Chapter 17: Viruses

Following the Big Ideas• Evolution-

– Changes in DNA can lead to phenotypic variation which can be acted upon by natural selection.

– Viral and bacterial genomes benefit from their ability to evolve rapidly.

• Information and Signaling- – DNA, and in some cases RNA, is the primary source of inheritable

information.– A variety of intercellular and intracellular signal transmissions mediate

gene expression.– Gene regulation results in differential gene expression, leading to cell

specialization.– Biological systems have multiple processes that increase genetic

variation.– Viral replication results in genetic variation, and viral infection can

introduce genetic variation into the hosts.• Interactions and Systems-

– Regulation of gene activity involves intricate interactions with internal and external factors.

Overview: A Borrowed Life

• A virus is an infectious particle consisting of little more than genes packaged into a protein coat

• Viruses lead “a kind of borrowed life,” existing in a shady area between life-forms and chemicals

Viral Structures

Microbial Model Systems• Viruses called bacteriophages can infect and set

in motion a genetic takeover of bacteria, such as Escherichia coli

• E. coli and its viruses are called model systems because of their frequent use by researchers in studies that reveal broad biological principles

• Beyond their value as model systems, viruses and bacteria have unique genetic mechanisms that are interesting in their own right

A virus has a genome but can reproduce only within a host cell

• Bacteria are prokaryotes with cells much smaller and more simply organized than those of eukaryotes

• Viruses are smaller and simpler than bacteria• Scientists detected viruses indirectly long before

they could see them• The story of how viruses were discovered begins

in the late 1800s

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Virus

Bacterium

Animalcell

Animal cell nucleus0.25 µm

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)

Concept 17.1: A virus consists of a nucleic acid surrounded by a protein coat

• Even the largest known virus is barely visible under the light microscope

• Some viruses can be crystalized• See Bozeman video on viruses:

– https://www.youtube.com/watch?v=L8oHs7G_syI

Structure of Viruses/ Viral Genome• Viruses are not cells• Viruses are very small infectious particles

consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope

• Viral genomes may consist of– Double- or single-stranded DNA– Double- or single-stranded RNA

• Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus

LE 18-4aCapsomereof capsid

RNA

18 250 mm

Tobacco mosaic virus20 nm

Capsids and Envelopes♥ A capsid is the protein shell that encloses the viral genome♥ A capsid can have various structures♥ Capsomeres make up capsids!

LE 18-4bCapsomere

Glycoprotein

70–90 nm (diameter)

DNA

Adenoviruses50 nm

• Some viruses have structures have membranous envelopes that help them infect hosts

• These viral envelopes surround the capsids of influenza viruses and many other viruses found in animals

• Viral envelopes, which are derived from the host cell’s membrane, contain a combination of viral and host cell molecules

LE 18-4c

Glycoprotein

80–200 nm (diameter)

RNA

Capsid

Influenza viruses50 nm

Membranousenvelope

• Bacteriophages, also called phages, are viruses that infect bacteria

• They have the most complex capsids found among viruses

• Phages have an elongated capsid head that encloses their DNA

• A protein tailpiece attaches the phage to the host and injects the phage DNA inside

LE 18-4d

80 225 nm

DNAHead

TailsheathTailfiber

Bacteriophage T450 nm

Concept 17.2: Viruses replicate only in host cells

• Viruses are obligate intracellular parasites, which means they can reproduce only within a host cell

• Each virus has a host range, a limited number of host cells that it can infect, recognized by compatible receptor proteins on the virus and the host

• Viruses use enzymes, ribosomes, and small host molecules to synthesize progeny viruses

Animation: Simplified Viral Reproductive CycleAnimation: Simplified Viral Reproductive Cycle

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DNAVIRUS

Capsid

HOST CELL

Viral DNA

Replication

Entry into cell anduncoating of DNA

Transcription

Viral DNA

mRNA

Capsidproteins

Self-assembly ofnew virus particlesand their exit from cell

Viral Envelopes• Many viruses that infect animals have a

membranous envelope• Viral glycoproteins on the envelope bind to specific

receptor molecules on the surface of a host cell

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RNA

ER

Capsid

HOST CELL

Viral genome (RNA)

mRNA

Capsidproteins

Envelope (withglycoproteins)

Glyco-proteins Copy of

genome (RNA)

Capsid and viral genomeenter cell

New virus

Template

Reproductive Cycles of Phages• Phages are the best understood of all viruses• Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle• See Bozeman video on viral replication:

