Ch 18 Microbial Models: Genetics of Viruses and Bacteria

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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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