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Chapter 18
The Genetics of
Viruses and Bacteria
Introduction
• Viral/bacterial studies led to understanding
mechanisms of heredity due to simplicity
(viruses – nucleic acid & protein coat only)
Studies led to:
• better understanding of disease
• emergence of biotechnology
Discovery of viruses
• 1883 A. Mayer – studying tobacco mosaic
disease (thought to be carried by unusually
small bacteria unseen through microscope)
• 1897 Beijerenck – discovered infectious agent
could reproduce, therefore, not just a bacterial
toxin
• 1935 Stanley – crystallized infectious particle;
then others could be identified with the aid of
the electron microscope
Structure of viruses
• very small (20 nm smallest)
• virion: nucleic acid & protein coat called a
capsid
• genome can be ds DNA, ssDNA, dsRNA,
ssRNA
• linear or circular NA (4 - 700 genes)
• acellular
Identified by shape
•
capsid – protein coat may be:
1) rod (helical)
ex: TMV
2) polyhedral
ex: adenovirus
3) complex (combination)
ex: T4 bacteriophage
• Some have viral envelopes,
membranes cloaking their
capsids derived from
membrane of host cell
ex: HIV
Viral Reproduction
Lytic cycle (virulent viruses) p.332
1) adsorption (attachment) – virus
attaches to host cell
2) entry – viral NA enters host cell
3) replication – virus NA takes over
host NA & makes new viral NA &
viral protein
4) assembly – viral NA & proteins
are joined to make new viruses
5) release – viruses break out &
destroy host cell (cell is lysed)
Lysogenic cycle
(temperate viruses) p. 333
1) adsorption
2) entry – viral NA is integrated into
host NA called a prophage or
provirus
-may remain latent in host cell or
may be triggered to complete
steps like the lytic cycle
Possible triggers may include:
• Stress
• Increased temperature
ex: herpes simplex I virus
(oral)
Animal viruses
• Enveloped viruses may have easier
access into cells
• Herpes viruses derive the envelope from
nuclear membrane
Retroviruses have the most complicated life
cycles
• Contain reverse transcriptase
RNA a DNA
ex: HIV
• In some cases, viral damage is easily repaired
(respiratory epithelium after a cold), but in
others, infection causes permanent damage
(nerve cells after polio)
Emergent viruses
• HIV
• New influenza viruses
• Ebola
due to :
1) mutation of existing viruses (flu strains)
2) spread of existing viruses from 1
species to another (bird flu, hantavirus)
3) dissemination of a viral disease from a
small isolated population (AIDS)
• Viruses appear to cause certain human
cancers
-hepatitis B – liver cancer
-Epstein-Barr virus – several cancers
including Burkitt’s lymphoma
-Papilloma viruses – cervical cancer
-HTLV -1 – adult leukemia
Plant viruses
• serious agricultural pests (stunt growth,
diminish yield)
• spread easily
• most are RNA viruses
Origin
• taxonomic puzzle;
they do evolve, but they are not cells
Bacterial genome
• circular chromosome with a few
associated proteins
• accessory genes found on smaller rings of
DNA called plasmids
• replication is bidirectional from a single
origin
Reproduction in bacteria
• Asexually:
binary fission – splitting in two
• Sexually:
1) transformation – gene transfer where
bacterial cell assimilates foreign DNA
from its environment
Reproduction in bacteria
2) transduction – gene transfer from one
bacterium to another by a phage
a) generalized – random pieces of
host’s DNA are packaged within
phage capsid
b) specialized – prophage takes piece
of bacterial chromosome with it
Reproduction in Bacteria
3) conjugation – direct transfer of genes
between 2 temporarily joined bacteria
Antibiotic resistance
• bacterial genes may code for enzymes that
destroy certain antibiotics
• these genes are carried by plasmids known as
R plasmids (R = resistance)
• resistant bacteria survive to pass on these
genes (causes increase in bacterial strains that
are antibiotic resistant)
• R plasmids can be transferred by conjugation
episomes – plasmids that can integrate into
bacterial chromosome
Transposons
transposons – described by Barbara McClintock
mobile segments of DNA that may move
within a chromosome & to & from plasmids
a) conservative – changes location
without replicating first
b) replicative - replicates, remaining in
its original position, also inserting in a
new location (can move genes to totally
new areas)
Insertion sequences
• simplest transposons
• consist only of DNA necessary for
transposition (transposase)
• IS flanked by inverted repeats (noncoding
segments 20-40 nucleotides long)
• enzyme (transposase) recognizes these
inverted repeats; enzyme binds to catalyze
cutting and resealing
• While insertion sequences may not benefit
bacteria in any specific way, composite
transposons may help bacteria adapt to
new environments.
