CH. 19 VIRUSESObjective:

EK 3.C.3: Viral replication results in genetic variation,

and viral infection can introduce genetic variation into the hosts.

L.O. 3.29 The student is able to construct an explanation of how viruses introduce genetic variation in host organisms.

19.1 A Virus Consists of a Nucleic Acid Surrounded By a Protein Coat• 1st discovered viruses – Tobacco Mosaic Virus

• Filtered plant sap still infected plants.• Something smaller (viruses) was infected them.

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• Nucleic Acid: typically 1 linear strand or circular molecule

• Double stranded DNA• Single stranded DNA• Double stranded RNA• Single stranded RNA

• Protein shell (capsid) encloses nucleic acid. These come in many shapes/sizes.

• Some have accessory structures:• Viral envelopes• Tail fibers

Capsomereof capsid

RNA CapsomereDNA

Glycoprotein Glycoproteins

Membranousenvelope RNA

CapsidHead

DNA

TailsheathTailfiber

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

19.2 Viruses Replicate Only in Host Cells

• Viruses do not have organelles to build proteins, replicate nucleic acids, metabolize, etc. Needs a host cell to do it.

• Viruses can only infect certain hosts (host range)• Only certain species• Only specific cells/tissues within species

• Ex: flu only infects cells of the respiratory system

General Features of Viral Replicative Cycles

• Infection begins with the insertion of the viral nucleic acid into the host cell:• Use tail fibers to inject DNA into host cell (like a needle)• Endocytosis of entire virus

• Viral DNA commandeers cells to make more viruses using the cell’s own machinery.

• Kills cells when done.

The Lytic Cycle• The virus injects DNA

into the host cell• Host cells starts

copying viral DNA and making capsid and accessory parts.

• Once done, the cell lyses (bursts) releasing new viruses to infect other cells.

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

© 2011 Pearson Education, Inc.

Animation: Phage T4 Lytic CycleRight-click slide / select “Play”

The Lysogenic Cycle• Same beginning and end

as lytic cycle• Insert DNA …… make

copies and cell lyses

• After the DNA is inserted, it is integrated into host cell’s DNA.• Prophage; 1 gene codes

for a protein that prevents transcription of the rest of its genes.

• Cell grows and replicates (binary fission) normally with viral DNA still “hiding” in host DNA.

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.

© 2011 Pearson Education, Inc.

Animation: Phage Lambda Lysogenic and Lytic CyclesRight-click slide / select “Play”

Replicative Cycles of Animal Viruses

• Be aware of (but don’t memorize) that animal viruses are classified according to type of nucleic acid and presence of an envelope.

RNA as Viral Genetic Material

• Class IV – viral RNA serves as mRNA to immediately make proteins.

• Class V – viral RNA used to make many mRNAs.• Class VI (retroviruses) –

• RNA + reverse transcriptase DNA• Viral DNA embeds in host’s DNA• Viral proteins made with host’s

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

Figure 19.8

© 2011 Pearson Education, Inc.

Animation: HIV Reproductive CycleRight-click slide / select “Play”

Evolution of Viruses• Not living but effects all living things.• Evolved after life.• Naked DNA infected injured cells at first, then evolved to have an envelope and could infect healthy cells.

• Viral nucleic acids are more similar to host’s nucleic acids but there are some viral genes that are similar for all viruses.