The Genetics of Viruses and Bacteria

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The Genetics of Virusesand Bacteria
Chapter 18
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Microbial Model Systems

• Recall that bacteria are prokaryotes
• With cells much smaller and more simply organized
  than those of eukaryotes
• Viruses
• Are smaller and simpler still

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Viruses

• Viruses called bacteriophages
• Can infect and set in motion a genetic takeover
  of bacteria, such as Escherichia coli

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Obligate Intracellular Parasites

• A virus has a genome but can reproduce only
  within a host cell

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The Discovery of Viruses Scientific Inquiry

• Tobacco mosaic disease
• Stunts the growth of tobacco plants and gives
  their leaves a mosaic coloration

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TMV

• In the late 1800s
• Researchers hypothesized that a particle smaller
  than bacteria caused tobacco mosaic disease
• In 1935, Wendell Stanley
• Confirmed this hypothesis when he crystallized
  the infectious particle, now known as tobacco
  mosaic virus (TMV)

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Viruses

• Are very small infectious particles consisting of
  nucleic acid enclosed in a protein coat and, in
  some cases, a membranous envelope

• Viral genomes may consist of
• Double- or single-stranded DNA
• Double- or single-stranded RNA

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Capsids and Envelopes

• A capsid
• Is the protein shell that encloses the viral
  genome

TMV Adenovirus
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Capsids and Envelopes

• Envelopes
• Membranous coverings derived from the membrane of
  the host cell

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Viral Envelopes

• Many animal viruses
• Have a membranous envelope
• Viral glycoproteins on the envelope
• Bind to specific receptor molecules on the
  surface of a host cell

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Bacteriophages

• A.K.A. phages
• Have the most complex capsids found among viruses

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General Features of Viral Reproductive Cycles

• Viruses are obligate intracellular parasites
• They can reproduce only within a host cell
• Each virus has a host range
• A limited number of host cells that it can infect

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General Features of Viral Reproductive Cycles

• Viruses use enzymes, ribosomes, and small
  molecules of host cells to synthesize progeny
  viruses

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Viral Reproductive Mechanisms

• Lytic cycle
• Is a phage reproductive cycle that culminates in
  the death (lysis) of the host
• Lysogenic cycle
• Replicates the phage genome without destroying
  the host

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Lytic Cycle (Viral Reproduction)
DOCKING with the host receptor protein PENETRATION
of the viral nucleic acid into the host
cytoplasm (Restriction Endonucleases, A.K.A.
restriction enzymes break up host DNA)
BIOSYNTHESIS of the viral components Assembly
(MATURATION) of the viral components into
complete viral units RELEASE of the completed
virus from the host cell
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Lysogenic Cycle
Lambda

• Temperate phages
• Are capable of using both the lytic lysogenic
  cycles of reproduction

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Prophage
When viral DNA is integrated into the bacterial
chromosome (Plasmid)
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• Retroviruses, such as HIV, use the enzyme reverse
  transcriptase
• To copy their RNA genome into DNA, which can then
  be integrated into the host genome as a provirus
  (Integrates into host DNA)

2 of each
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Evolution of Viruses

• Viruses do not really fit our definition of
  living organisms since viruses can reproduce only
  within cells
• They probably evolved after the first cells
  appeared, perhaps packaged as fragments of
  cellular nucleic acid

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Viral Diseases in Animals

• Viruses may damage or kill cells
• By causing the release of hydrolytic enzymes from
  lysosomes
• Some viruses cause infected cells
• To produce toxins that lead to disease symptoms

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Viral Diseases in Animals

• Viruses may damage or kill cells (Amount
  of damage depends on the ability of infected
  tissue to regenerate by mitosis)
• -Respiratory tract epithelium repairs
  quickly from adenovirus infection
• - Nerve tracts affected by polio virus is
  permanent
• Find host cells using lock key fit with
  proteins on virus host cell receptors

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Prions

• Protein infectious particles
• Contain no RNA or DNA
• Long incubation period (10 years)

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Prions

• Are slow-acting, virtually indestructible
  infectious proteins that cause brain diseases in
  mammals
• Propagate by converting normal proteins into the
  prion version

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Emerging Viruses

• Are those that appear suddenly or suddenly come
  to the attention of medical scientists
• 3 processes contribute to emerging viruses
• Mutation of existing viruses as RNA is not
  corrected by proofreading e.g. SARS
• Spread from one host species to another e.g.
  Hanta Virus
• Dissemination from a small isolated population
  e.g. HIV

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SARS Severe Acute Respiratory Syndrome
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Emerging Viruses are NOT new

• They are existing viruses that
• Mutate
• Spread to new host species
• Disseminate more widely in the host species

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Small Pox
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Polio
Polio
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Herpes Simplex
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Hepatitis
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Varicella Zoster
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Mumps
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Measles - Rubeola
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Other Viruses that affect humans

• Influenza Virus
• Rubella
• Parvo-virus
• Epstein Barr Virus
• Hanta Virus
• (HPV) Human Papilloma Virus
• (RSV) Respiratory Syncytial Virus
• Rabies
• Rhinovirus
• Rotavirus
• West Nile Virus

