Viruses and Bacteria What is Microbiology Microbiology is the science that studies microorganisms Microorganisms roughly are those living things that are too small to be seen with the naked eye ID: 780403
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Slide1
Chapter 18:
The Genetics of
Viruses and
Bacteria
Slide2What is Microbiology?
Microbiology is the science that studies microorganisms
Microorganisms, roughly, are those living things that are too small to be seen with the naked eye
Microorganisms cannot be distinguished phylogenetically from “Macroorganisms”, e.g., includes fungi as well as bacteria, etc. (that is, they are not, as a whole, a closely related group of organisms)
Microbiology is more a collection of techniques:
Aseptic technique, Pure culture technique, Microscopic observation of whole organisms, etc.
A microbiologist usually first isolates a specific microorganism from a population and then cultures it
Slide3Importance of Microbes
Microbes are producers—they provide energy to ecosystems
Microbes are fixers—they make nutrients available from inorganic sources, e.g., nitrogen
Microbes are decomposers—they free up nutrients from no longer living sources
Microbes form symbioses (such as mycorrhizal fungi associated with plant roots—though these are somewhat macroscopic; also the bacteria found in legume root nodules, etc.)
Microbes serve as endosymbionts (e.g., chloroplasts and mitochondria)
Microbes make fermentation products (ethanol!), food (beer! Cheese! Yogurt! Half-sour pickles!), Biotech products (e.g., recombinant insulin), etc.
Germ theory of disease; Normal flora
Slide4Relative Microbe Sizes
Slide5Examples of Types of Viruses
Slide6What is a Virus?
Viruses consist of protein capsids and nucleic acid (DNA or RNA) plus some viruses (virions) have a lipid envelope (enveloped viruses)
Viruses are… “...infectious agents of small size and simple composition that can multiply only in living cells of animals, plants and bacteria
[plus fungi & protozoa]
.
Viruses are obligate parasites that are metabolically inert when they are outside their hosts. They all rely, to varying extents, on the metabolic processes of their hosts to reproduce themselves.
The viral diseases we see are due to the effects of this interaction between the virus and its host cell (and/or the host’s response to this interaction).”
Encyclopedia Britannica
Slide7Virus (Virion Particle)
The Virion is what defines a virus as a virus
A Virion is the extracellular state of a virus
The
job
of Virions is to find new cells to infect
As such, Virions are a durable state that is “designed” to attach to susceptible cells
The Virion is then responsible for translocation of the virus genome into the cell
The Virion consists of a DNA (or RNA) genome surrounded by Protein that, in turn, may be surrounded by a Lipid Bilayer
The Protein layer is called a Capsid
The Lipid Bilayer is called an Envelope
Slide8Steps of Virus Replication
Adsorption
(attachment)
Penetration
(nucleic-acid release)
Synthesis
(of RNA and proteins, as well as DNA if DNA genome)
Maturation
(assembly of virion)
Release
(lysis or chronic release, e.g., budding, with the latter coinciding with release for various enveloped viruses)
Caveat: It is important to realize that variation among viruses is between virus strains/species; any one kind of virus cannot replicate in multiple ways, have more than one virion morphology, or vary in genome type, etc.
Slide9DNA Virus Life Cycle
Lysis
Slide10Bacteriophage Lytic Cycle
Lysis
Slide11Lysogenic Cycle (Temperate Phage)
Only
temperate
phage
are able to display lysogeny
Lysis
Slide12Enveloped RNA Virus
An example of an animal virus
Acquisition of plasma membrane as envelope
Budding
Slide13HIV Life Cycle
Slide14HIV Life Cycle
Budding
Slide15Bacteria Sex
Viruses move genetic material from cell to cell
Mostly this material is their own genomes, i.e., genes that collectively code for the production of new viruses
Bacteria DNA also can move from cell to cell
Once received by a cell, this DNA may be incorporated into the bacterial genome via recombination
This idea of DNA sourced from different parents recombining into a single chromosome is equivalent to eukaryotic sex (i.e., fertilization followed by recombination)
Transformation, Transduction, Conjugation
Slide16Why study bacterial genetics?
Its an easy place to start
history
we know more about it
systems better understood
simpler genome
good model for control of genes
build concepts from there to eukaryotes
bacterial genetic systems are exploited in biotechnology
Slide17Bacteria
Bacteria review
one-celled organisms
prokaryotes
reproduce by binary fission
rapid growth
generation every ~20 minutes
10
8
(100 million) colony overnight!
dominant form of life on Earth
incredibly diverse
Slide18Bacterial genome
Single circular chromosome
haploid
naked DNA
no histone proteins
~4 million base pairs
~4300 genes
1/1000 DNA in eukaryote
Intro to Bacteria video
Slide19No nucleus
!
No nuclear membrane
chromosome in cytoplasm
transcription & translation are coupled together
no processing of mRNA
no introns
but Central Dogma
still applies
use same
genetic code
Slide20Binary fission
Replication of bacterial chromosome
Asexual reproduction
offspring genetically identical to parent
where does variation come from?