– https://www.youtube.com/watch?v=EqK1CYYQIug&list=PLFCE4D99C4124A27A&index=44

The Lytic Cycle• The lytic cycle is a phage reproductive cycle that culminates in

the death of the host cell• The lytic cycle produces new phages and digests the host’s cell

wall, releasing the progeny viruses• A phage that reproduces only by the lytic cycle is called a

virulent phage• Bacteria have defenses against phages, including restriction

enzymes that recognize and cut up certain phage DNA

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Attachment

Entry of phage DNAand degradation of host DNA

Synthesis of viralgenomes and proteins

Assembly

ReleasePhage assembly

Head Tails Tail fibers

The Lysogenic Cycle• The lysogenic cycle replicates the phage genome

without destroying the host• The viral DNA molecule is incorporated by genetic

recombination 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• Phages that use both the lytic and lysogenic cycles

are called temperate phages

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Phage

Phage DNA

The phage attaches to ahost cell and injects its DNA.

Phage DNAcircularizes

Bacterial chromosome

Lytic cycle

The cell lyses, releasing phages.Lytic cycleis induced

or Lysogenic cycleis entered

Certain factorsdetermine whether

Lysogenic cycle

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

The bacterium reproducesnormally, copying the prophageand transmitting it to daughter cells.

Prophage

Many cell divisionsproduce a large population of bacteria infected withthe prophage.

Daughter cellwith prophage

Phage DNA integrates into thebacterial chromosomes, becoming aprophage.

New phage DNA and proteins aresynthesized and assembled into phages.

Reproductive Cycles of Animal Viruses

• Two key variables in classifying viruses that infect animals:– DNA or RNA?– Single-stranded or double-stranded?

Class/Family Envelope Examples/Disease

I. Double-stranded DNA (dsDNA)

Adenovirus No Respiratory diseases, animal tumors

Papovavirus No Papillomavirus (warts, cervical cancer): polyomavirus (animal tumors)

Herpes virus Yes Herpes simplex I and II (cold sores, genital sores); varicella zoster (shingles, chicken pox); Epstein-Barr virus (mononucleosis, Burkitt’s lymphoma)

Pox virus Yes Smallpox virus, cowpox virus

Class/Family Envelope Examples/Disease

II. Single-stranded DNA (ssDNA)

Parvovirus No B19 parvovirus (mild rash)

III. Double-stranded RNA (dsRNA)

Reovirus No Rotavirus (diarrhea), Colorado tick fever virus

Class/Family Envelope Examples/Disease

IV. Single-stranded RNA (ssRNA); serves as mRNA

Picornavirus No Rhinovirus (common cold); poliovirus, hepatitis A virus, and other enteric (intestinal) viruses

Coronavirus Yes Severe acute respiratory syndrome (SARS)

Flavivirus Yes Yellow fever virus, West Nile virus, hepatitis C virus

Togavirus Yes Rubella virus, equine encephalitis viruses

Class/Family Envelope Examples/Disease

V. ssRNA; template for mRNA synthesis

Filovirus Yes Ebola virus (hemorrhagic fever)

Orthomyxovirus Yes Influenza virus

Paramyxovirus Yes Measles virus; mumps virus

Rhabdovirus Yes Rabies virus

VI. ssRNA; template for DNA synthesis

Retrovirus Yes HIV (AIDS); RNA tumor viruses (leukemia)

RNA as Viral Genetic Material• The broadest variety of RNA genomes is found in viruses that

infect animals• RNA viruses lack replication error-checking mechanisms, thus

have higher rates of mutation.• Related viruses can combine/recombine information if they

infect the same host cell. (flu)• Retroviruses use reverse transcriptase to copy their RNA

genome into DNA• Reverse transcriptase is a type of polymerase that transcribes

DNA from an RNA template.• HIV is the retrovirus that causes AIDS- rapidly evolves!• Retroviruses have the most complicated reproductive cycles

because of the reverse transcriptase stage.

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Capsid

Viral envelopeGlycoprotein

Reversetranscriptase

RNA(two identicalstrands)

HIV

• The viral DNA that is integrated into the host genome is called a provirus

• Unlike a prophage, a provirus remains a permanent resident of the host cell

• The host’s RNA polymerase transcribes the proviral DNA into RNA molecules

• The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell

LE 18-10

HOST CELL

ReversetranscriptionViral RNA

RNA-DNAhybrid

DNA

NUCLEUS

ChromosomalDNA

Provirus

RNA genomefor thenext viralgeneration

mRNA

New HIV leaving a cell

HIV entering a cell

0.25 µm

HIVMembrane ofwhite blood cell

Reverse transcriptase catalyzes the synthesis of a DNA strand complementary to the viral RNA.

Reverse Transcriptase then catalyzes the synthesis of a second DNA strand complementary to the first.

The double stranded DNA incorporates as a provirus.

