chapter 19: the genetics of viruses and bacteria
TRANSCRIPT
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Chapter 19:
The Genetics of
Viruses and
Bacteria
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What is Microbiology?
Microbiology is the science that studies
microorganisms = living things that are too
small to be seen with the naked eye
Microorganisms cannot be distinguished
phylogenetically from “Macroorganisms”
Includes fungi, protists, and bacteria
A microbiologist usually first isolates a
specific microorganism from a population
and then cultures it
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Relative Microbe Sizes
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What are the different types of
DNA? • Eukaryotic DNA – linear
• Prokaryotic DNA – circular
• Mitochondrial DNA
• Plasmid – small circular DNA in bacteria
• Viral DNA
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What is a Virus?
Viruses are infectious agents of a small size and
simple composition that can multiply in living cells
of animals, plants and bacteria
Obligate parasites that are metabolically inert when
they are outside their hosts (rely on the host for its
reproduction)
Motto: Get In, Take Over, Get Out!!
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Structure of Viruses
Viral Genome = nucleic acid (DNA or RNA, single
or double stranded)
4 genes to several thousand in their genomes
Capsids and Envelopes
Capsid = protein shell
Most complex are found in bacteriophages
Envelopes = derived from host membrane and contains
host’s phospholipids and membrane proteins (influenza
virus)
Glycoproteins
Aids in attachment to host cell
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Examples of Types of Viruses
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Steps of Virus Replication
1. Attachment – binds to cell
2. Entry (nucleic acid release)
3. Synthesis (viral genome and proteins)
4. Assembly (build virus)
5. Release (lysis or chronic release, e.g.
budding – includes release of various
enveloped viruses)
Host range = limited variety of hosts to infect
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DN
A V
iru
s L
ife
Cycle
Lysis
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Viral Replication
• Viral replication differs from other
reproductive strategies and generates
genetic variation via various mechanisms
– Viruses have highly efficient replicative
capabilities that allow for rapid evolution and
acquisition of new phenotypes
– Viruses replicate via a component assembly
model allowing one virus to produce many
progeny simultaneously via the lytic cycle
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Viral Replication – Virus replication allows for mutations to occur
through usual host pathways
– RNA viruses lack replication error-checking
mechanisms, and thus have higher rates of
mutations
– Related viruses can combine/recombine
information if they infect the same host cell
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Viral Replication
• The reproductive cycles of viruses
facilitate transfer of genetic information
– Viruses transmit DNA or RNA when they
infect a host cell
• Ex: transduction in bacteria
• Ex: transposons present in incoming DNA
– “Jumping genes”
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Viral Replication
– Some viruses are able to integrate into the
host DNA and establish a latent (lysogenic)
infection. These latent viral genomes can
result in new properties for the host such as
increased pathogenicity in bacteria
• Pathogenicity = whether a virus causes a disease
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Bacteriophage Lytic Cycle
(Virulent Phage)
Lysis
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Lysogenic Cycle
(Temperate Phage)
Only
temperate
phage are
able to
display
lysogeny
Lysis
Prophage
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En
ve
lop
ed
RN
A V
iru
s
An
example of
an animal
virus
(Influenza)
Acquisition of
plasma membrane
as envelope
Budding
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HIV Life Cycle
• HIV is a well-studied system where the rapid
evolution of a virus within the host contributes to
the pathogenicity of viral infection
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HIV
Life
Cycle
Budding
Provirus
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Animal Pathogens
• Emergence of new viruses
– Mutations can lead to epidemics = new strains
of a virus arise and people don’t have an
immunity for it
– Dissemination of disease from small, isolated
human population
– Spread from other animals
– Can lead to pandemics = global epidemic
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Viroids and Prions
• Viroids = small, circular RNA molecule that
infects plants
– Not enclosed in proteins and disrupts growth
of plants
• Prions = infectious proteins
– Converts normal proteins into infectious
proteins and are virtually indestructible
– Causes brain deterioration
– Ex: Mad cow disease
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Ba
cte
ria
Re
pro
du
ctio
n Viruses move genetic material from cell to
cell
Mostly this material is their own genomes
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
sexual reproduction
Transformation, Transduction, Conjugation
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Transformation
Transformation: DNA picked up
directly from the medium and
recombined into the genome
Competent cell:
capable of
picking up DNA
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Pla
sm
ids
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Conjugation
Moves plasmid
more often than
chromosomal
DNA
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Ge
ne
raliz
ed
Tra
nsd
uctio
n
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Learning Objectives
• The student is able to construct an
explanation of how viruses introduce
genetic variation in host organisms.
• The student is able to use representations
and appropriate models to describe how
viral replication introduces genetic
variation in the viral population.