Slide21Variation in bacteria
Sources of variation
spontaneous mutation
transformation
plasmids
DNA fragments
transduction
conjugation
transposons
bacteria shedding DNA
Slide22Transformation
Transformation: DNA picked up directly from the medium and recombined into the genome
Competent cell: capable of picking up DNA
Slide23Generalized Transduction
Slide24Plasmids
Slide25Conjugation
Moves plasmid more so than chromosomal DNA
Slide26Bacterial Genetics
Regulation of Gene Expression
Slide27Bacterial metabolism
Bacteria need to respond quickly to changes in their environment
if have enough of a product,
need to stop production
why?
waste of energy to produce more
how?
stop production of synthesis enzymes
if find new food/energy source,
need to utilize it quickly
why?
metabolism, growth, reproduction
how?
start production of digestive enzymes
Slide28Regulation of Metabolism
e.g., transcription
Slide29Reminder: Regulation of metabolism
Feedback inhibition
product acts
as an allosteric inhibitor of
1
st
enzyme in tryptophan pathway
= inhibition
-
Slide30Another way to Regulate metabolism
Gene regulation
block transcription of genes for all enzymes in tryptophan pathway
saves energy by
not wasting it on unnecessary protein synthesis
= inhibition
-
Slide31Gene regulation in bacteria
Control of gene expression enables individual bacteria to adjust their metabolism to environmental change
Cells vary amount of specific enzymes by
regulating gene transcription
turn
genes on
or turn
genes off
ex.
if you have enough tryptophan in your cell then you don’t need to make enzymes used to
build
tryptophan
waste of energy
turn off genes which codes for enzymes
Slide32Control of Gene Expression
Operons- sequence of DNA that directs particular biosynthetic pathways
4 Major Components of an operon
Regulatory gene-
produces a repressor protein that prevents gene expression by blocking DNA polymerase
Promotor region-
sequence of DNA where RNA Polymerase attaches for transcription
Operator region-
can block action of RNA Polymerase if region is occupied by repressor protein
Structural gene-
contain DNA sequence that code for several related enzymes that direct production of an end product.
Slide33Control of Gene Expression
It makes energetic sense to make or use proteins responsible for certain metabolic processes only when those processes are needed.
Trp Operon-
enzymes make needed tryptophan
Repressor inactivated in response to presence of tryptophan
Tryptophan acts as Corepressor
“Repressable enzymes”- Usually turned on and has to be turned off.
Lac Operon
Controls breakdown of lactose
Lactose presence needed to turn on Operon
“inducible enzymes”- Usually turned off and needs to be turned on.
Slide34So how can genes be turned off?
First step in protein production?
transcription
stop RNA polymerase!
Repressor protein
binds to DNA near promoter region blocking RNA polymerase
binds to
operator
site on DNA
blocks transcription
Slide35Genes grouped together
Operon
genes grouped together with related functions
ex.
enzymes in a synthesis pathway
promoter = RNA polymerase binding site
single
promoter controls transcription of all genes in operon
transcribed as 1 unit & a single mRNA is made
operator = DNA binding site of regulator protein
Slide36Trp Operon (low trp densities)
Don’t worry about the names of these genes and products
Recall that the promoter is the site of RNA polymerase binding
Slide37Trp Operon (higher trp densities)
Equilibrium: Likelihood of being in bound state depends on trp density
Negative regulation
Corepression
Slide38operator
promoter
Repressor protein
model
DNA
TATA
RNA
polymerase
repressor
repressor
repressor protein
Operon
:
operator, promoter & genes they control
serve as a model for gene regulation
gene
1
gene
2
gene
3
gene
4
RNA
polymerase
Repressor protein
turns off gene by blocking RNA polymerase binding site.
Slide39operator
promoter
Repressible operon: tryptophan
DNA
TATA
RNA
polymerase
repressor
tryptophan
repressor
repressor protein
repressor
tryptophan – repressor protein
complex
Synthesis pathway model
When excess tryptophan is present, binds to
tryp
repressor protein
& triggers repressor to
bind
to DNA
blocks (represses) transcription
gene
1
gene
2
gene
3
gene
4
RNA
polymerase
conformational change in repressor protein!
Slide40Tryptophan operon
What happens when tryptophan is present?
Don’t need to make tryptophan-building enzymes
Tryptophan binds allosterically to regulatory protein
Slide41operator
promoter
Inducible operon: lactose
DNA
TATA
RNA
polymerase
repressor
repressor protein
repressor
lactose – repressor protein
complex
lactose
repressor
gene
1
gene
2
gene
3
gene
4
Digestive pathway model
When lactose is present, binds to
lac
repressor protein
& triggers repressor to
release
DNA
induces transcription
RNA
polymerase
conformational change in repressor protein!
Slide42Lactose operon
What happens when lactose is present?
Need to make lactose-digesting enzymes
Lactose binds allosterically to regulatory protein