+

Genetics of Viruses

+Viruses

-small infectious particles made of nucleic acid (DNA or RNA) enclosed in a protein coat

obligate intracellular parasites: can only reproduce within a host cell

much smaller than bacteria

http://learn.genetics.utah.edu/content/cells/scale/

+Types of Viruses

Figure 17.2

RNA

Capsomereof capsid

Glycoproteins

Capsomere Membranousenvelope

RNA

Capsid HeadDNA

DNA

Tailsheath

Tailfiber

Glycoprotein80 225 nm80–200 nm (diameter)70–90 nm (diameter)18 250 nm

(a) Tobacco mosaicvirus

20 nm(b) Adenoviruses

50 nm(c) Influenza viruses

50 nm(d) Bacteriophage T4

50 nm

(Protein)

+Capsids are built from protein subunits called capsomeres-host range, a limited number of host cells that it can infect

Figure 17.2a

RNA

Capsomereof capsid

CapsomereDNA

Glycoprotein

70–90 nm (diameter)18 250 nm

(a) Tobacco mosaicvirus

20 nm(b) Adenoviruses

50 nm

+

Some viruses have membranous envelopes that help them infect hosts

host range, a limited number of host cells that it can infect

Figure 17.2b

Membranousenvelope

RNA

CapsidHead

DNA

Tailsheath

Tailfiber

80 225 nm80–200 nm (diameter)

(c) Influenza viruses50 nm

(d) Bacteriophage T450 nm

Glycoproteins

+Bacteriophages (phages)

viruses that infect bacteria

DNA injected into host

Icosahedral head 20 sides

+

Viral Replication

Figure 17.3VIRUS

Replication

Entry anduncoating

DNA

Capsid Transcription andmanufacture ofcapsid proteins

HOSTCELL

Viral DNA

Viral DNA

mRNA

Capsidproteins

1

2

3

4 Self-assembly ofnew virus particlesand their exit fromthe cell

+Lytic Cycle

Figure 17.4-5

Attachment1

Entry of phageDNA anddegradationof host DNA

2

Synthesis ofviral genomesand proteins

3Assembly

Phage assembly

Head Tail Tailfibers

4

Release5

• New phages lyse cell wall, killing cell• Virulent phages (cause cell death)

+ replicates the phage

genome without destroying the host (temperate phages)

viral DNA molecule is incorporated into the host cell’s chromosome (prophage)

Every time the host divides, it copies the phage DNA and passes the copies to daughter cells

Figure 17.5bDaughter cellwith prophage

Many celldivisionscreate manyinfectedbacteria.

Prophage is copiedwith bacterialchromosome.

Phage DNA integrates intobacterial chromosome.

Lysogenic cycle

Prophage exitschromosome.

Prophage

lysogenic cycle

+Lytic vs lysogenic

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

The phage injects its DNA.Daughter cellwith prophage

Many celldivisionscreate manyinfectedbacteria.

Prophage is copiedwith bacterialchromosome.

Phage DNA integrates intobacterial chromosome.

Phage DNA and proteins aresynthesized and assembled.

The cell lyses, releasing phages.

Lytic cycle Lysogenic cycle

Prophage exitschromosome.

Phage DNAcircularizes.

PhageDNA

PhageBacterialchromosome

Prophage

+Viral envelopes

Used to enter host cell

Figure 17.6

Capsid

RNA

Envelope (withglycoproteins)

HOST CELL

(RNA)

New virus

Copy of genome

Viral genome(RNA)Template

mRNA

ER

Glycoproteins

Capsidproteins

Influenza-RNA

+RNA viruses

Retroviruses use reverse transcriptase to copy their RNA genome into DNA

HIV

Viral DNA that is integrated into the host genome permanently – provirus

Figure 17.7

Reversetranscriptase

HIV

GlycoproteinViral envelope

Capsid

RNA (twoidenticalstrands)

HOSTCELL

Reversetranscriptase

Viral RNA

RNA-DNAhybrid

DNA

NUCLEUS

ChromosomalDNA

RNA genomefor the nextviral generation mRNA

HIVMembrane ofwhite blood cell

HIV entering a cell0.25 m

Provirus

New virusNew HIV leaving a cell

http://www.galaxygoo.org/biochem/hiv/hiv_lifecycle.html

+Chemotherapy

http://pharmaxchange.info/press/2011/01/hiv-life-cycle/

+Evolution of Viruses

Are viruses alive?

Since viruses can replicate only within cells, they probably evolved after the first cells appeared