+ genetics of viruses. + viruses -small infectious particles made of nucleic acid (dna or rna)...
TRANSCRIPT
+
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