Scene from the 1918 influenza pandemic.

Scene from the 2003 SARS Scare

Comparing the size of a virus, a bacterium, and an animal cell

0.25 μm

Virus

Animalcell

Bacterium

Animal cell nucleus

4

T4 bacteriophage infecting an E. coli cell

0.5 μm

Infection by tobacco mosaic virus (TMV)

Viral infection of plants

Viral structure

18 250 mm 70–90 nm (diameter) 80–200 nm (diameter) 80 225 nm

20 nm 50 nm 50 nm 50 nm

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

RNA

RNACapsomereof capsid

DNACapsomere

Glycoprotein Glycoprotein

Membranousenvelope

CapsidDNA

Head

Tail fiber

Tail sheath

Classes of

Animal Viruses

A simplified viral

reproductive cycle

VIRUS

Capsid proteins

mRNA

Viral DNA

HOST CELL

Viral DNA

Entry into cell anduncoating of DNA

Replication Transcription

DNA

Capsid

Self-assembly of new virus particles and their

exit from cell

The lytic cycle of phage T4, a virulent phageAttachment. The T4 phage uses

its tail fibers to bind to specificreceptor sites on the outer surface of an E. coli cell.

Entry of phage DNA and degradation of host DNA.The sheath of the tail contracts,

injecting the phage DNA intothe cell and leaving an empty

capsid outside. The cell’sDNA is hydrolyzed.

Synthesis of viral genomes and proteins. The phage DNA

directs production of phageproteins and copies of the phagegenome by host enzymes, using

components within the cell.

Assembly. Three separate sets of proteinsself-assemble to form phage heads, tails,

and tail fibers. The phage genome ispackaged inside the capsid as the head forms.

Release. The phage directs productionof an enzyme that damages the bacterialcell wall, allowing fluid to enter. The cellswells and finally bursts, releasing 100

to 200 phage particles.

12

4 3

5

Phage assembly

Head Tails Tail fibers

The lytic and lysogenic cycles of phage λ, a temperate phage

Many cell divisions produce a large

population of bacteria infected with the

prophage.

The bacterium reproducesnormally, copying the prophage

and transmitting it to daughter cells.

Phage DNA integrates into the bacterial chromosome,

becoming a prophage.

New phage DNA and proteins are synthesized

and assembled into phages.

Occasionally, a prophage exits the bacterial chromosome,

initiating a lytic cycle.

Certain factorsdetermine whether

The phage attaches to ahost cell and injects its DNA.

Phage DNAcircularizes

The cell lyses, releasing phages.

Lytic cycleis induced

Lysogenic cycleis entered

Lysogenic cycleLytic cycle

or Prophage

Bacterialchromosome

Phage

PhageDNA

12

Bacterial Defense• What defense do bacteria have against

phage infection?• Restriction enzymes• What do restirciton enzymes do?• They cut up DNA The bacterial DNA is

modified to protect it from the restriction endonucleases.

14

Herpesvirus• Consists of double stranded DNA• Envelope derived from host cell nuclear

envelope not from plasma membrane• It, therefore, reproduces within the

nucleus• May integrate its DNA as a provirus• Tends to recur throughout lifetime of

infected individual.• Why do you think your immune system

never removes this virus from your body?

 SARS (severe acute respiratory syndrome), a recently emerging viral disease

(a) Young ballet students in Hong Kong wear face masks to protect themselves

from the virus causing SARS.

(b) The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glycoprotein spikes protruding from

the envelope.

The reproductive cycle of an enveloped RNA virus

Capsid and viral genomeenter cell

2

The viral genome (red)functions as a template for

synthesis of complementary RNA strands (pink) by a viral

enzyme.

3

New virus8

RNA

Capsid

Envelope (withglycoproteins)

Glycoproteins on the viral envelope bind to specific receptor molecules

(not shown) on the host cell, promoting viral entry into the cell.

1

New copies of viralgenome RNA are made

using complementary RNAstrands as templates.

4

Vesicles transportenvelope glycoproteins to

the plasma membrane.

6

A capsid assemblesaround each viralgenome molecule.

7

Complementary RNAstrands also function as mRNA,

which is translated into bothcapsid proteins (in the cytosol)and glycoproteins for the viral

envelope (in the ER).

5

HOST CELL

Viral genome (RNA)

Template

Capsidproteins

Glyco-proteins

mRNA

Copy ofgenome (RNA)

ER

19

RNA Viruses• Different classes of RNA viruses: single

stranded range from class IV to class VI• Class IV: invades as mRNA, is ready for

translation• Clas V: RNA serves as template for

mRNA synthesis• RNA is transcribed and serves as

template for mRNA and genomic RNA• Viral replicase enzyme is packages

within viral capsid.

20

Class VI: Retroviruses

 The structure of HIV, the retrovirus that causes AIDS

Reversetranscriptase

Viral envelope

Capsid

Glycoprotein

RNA(two identical

strands)

Why are RNAViruses

More deadly than DNA viruses?

The reproductive cycle of HIV, a retrovirus

Vesicles transport theglycoproteins from the ER tothe cell’s plasma membrane.

7

The viral proteins include capsid proteins and reverse transcriptase

(made in the cytosol) and envelope glycoproteins (made in the ER).

6

The double-stranded DNA is incorporated

as a provirus into the cell’s DNA.

4

Proviral genes are transcribed into RNA molecules, which serve as genomes for the

next viral generation and as mRNAs for translation into viral

proteins.

5

Reverse transcriptasecatalyzes the synthesis of

a second DNA strandcomplementary to the first.

3

Reverse transcriptasecatalyzes the synthesis of aDNA strand complementary

to the viral RNA.

2

New viruses budoff from the host cell.9

Capsids areassembled aroundviral genomes and

reverse transcriptase molecules.

8

mRNA

RNA genomefor the next

viral generation

Viral RNA

RNA-DNAhybrid

DNA

ChromosomalDNA

NUCLEUSProvirus

HOST CELL

Reverse transcriptase

New HIV leaving a cell

HIV entering a cell

0.25 µm

HIV Membrane of white blood cell

The virus fuses with thecell’s plasma membrane.The capsid proteins areremoved, releasing the viral proteins and RNA.

1

Virus videos

24

Vaccines / Drugs

• What are vaccines and how do they work?

• Introduce body to harmless or weakened strain of the virus, so that your immune system learns to recognize the virus prior to invasion

• Few drugs around to fight viruses, most interfere with DNA, RNA or protein synthesis

25

Viroids and Prions

• Viroids are naked circular RNA that infect plants

• Prions are proteins that infect cells• Examples of prions seen in scrapies

in sheep, mad-cow disease, and Creutzfeldt-Jakob disease in humans

• How can a prion spread infection?• Altered versions of proteins that can

alter other proteins

Model for how prions propagate

Prion

Normalprotein

Originalprion

Newprion

Many prions

Viruses and Cancer

Viral Defenses

Dengue Invasion Video

Review Questions

1. Viruses are not classified as prokaryotes because:

A. They contain membrane bound organellesB. They are multicellularC. They are unicellularD. They are acellularE. They lack genetic material

2. Which of the following can be found in all viruses:

A. RibosomesB. RNAC. DNAD. A Protein CoatE. Cell membrane

3. The virus HIV replicates using reverse transcriptase. Thus it can be inferred that the virus

A. Uses only DNAB. Uses only RNAC. Uses RNA as a template for DNAD. Uses DNA as a template for RNAE. Replicates continuously