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Viral Diseases in Plants

• More than 2,000 types of viral diseases of plants
  are known
• Common symptoms of viral infection include
• Spots on leaves and fruits, stunted growth, and
  damaged flowers or roots

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Viral Diseases in Plants

• Plant viruses spread disease in two major modes
• Horizontal transmission, entering through damaged
  cell walls
• Vertical transmission, inheriting the virus from
  a parent

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Viroids -The Simplest Infectious Agent

• Are circular RNA molecules that infect plants and
  disrupt their growth

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Bacteria

• Rapid reproduction, mutation, and genetic
  recombination contribute to the genetic diversity
  of bacteria
• Bacteria allow researchers
• To investigate molecular genetics in the simplest
  true organisms

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The Bacterial Genome and Its Replication

• The bacterial chromosome
• Is usually a circular DNA molecule with few
  associated proteins
• In addition to the chromosome
• Many bacteria have plasmids, smaller circular DNA
  molecules that can replicate independently of the
  bacterial chromosome

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The Bacterial Genome and Its Replication

• Bacterial cells divide by binary fission
• Which is preceded by replication of the bacterial
  chromosome

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Mutation and Genetic Recombination as Sources of
Genetic Variation

• Since bacteria can reproduce rapidly
• New mutations can quickly increase a populations
  genetic diversity
• Genetic diversity
• Can also arise by recombination of the DNA from
  two different bacterial cells
• Remember that prokaryotes dont undergo meiosis
  or fertilization

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Recombination in Bacteria

• Three processes bring bacterial DNA from
  different individuals together
• Transformation
• Transduction
• Conjugation

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Transformation
Is the alteration of a bacterial cells genotype
and phenotype by the uptake of naked, foreign DNA
from the surrounding environment
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Transduction
Phages carry bacterial genes from one host cell
to another
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Conjugation and Plasmids

• Conjugation
• Is the direct transfer of genetic material
  between bacterial cells that are temporarily
  joined

DNA transfer is one way
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The F Plasmid and Conjugation

• Cells containing the F plasmid, designated F
  cells
• Function as DNA donors during conjugation
• Transfer plasmid DNA to an F? recipient cell

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F Plasmid recombination

• Chromosomal genes can be transferred during
  conjugation when the donor cells F factor is
  integrated into the chromosome
• Hfr cell
• A cell with the F factor built into its
  chromosome
• The F factor of an Hfr cell
• Brings some chromosomal DNA along with it when it
  is transferred to an F cell

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R plasmids and Antibiotic Resistance
Confer resistance to various antibiotics
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Transposition of Genetic Elements

• Transposable elements
• Can move around within a cells genome
• Are often called jumping genes
• Contribute to genetic shuffling in bacteria by
  folding the DNA

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Insertion Sequences

• An insertion sequence contains a single gene for
  transposase
• An enzyme that catalyzes movement of the
  insertion sequence from one site to another
  within the genome

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Transposons

• Bacterial transposons
• Also move about within the bacterial genome
• Have additional genes, such as those for
  antibiotic resistance

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Prokaryotic Gene Expression

• Individual bacteria respond to environmental
  change by regulating their gene expression
• E. coli, a type of bacteria that lives in the
  human colon
• Can tune its metabolism to the changing
  environment and food sources

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Response to the environment

• This metabolic control occurs on two levels
• Adjusting the activity of metabolic enzymes
  already present
• Regulating the genes encoding the metabolic
  enzymes

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Feedback Inhibition
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Operons The Basic Concept

• In bacteria, genes are often clustered into
  operons, composed of
• An operator, an on-off switch
• A promoter
• Genes for metabolic enzymes
• An operon
• Is usually turned on
• Can be switched off by a protein called a
  repressor

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Operon Parts

• The regulatory gene codes for the repressor
  protein.
• The promoter site is the attachment site for RNA
  polymerates.
• The operator site is the attachment site for the
  repressor protein.
• The structural genes code for the proteins.
• The repressor protein is different for each
  operon and is custom fit to the regulatory
  metabolite. Whether or not the repressor protein
  can bind to the operator site is determined by
  the type of operon.
• The regulatory metabolite is either the product
  of the reaction or the reactant depending on the
  type of operon.

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Operon Parts

• The repressor protein is different for each
  operon and is custom fit to the regulatory
  metabolite. Whether or not the repressor protein
  can bind to the operator site is determined by
  the type of operon.
• The regulatory metabolite is either the product
  of the reaction or the reactant depending on the
  type of operon.

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The trp operon regulated synthesis of
repressible enzymes
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Trp Operon
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Repressible and Inducible Operons Two Types of
Negative Gene Regulation

• In a repressible operon
• Binding of a specific repressor protein to the
  operator shuts off transcription (Found in
  anabolic pathways)
• In an inducible operon
• Binding of an inducer to an innately inactive
  repressor inactivates the repressor and turns on
  transcription (Found in catabolic pathways)

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Lac Operon
The lac operon regulated synthesis of inducible
enzymes
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Lac Operon off
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Lac Operon on
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Types of Operons

• Inducible enzymes
• Usually function in catabolic pathways
• Repressible enzymes
• Usually function in anabolic pathways