Transcript APCh. 18virus & bacteriaShort For Class

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Chapter 18~
Microbial Models:
The Genetics of
Viruses and Bacteria
Viral structure
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Virus: “poison” (Latin);
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infectious particles
nucleic acid in a protein coat
(capsid)
DNA or RNA
Bacteriophages (phages)
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Some viruses have viral
envelopes, membranes
cloaking their capsids.
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Mostly, these envelopes are
derived from the membrane
of the host cell.
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They also have some viral
proteins and glycoproteins.
Fig. 18.2c
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Viral infection
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Host range:
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Each type of virus can only infect and parasitize a
limited range of host cells by a “lock and key” fit
between viral proteins and receptors on host cell
membrane.
May be narrow (1 species) or wide ( several species)
In eukaryotes, viruses are tissue specific
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HIV: Helper T cells
Cold virus-cells lining URT
Viral reproduction: Lytic Cycle
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Results in death of host
cell
1. The lytic cycle:
1- attachment
2- injection
3- hydrolyzation
4- assembly
5- release
 Virulent virus (phage
reproduction only by the
lytic cycle)
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Viral reproduction: Lysogenic Cycle
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2.Lysogenic: Genome
replicated w/o destroying
the host cell
Genetic material of virus
becomes incorporated into
the host cell DNA
(prophage DNA).
3. Temperate virus
(phages capable of using
the lytic and lysogenic
cycles)
Lysogenic Cycle: various Herpes
viruses
Herpes Simplex ( cold sores)
Herpes Varicella-zoster ( chickenpox
& shigles)
Epstein Barr ( mononucleosis)
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RNA viruses: a look at the
Retroviruses
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Retroviruses: transcribe
DNA from an RNA
template (RNA--->DNA).
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Reverse transcriptase
(catalyzing enzyme)
HIV--->AIDS
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Viroids and prions
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Viroids: tiny, naked
circular RNA that infect
plants; do not code for
proteins, but use cellular
enzymes to reproduce;
stunt plant growth
Prions: “infectious
proteins”; “mad cow
disease”; trigger chain
reaction conversions; a
transmissible protein
Bacterial genetics
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Nucleoid:
region in bacterium
densely packed with DNA
(no membrane)
Plasmids:
small circles of DNA
Reproduction:
binary fission (asexual)
Bacterial DNA-transfer processes
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Transformation: genotype alteration by
the uptake of naked, foreign DNA from
the environment (Griffith expt.)
Transduction: phages that carry
bacterial genes from 1 host cell to
another •generalized~ random transfer
of host cell chromosome •specialized~
incorporation of prophage DNA into
host chromosome
Conjugation: “baxterial sex”
direct transfer of genetic material;
cytoplasmic bridges; pili; sexual
Conjugation transfers genetic material between
two bacterial cells that are temporarily joined.
 One cell (“male”) donates DNA and its “mate”
(“female”) receives the genes.
 A sex pilus from the male initially joins the two
cells and creates a cytoplasmic bridge between
cells.
 “Maleness,” the ability to form
a sex pilus and donate DNA,
results from an F factor as a
section of the bacterial
chromosome or as a plasmid.
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Fig. 18.14
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Transduction occurs when a phage carries
bacterial genes from one host cell to another.
 In generalized transduction, a small piece of
the host cell’s degraded DNA is packaged within
a capsid, rather than the phage genome.
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When this pages attaches to another bacterium, it will
inject this foreign DNA into its new host.
Some of this DNA can subsequently replace the
homologous region of the second cell.
This type of transduction transfers bacterial genes
at random.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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Specialized transduction occurs via a
temperate phage.
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When the prophage viral genome is excised from the
chromosome, it sometimes takes with it a small
region of adjacent bacterial DNA.
These bacterial genes are injected along with the
phage’s genome into the next host cell.
Specialized transduction only transfers those genes
near the prophage site on the bacterial chromosome.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Bacterial Plasmids
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Small, circular, self-replicating DNA separate from the
bacterial
chromosome
F (fertility) Plasmid: codes for the production of sex
pili (F+ or F-)
R (resistance) Plasmid: codes for antibiotic drug
resistance
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Transposons
jumping genes”
 Barbara McClintock
transposable genetic element
piece of DNA that can move from
location to another in a cell’s genome
(chromosome to plasmid, plasmid to
plasmid, etc.);
May alter gene expression
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Operons
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Unit of genetic function consisting of coordinately related
clusters of genes with related functions (transcription unit)
In Prokaryotes
One Promoter
Advantage: one “Off/on” switch controls related genes.
Operator: these are the switches. By itself, operator is
“on”. Repressor turns it “off”
Located between the promoter and the genes
Repressible & Inducible Operons
Repressors (repressible operons)
Or
Inducers (inducible operons)
Operons
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Repressible (trp operon):
tryptophan (a.a.) synthesis
Tryptophan: corepressor
promoter: RNA polymerase binding
site; begins transcription
operator: controls access of RNA
polymerase to genes
(tryptophan not present)
repressor: protein that binds to
operator and prevents attachment
of RNA polymerase ~ coded from
a regulatory gene (tryptophan
present ~ acts as a corepressor)
transcription is repressed when
tryptophan binds to a regulatory
protein
Operons, II
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Inducible (lac operon):
lactose metabolism
lactose not present:
repressor active, operon off;
no transcription for lactose
enzymes
lactose present:
repressor inactive, operon on;
inducer molecule inactivates
protein repressor (allolactose)
transcription is stimulated when
inducer binds to a regulatory
protein
Def: Unit of genetic function consisting of
coordinately related clusters of genes with
related functions (transcription unit)
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An Inducer (Allolactose—isomer of
lactose) inactivates the repressor (turning on
these genes so lactose can be digested)
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More active in absence of glucose