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Viruses
Lecture 16
Fall 2008
Viruses
• What is a virus?
• Are viruses alive?
• Read Discovery of Viruses
pgs. 381-382 and Fig. 19.2
1
Virus Structure
• Capsid - protein shell
– Capsomere – protein subunits
• Viral envelopes - Membranous envelope
– Host phospholipids and membrane proteins
– Viral proteins & glycoproteins
Fig. 19.3
2
Virus Structure
• Nucleic acids
– Double-stranded DNA
– Single stranded DNA
– Double-stranded RNA
– Single-stranded RNA
• Single linear or circular molecule
– 4 to ~ 1000 genes
3
4
Virus Reproduction
• Obligate intercellular parasites
• Host range
– Limited range of host cells
each type of virus can infect
– ~ one cell type in multicellular
organisms
• Host specificity
– Recognition proteins “lock &
key”
Virus Reproduction
• Recognition
– Virus binds to host
• Entry
– DNA only or entire virus
• Injection
• Endocytosis
• Fusion of viral envelope with host
membrane
• Reproduction
– Genome copied
• DNA virus - host’s DNA
polymerase
• RNA virus – virally encoded
polymerases
– Genome transcribed/translated
Fig. 19.4
5
6
Reproductive Cycles of Phages
• Lytic cycle
– Results in the
release of new
phases by lysis of the
host cell
– New phages can
infect other cells
• Virulent phage
– Reproduces only by
lytic cycle
Fig. 19.5
7
Reproductive Cycles of Phages
• Bacterial defense against phages
– Natural selection for different receptor
proteins
– Restriction enzymes
• Enzymes that cut DNA at specific sequences
• Host DNA methylated, so virus DNA recognized as
foreign
– Co-evolution
Reproductive Cycles of Phages
8
• Lysogenic cycle
– Viral genome becomes
incorporated into the
bacterial host
chromosome as a
prophage and does not
kill the host
• Prophage
– A phage genome that
has been inserted into
a specific site on a
bacterial chromosome
Fig. 19.6
9
Reproductive Cycles of Phages
• Temperate phages
– Use both modes of reproduction
• Environmental cues stimulate switch from
lysogenic to lytic cycle
Fig. 19.6
10
Reproductive Cycles of Phages
• Prophage mostly silent
– One gene codes to block transcription of most
other genes
• Some genes may be transcribed and alter
bacteria
– Prophage genes code for toxins in bacteria
causing diphtheria, botulism and scarlet fever
– Prophage in harmless E. coli strain causes
food poisoning
11
Animal Viruses
• Many animal viruses have
envelopes
• Glycoproteins in envelope
bind to receptor proteins of
host
• Production of new
glycoproteins
–
–
–
–
Coded by viral genome
Proteins made in ER
Sugars added in ER & GA
Transported to host plasma
membrane
• New capsids w/viral DNA
bud from host cell
• May not kill the host cell
Fig. 19.7
12
Animal Viruses
• Single-stranded RNA viruses
– Serves as RNA
• Can be translated into proteins right away
– Template for mRNA synthesis
– Template for DNA synthesis
– See table 19.1
• RNA-RNA synthesis requires viral enzymes
– Carried with genome in capsid
13
Animal Viruses
Retrovirus
• An RNA virus that
reproduces by
transcribing its RNA
into DNA and then
inserting the DNA into a
cellular chromosome
• Reverse transcriptase
– Enzyme that transcribes
RNA template to DNA
Fig. 19.8
14
Animal Viruses
Retrovirus
• E.g., HIV (human
immunodeficiency
virus)
– Capsid contains two
molecules of ssRNA
and two molecules of
reverse transcriptase
• Provirus
– Integrated viral DNA
– Never leaves cell latency
Fig. 19.8
15
• Read Pg. 390 Evolution of Viruses
16
Viral Diseases
• Viral diseases
–
–
–
–
Cell death
Cell releases toxins
Immune responses to infected cells
Ability of cells to regenerate
• Polio - mature nerve cells don’t divide
• Vaccine
– A harmless variant or derivative of a pathogen that
stimulates a host’s immune system to mount
defenses against the pathogen
– E.g., smallpox vaccines
Emerging Viruses
• Ability of virus to mutate
– RNA viruses have very high mutation rate –
copy errors are not corrected by proofreading
• Dissemination from small isolated human
populations
• Virus spreads from other animals to human
population
– “Spanish flu” pandemic of 1918
• 40 million people died
17
18
Other Infectious Agents
• Viroids
– Molecule of naked, circular RNA a few
hundred nucleotides long
– Specific to plants
– Replicate in host cells
– Do not encode proteins
– Interfere with plant growth regulatory system
19
Other Infectious Agents
• Prions
– Infectious agent that is a misfolded version of
a normal cellular protein
– Cause degenerative brain diseases
• E.g., scrapie (sheep), mad cow disease,
Creutzfeldt-Jakob disease (human)
– Can be transmitted by eating infected meat
20
Other Infectious Agents
Prions
• Long incubation period (~10 years)
• Not destroyed by normal cooking temperatures
• Prion propagation
– Prion converts normal proteins to misfolded proteins
and forms aggregates
Fig. 19.11
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