Transcript Viruses and Bacteria

Viruses and Bacteria
Ch. 18
Viruses
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Parasite that requires a host cell in order to live
They take the host cell hostage and use the cell to
create the proteins it needs to make new viruses
 Typically creates thousands of new viruses and
eventually kills the host cell
The range of organisms that a virus can attack is
referred to as its host range
A new viral disease that harms humans (ex. Hanta virus)
may result from a mutation in the virus that expanded its
host range
Structure
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Made of either DNA or RNA enclosed in a protein coat
called a capsid
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Many viruses also have a viral envelope which is a
membrane that cloaks the capsids
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A virus can only infect one specific type of cell
 The reason for this is that the virus binds to specific
receptors on the surface of the cell
Bacteriophages
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Most complex virus; Infects bacteria
Also known as the shortened term “phages”
Can reproduce by either the lytic cycle or lysogenic
cycle
Lytic Cycle:
 Phage enters host cell
 Takes control
 Replicates itself
 Causes cell to burst
 This releases a new generation of phage viruses to
go and do the same thing
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Lysogenic Cycle
 Replicate without killing the host cell
 Incorporated into the host’s DNA
 Stays dormant and is called a prophage
 When host cell divides, prophage is replicated with it
 Leads to a population of infected cells
 Environmental trigger causes prophage to switch to the
lytic phase
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Viruses capable of both types of reproducing are called
temperate viruses
Retroviruses
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Viruses that contain RNA instead of DNA and replicate
unusually
Virus infects host cell, uses its own RNA to create
complementary DNA (cDNA)
Reverse the usual flow of information from DNA to RNA
 This is why they are called retroviruses
Reverse transcription is made possible by an enzyme
called reverse transcriptase
Newly made DNA integrates into a chromosome in the
nucleus
The integrated viral DNA is called a provirus
Transduction
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Phages carry bacterial genes from one host cell to
another
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There are two forms of transduction:
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Generalized Transduction: moves random pieces of
bacterial DNA as a cell is lysed by the phage and
infects another (lytic cycle)
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Restricted Transduction: moves specific pieces of
DNA that were near the prophage site on the bacterial
chromosome. Can only be done by a temperate
phage.
Prions
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Infectious proteins that can cause several brain diseases like
“mad cow” disease in cattle
Prion is a misfolded version of a protein normally found in the
brain
Causes normal proteins to misfold in same way
Prions are not considered cells or viruses
Bacteria
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Chromosomes are circular, double-stranded DNA tightly
packed into a nucleoid
A nucleoid has no nuclear membrane
Bacteria replicate DNA in both directions from a single
origin of replication
 This is called theta replication
Reproduction:
 Conjugation: direct transfer of genetic material
between two bacterial cells that are temporarily
connected
 Binary Fission: asexual reproduction
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Plasmid: foreign, small, circular, self-replicating DNA
molecule in a bacterium
A bacterium can have several plasmids
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The first plasmid discovered is the F plasmid
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F stands for fertility
Bacteria that contain the plasmid are F+
Bacteria that do NOT contain the plasmid are FThe F plasmid has the genes that produce pili
Pili are cytoplasmic bridges that connect cells and
allow conjugation
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Another plasmid worth knowing is the R plasmid
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Resistance to antibiotics for the host bacteria
R plasmid can also be transferred from one bacteria
to another by conjugation
Bacteria that have the R plasmid have a big
advantage because they are resistant to antibiotics
Resistant bacteria are more likely to survive and
reproduce quickly
This is bad news for doctors!
The Operon
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Important to gene regulation
Operon is a set of genes and “switches” that control the
expression of those genes
Discovered in 1940’s by Jacob and Monod
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Two types of operons:
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Inducible: switched off until it is “induced” to turn on
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Repressible: always on until it is not needed and
becomes “repressed” (turned off)
Tryptophan Operon
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A repressible operon
Continuously turned on unless turned off by a corepressor
Repressor is encoded by the regulatory gene and is initially
inactive
This means RNA polymerase is free to bind to the promoter
and transcribe the structural genes, producing tryptophan
When the inactive repressor combines with the corepressor
(tryptophan) it changes its shape and binds to the operator,
stopping transcription
If tryptophan levels are high, no more is needed, so the
production is turned off
Tryptophan is an allosteric effector
Lac Operon
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Inducible Operon
Turned off until it is “induced” to turn on
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Repressor is initially active; binds to the operator located on
the promoter, which blocks RNA polymerase from binding
there, stopping production
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Allolactose (derivative of lactose) acts as the inducer
 It binds to the repressor, changing its conformation (shape)
 Now that the shape has changed the repressor cannot bind
to the operator, which turns the operon on
Transposons
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Transposable genetic elements
Also called “jumping genes”
Discovered by Barbara McClintock
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Two types:
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Insertion sequences: made of only one gene, which
codes for transposase, the enzyme responsible for
moving the sequence from one place to another
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Complex transposons: longer and include extra
genes. One or more genes located between two
insertion sequences