularizes.

Certain factorsdetermine whether

lysogenic cycleis entered

Lysogenic cycle

Prophage

Daughter cellwith prophage

Occasionally, a prophageexits the bacterial chromosome,initiating a lytic cycle.

Cell divisionsproduce apopulation ofbacteria infectedwith the prophage.

The bacterium reproduces,copying the prophage andtransmitting it to daughtercells.

Phage DNA integrates intothe bacterial chromosome,becoming a prophage.

Capsid

RNA

Envelope (withglycoproteins)

Capsid and viral genomeenter the cell

HOST CELL

Viral genome(RNA)Template

mRNA

ERCapsidproteins

Copy ofgenome(RNA)

New virus

Glyco-proteins

The replicative cycle of an enveloped RNA virus.

RNA as Viral Genetic Material

• The broadest variety of RNA genomes is found in viruses that infect animals

• Retroviruses use reverse transcriptase to copy their RNA genome into DNA

• HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)

© 2011 Pearson Education, Inc.

Glycoprotein

Reversetranscriptase HIV

Viral envelope

Capsid

RNA (twoidenticalstrands)

HOSTCELL

Viral RNAReversetranscriptase

RNA-DNAhybrid

DNA

NUCLEUSProvirus

ChromosomalDNA

RNA genomefor thenext viralgeneration

mRNA

New virus

HIV

Membraneof whiteblood cell

0.25 m

HIV entering a cell

New HIV leaving a cell

HIV: a retrovirus

© 2011 Pearson Education, Inc.

Animation: HIV Reproductive Cycle

Right-click slide / select “Play”

Concept 19.3: Viruses, viroids, and prions are formidable pathogens in animals and plants

© 2011 Pearson Education, Inc.

Why viruses cause disease in animals, other than by lysis

• Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes

• Some viruses cause infected cells to produce toxins that lead to disease symptoms

• Others have molecular components such as envelope proteins that are toxic

© 2011 Pearson Education, Inc.

• Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the harmful pathogen– Vaccines can prevent certain viral illnesses

• Viral infections cannot be treated by antibiotics• Antiviral drugs can help to treat, though not cure,

viral infections

© 2011 Pearson Education, Inc.

Emerging Viruses

• Mostly spread from animals or massively mutated pre-existing viruses– The 2009 flu pandemic was likely passed to humans

from pigs; for this reason it was originally called the “swine flu”

© 2011 Pearson Education, Inc.

Figure 19.9

(c) 1918 flu pandemic

2009 pandemicscreening

(b)2009 pandemic H1N1influenza A virus

(a)

1 m

Viroids and Prions: The Simplest Infectious Agents

• Viroids are small circular RNA molecules that infect plants and disrupt their growth

• Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals– Prions propagate by converting normal proteins into the

prion version– Scrapie in sheep, mad cow disease, and Creutzfeldt-

Jakob disease in humans are all caused by prions

© 2011 Pearson Education, Inc.

Figure 19.11

Prion

Normal protein

Originalprion

Newprion

Aggregatesof prions

Test your understanding

Test Your Understanding, question 8

Time Time

A B

Nu

mb

er o

f b

acte

ria

Nu

mb

er o

f vi

ruse

s

Test Your Understanding, question 6