• For example, repeated movements of
resistance genes by composite transposition
may concentrate several genes for antibiotic
resistance onto a single R plasmid.
• In an antibiotic-rich environment, natural
selection factors bacterial clones that have
built up composite R plasmids through a
series of transpositions.
• Transposable genetic elements are important
components of eukaryotic genomes as well.
• In the 1940s and 1950s Barbara McClintock
investigated changes in the color of corn
kernels.
– She postulated that the changes in kernel color only
made sense if mobile genetic element moved from
other locations in the genome to the genes for kernel
color.
– When these “controlling elements” inserted next to
the genes responsible for kernel color, they would
activate or inactivate those genes.
– In 1983, more than 30 years after her initial breakthrough, Dr. McClintock received a Nobel Prize for
her discovery.
• The control of gene expression
enables individual bacteria to adjust
their metabolism to environmental
change
• An individual bacterium, locked into the
genome that it has inherited, can cope
with environmental fluctuations by exerting
metabolic control.
– First, cells vary the number of specific enzyme
molecules by regulating gene expression.
– Second, cells adjust the activity of enzymes
already present (for example, by feedback
inhibition).
• For example, the tryptophan biosynthesis
pathway demonstrates both levels of control.
– If tryptophan levels are high, some of the
tryptophan molecules can inhibit the first
enzyme in the pathway.
– If the abundance of
tryptophan continues,
the cell can stop
synthesizing additional
enzymes in this pathway
by blocking transcription
of the genes for these
enzymes.
Fig. 18.19
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Operator
• On – binding of activator protein stimulates
transcription (usually the case)
• Off – binding of specific repressor protein
shuts off transcription
constitutive genes – unregulated genes
(always needed by cell)
Repressible operon
• Inhibited by an anabolic end product (such
as trp - tryptophan)
• trp operon is an example
• tryptophan is a corepressor that binds to
repressor protein, changing it to active state
which switches off trp operon
• When trp decreases, repressor protein is no
longer bound; operator not repressed, RNA
polymerase attaches to promoter & trp
synthesis continues
Inducible operon
• function in catabolic pathways
• inducer (substrate for pathway) derepresses
operon by inhibiting the otherwise active
repressor protein
• example is lac operon (lactose)
• has 3 structural genes, repressor innately
active, binding to lac operator & switching off
the operon
• allolactose (lactose isomer) acts as inducer,
binds to and inactivates repressor protein so
operon can be transcribed
Repressible vs. Inducible
Repressible
•
•
•
•
•
inhibited by anabolic end product
inherently “on”
innately inactive repressor
under negative control
ex: trp operon
Repressible vs. inducible
Inducible
•
•
•
•
•
function in catabolic pathway
inherently “off”
innately active repressor
under negative & positive control
ex: lac operon
• Both of these are examples of negative
control – operons are switched off by the
active form of the repressor protein
• Positive control – when an activator
molecule interacts directly with the
genome to switch transcription on
• Positive gene control occurs when an activator
molecule interacts directly with the genome to
switch transcription on.
• Even if the lac operon is turned on by the
presence of allolactose, the degree of
transcription depends on the concentrations of
other substrates.
– If glucose levels are low (along with overall energy
levels), then cyclic AMP (cAMP) binds to cAMP
receptor protein (CRP) which activates
transcription.
• The cellular metabolism is biased toward the
utilization of glucose.
• If glucose levels are sufficient and cAMP
levels are low (lots of ATP), then the CRP
protein has an inactive shape and cannot
bind upstream of the lac promotor.
– The lac operon will
be transcribed but
at a low level.
Fig. 18.22b
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• For the lac operon, the presence / absence of
lactose (allolactose) determines if the operon is
on or off.
• Overall energy levels in the cell determine the
level of transcription, a “volume” control, through
CRP.
• CRP works on several operons that encode
enzymes used in catabolic pathways.
– If glucose is present and CRP is inactive, then the
synthesis of enzymes that catabolize other
compounds is slowed.
– If glucose levels are low and CRP is active, then the
genes which produce enzymes that catabolize
whichever other fuel is present will be transcribed at
high levels.