T4 bacteriophage infecting an E. coli cell

0.5 m

Science as a Process Research into tobacco mosaic disease

led to the conclusion that the pathogen was smaller than a bacterial cell

The pathogen was named virus Characteristics of viruses:

Smaller than bacteria Not cellular Composed of nucleic acid and protein Obligate intracellular parasites

Comparing the size of a virus, a bacterium, and an animal cell 0.25 m

Virus

Animalcell

Bacterium

Animal cell nucleus

Infection by tobacco mosaic virus

Figure 18.4 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

Capsids and Envelopes

Capsid =

Capsids and Envelopes

Capsid = protein coat that surrounds the viral genome

viral envelope =

Capsids and Envelopes

Capsid = protein coat that surrounds the viral genome

viral envelope = derived from host cell or nuclear membranes, it helps the virus invade

Viral Genome

Double stranded DNA Single Stranded DNA Double stranded RNA Single stranded RNA A virus has only one of these types

of nucleic acids

Viral Replication What are the possible patterns of viral

replication? DNA --> DNA RNA --> RNA, where viral genes code for

viral RNA and proteins (class IV and V) RNA --> DNA --> RNA; where viral gene

uses reverse transcriptase to create a “provirus” in the nucleus that does not leave host cell…viral RNA and protein is also made (class VI)

Bacterial Viruses Which scientists used

bacteriophages to prove that DNA was the hereditary material?

Hershey and Chase What are the two mechanisms of

phage infection? Lytic and Lysogenic cycles (of

DNA viruses)

Lytic Cycle Virulent phage … example T4 phage Steps:

1. Attachment2. Entry of phage DNA and degradation of

host DNA3. Synthesis of viral genome and protein4. Assembly 5. Release … (host cell dies while releasing

100-200 phages)

Lysogenic Cycle Temperate phage … examplelambda phage Steps:

1. Entry2. Integration of viral DNA into bacterial chromosome

creating a prophage3. Bacterium reproduces normally copying prophage and

transmitting it to daughter cells4. Under certain environmental conditions, a switchover

to lytic cycle is triggered Other prophage genes may alter host’s

phenotype and have medical significance Ex. bacteria causing diphtheria is harmless unless

infected by a phage…phage experiences a lysogenic cycle and prophage causes host cell to make a toxin that causes illness!

Bacterial Defense

What defense do bacteria have against phage infection?

Restriction enzymes (a.k.a. restriction endonucleases)

What do restriction enzymes do? They cut up DNA. The bacterial

DNA is modified to protect it from the restriction endonucleases.

Animal Viruses

What is the viral envelope? An outer membrane (outside of

the capsid) that helps the virus to invade the animal cell.

The invasion of the virus has the following stages ...

1. Attachment

2. Entry

3. Uncoating

4. RNA and protein synthesis

5. Assembly and release

Herpes virus 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

(becoming like mini-chromosomes in nucleus)

Tends to recur throughout lifetime of infected individual. Often triggered by environmental situations.

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

Class V: RNA serves as template for mRNA synthesis

Class VI: Retrovirus RNA DNA (using enzyme reverse transcriptase) RNA

The structure of HIV, the retrovirus that causes AIDS

Reversetranscriptase

Viral envelope

Capsid

Glycoprotein

RNA(two identicalstrands)

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 incorporatedas 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 ofa second DNA strandcomplementary to the first.

3

Reverse transcriptasecatalyzes the synthesis of aDNA strand complementaryto 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 nextviral 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

Reasons for success of HIV

Has an envelope Creates a provirus which stays in

the nucleus of the host cell Is an RNA virus…high rate of

mutation

Viral Disease

Some viruses have toxic components

Some cause infected cells to release enzymes from lysosomes

Recovery involves ability to repair damaged region of the body.

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 Often mimic nucleosides that would allow for

nucleic acid synthesis Ex. AZT HIV replication

acyclovor herpes

Emerging Viruses

HIV, Ebola, SARS, West Nile Virus, Influenza, Hantavirus

From where do these viruses emerge? From mutated versions of current

viruses Jump from current host to new host Move from a previously isolated region

of the world

SARS (severe acute respiratory syndrome)

(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.

Viroids and Prions

Viroids are naked circular RNA that infect plants Prions are proteins that infect cells (cause tangles of

proteins in brain) Examples of prions seen in scrapies in sheep, mad-cow

disease, and Creutzfeldt-Jakob disease (CJD) in humans Timeline of Mad Cow Disease Outbreaks

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

proteins (altered protein is thought to be a result of a mutated gene)

Or can be ingested by eating contaminated meats…

Figure 18.13 Model for how prions propagate

Prion

Normalprotein

Originalprion

Newprion

Many prions

Viral Evolution How did viruses evolve? Because viruses depend on cells for

their own reproduction, they most likely evolved after the first cells appeared.

Possible link to mobile genetic elements. (transposons, plasmids)

Much debate about viral evolution…lots to learn about viruses!