http://www.cellsalive.com/howbig.htm
Jan 22, 2016
http://www.cellsalive.com/howbig.htm
Ch 18
Microbial Models: Genetics
of Viruses and
Bacteria
VirusVirus-
*biological particle (not a cell)*composed of nucleic
acid (RNA or DNA)
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VirusVirus-
*surrounded by capsid
(protein coat) and sometimes a
viral envelope (membrane surrounding)
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The head of a dress-maker's pin
can provide seating accommodation for
five hundred million
rhinoviruses (cause of the common cold)
!
Why not alive?•Lacks: nucleus, cytoplasm, organelles, cell membrane
•Why not alive? •No reproduction by mitosis No meiosis
(require a host cell in order to reproduce)
obligate intracellular parasites
•Why not alive? •Resistant to Resistant to alcoholalcohol
resistant to dehydration by
osmosisobligateobligate
intracellularintracellular parasitesparasites
•Why not alive? •virus virus crystallizationcrystallization
cells cannot form crystals
obligateobligate intracellularintracellular
parasitesparasites
Lytic Lytic CycleCycle
Lytic Cycle
“lock & key”
Lytic Cyclephage injects DNA
(hopefully, for the virus, it is resistant to the restriction nucleases!)
Lytic Cyclephage DNA hydrolysises bacterial DNA via. enzymes
Lytic Cyclephage proteins are produced, phage DNA reproduced
Lytic Cyclehost cell provides: nucleotides, ribosomes, tRNA etc.
Lytic CycleLysozyme produced and bacterial wall is digested
Lytic Cycle
Lytic Cycle
Lysogenic Lysogenic CycleCycle
Lysogenic Cycletemperate temperate virusesviruses
““phage” phage” inserts inserts DNA; via. DNA; via. “lock & “lock & key”key”
Lysogenic Cycle
encorporatencorporate into e into bacterial bacterial DNA via. DNA via. crossing crossing overover
Lysogenic Cycle
cellular cellular division division
w/ a w/ a “friend” “friend”
--replicatereplicates along s along with with
bacterial bacterial DNADNA))
Lysogenic Cycle
at some at some point, point, exits the exits the chromosomechromosome, & , & initiates initiates a LYTIC a LYTIC cyclecycle
Lysogenic Cycle
Lysogenic Cycle
Phages Phages manufacturmanufactureded
virulent
latent/ tempera
te
Why haven’t phages exterminated all
bacteria?• Restriction Enzymes in bacteria– Methylation of bacterial DNA prevents the restriction enzyme from attacking its own self
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Why haven’t phages exterminated all
bacteria?• Natural Selection favors the mutant bacteria w/ protein receptors that have an altered shape– no longer recognizable by phage
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Why haven’t phages exterminated all
bacteria?• LYSOGENY
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Viruses are Classified as:
1. DNA viruses
2. RNA viruses
1. membranous envelope
presentpresent 2. membranous
envelope absentabsent
Viruses are Classified as:
Reproduce in nucleusReproduce in nucleus
Envelope derived from Envelope derived from nuclear membranenuclear membrane
Typically, leave some Typically, leave some DNA behind in nerve DNA behind in nerve cellscells
ex.ex. of enveloped =of enveloped = herpes virusherpes virus
ex. Herpes Simplex Type 1 (cold sore)
Herpes Viruses•Use nuclear membrane to
derive new envelope.•Viral DNA integrated into Host DNA as a provirus.
•Shows both lytic and lysogenic life cycles.
RETROVIRUS ex. HIV
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human immunodificiency
virus
`
host is a WBC called a T-cell
http://www.youtube.com/watch?v=B7ITZgag6w0&feature=related
http://www.youtube.com/watch?v=dyxujBKHdWc
http://www.youtube.com/watch?v=9leO28ydyfU&NR=1
HIV needs to connect to TWO protein types on the outside of the
Immune System’s Helper T Cells
HIV needs to connect to TWO protein types on the outside of the
Immune System’s Helper T Cells (WBC)
LAST KIND
• DNA
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DNA VirusesDNA Viruses1.Insert their DNA into host/ the host produces mRNA compliment
2.2.Production of new viral proteins, or join with the DNA of the host cell
3.Directs production of new viruses
Viral Viral DiseasesDiseases MeaslesMeasles
PolioPolio SmallpoxSmallpox InfluenzaInfluenza S.A.R.S.S.A.R.S. Bird fluBird flu
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“The single biggest threat to man's continued dominance on the planet is
the virus!”
-Joshua Lederberg, Ph.D, Nobel laureate 1969
(won prize with Beadle and Tatum)
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Actually, he won the prize for discovering that bacteria can exchange genes… but what a great quote!
EMERGING VIRUSES: HOW? HOW?HOW? HOW?
Mutations of existing virus
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Dissemination from
a small, isolated
population
Increasing viral traffic
ie. new roads can allow
virus to spread between
previously isolated areas
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Mutated form
spread from
animals
How to How to combate combate Virus?Virus?VACCINES VACCINES - -- -**variants of variants of pathogen pathogen
*stimulates the *stimulates the immune immune system to “arm” system to “arm” and and “defend”“defend”
Note: Can Note: Can preventprevent some some viral viral illnesses, illnesses, Cannot Cannot curecure the the viral infectionviral infection
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Some viruses cause
Human papilloma virusHuman papilloma virus
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Viral nucleic acid becomes integrated
into the hosts DNA
Proto-oncogenes code for proteins that affect the cell cycle.
•ViroidViroid-- “naked” RNA
*disrupt metabolism of plants stunts growth
•ViroidViroid--
BIG NEWS b/c *molecules* can be an infectious agent and spread
disease!
•Prions- infectious *present in brain
cells *misfolded proteins *converts “regular” proteins to prion version Mad cow disease
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