Proviral genes are transcribed into RNA, the genomes for the next HIV generation and mRNA for the translation into viral proteins.

The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelop glycoproteins (made in the ER)

Evolution of Viruses• Viruses do not fit our definition of living organisms• Since viruses can reproduce only within cells, they probably

evolved as bits of cellular nucleic acid• Coevolution exists between virus and host.

– Defenses that bacteria have against invasion from phage infection-

• Mutant receptor sits no longer recognized by phage proteins

• Enzymes that digest foreign DNA evolved (restriction endonucleases- cut up foreign DNA but not host DNA)

• As bacteria evolve to resist viral infection, the virus evolves to become pathogenic again.

• Lysogenic cycle has evolved to allow bacteria and virus to co-exist for a time before virus becomes pathogenic.

Concept 17.3: Viruses are formidable pathogens in animals and plants

• Diseases caused by viral infections afflict humans, agricultural crops, and livestock worldwide– Damage or kill cells by releasing hydrolytic enzymes from

lysosomes.– Causes infected cells to produce toxins that lead to disease

symptoms– Some viruses have toxic components, such as envelope proteins– Many symptoms are due to the body’s attempt at defense.

Vaccines and Antiviral Drugs

•Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen•Vaccines can prevent certain viral illnesses•Antiviral drugs work by interfering with viral nucleic acid synthesis

Emerging Viruses• Emerging viruses are those that appear suddenly or

suddenly come to the attention of scientists• Emergent diseases are scientific evidence that

evolution continues to occur!• HIV is a classic example• The West Nile virus appeared in North America

first in 1999 and has now spread to all 48 contiguous states

• In 2009 a general outbreak, or epidemic, of a flu-like illness occurred in Mexico and the United States; the virus responsible was named H1N1

• H1N1 spread rapidly, causing a pandemic, or global epidemic• Severe acute respiratory syndrome (SARS) recently

appeared in China

Figure 17.8

(a) 2009 pandemic H1N1 influenza A virus

(b) 2009 pandemic screening

1 m

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Young ballet students in HongKong wear face masks toprotect themselves from thevirus causing SARS.

The SARS-causing agent is acoronavirus like this one(colorized TEM), so named forthe “corona” of glyco-proteinspikes protruding form theenvelope.

• Three processes contribute to the emergence of viral diseases– The mutation of existing viruses, which is especially

high in RNA viruses– Dissemination of a viral disease from a small, isolated

human population, allowing the disease to go unnoticed before it begins to spread

– Spread of existing viruses from animal populations; about three-quarters of new human diseases originate this way

• Strains of influenza A are given standardized names

• The name H1N1 identifies forms of two viral surface proteins, hemagglutinin (H) and neuraminidase (N)

• There are numerous types of hemagglutinin and neuraminidase, identified by numbers

Viruses and Cancer• Tumor viruses- All tumor viruses transform

cells into cancer cells through the integration of viral nucleic acid into host cell DNA. Through a variety of possible mechanisms, cell cycle control is altered

Viral Group Ex. /Diseases Cancer Type

Retrovirus HTLV-1 Adult leukemia

Herpesvirus Epstein-Barr/infectious mononucleosis

Burkitt’s Lymphoma

Papovavirus Papilloma/human warts

Cervical cancer

Hepatitis B virus Chronic hepatitis Liver cancer

Viral Diseases in Plants• More than 2,000 types of viral diseases of plants

are known; these have enormous impacts on the agricultural and horticultural industries

• Plant viruses have the same basic structure and mode of replication as animal viruses

• Most plant viruses known thus far have an RNA genome and many have a helical capsid

• Some symptoms are spots on leaves and fruits, stunted growth, and damaged flowers or roots

• Plant viruses spread disease in two major modes:– Horizontal transmission, entering through damaged

cell walls (infection is from an outside source- wind)– Herbivores, especially insects, pose a double threat

because they can both carry a virus and help it get past the plant’s outer layer of cells

– Vertical transmission, inheriting the virus from a parent (virus spreads through plasmodesmata).

Connecting the Concepts to the Big Ideas

• Evolution- – All living things, and viruses, are subject to genetic

mutations or changes in the base sequence of DNA (or RNA in the case of viruses); this is a source of genetic variability that is acted upon by natural selection, allowing for evolutionary change over time.

– The phenomena of emergent diseases supports present-day evolution.

Connecting the Concepts to the Big Ideas

• Interactions and Systems-– Environmental and internal cues may affect gene

regulation, which ultimately effects their survival.– Unicellular populations are capable of interactions which

affect their efficiency and productivity.

• Information and Signaling– Viruses evolve rapidly engaging in lytic and lysogenic

cycles as well as transduction.– Retroviruses use reverse transcriptase.