David Sadava H. Craig Heller Gordon H. Orians
William K. Purves David M. Hillis
Biologia.blu
B – Le basi molecolari della vita e
dell’evoluzione
The Genetics of Viruses
and Prokaryotes
The Genetics of Viruses and Prokaryotes
• How do viruses reproduce and transmit
genes?
• How do prokaryotes exchange genes?
• How is gene expression regulated in
prokaryotes?
The Genetics of Viruses and Prokaryotes - How do viruses reproduce and
transmit genes?
Classification:
• genome of DNA or RNA;
• nucleic acid is single- or double-stranded;
• simple or complex shape;
• whether virion is surrounded by a
membrane or not;
• type of organism it infects;
• manner of the infection.
The Genetics of Viruses and Prokaryotes - How do viruses reproduce and
transmit genes?
Virions come in various shapes
The Genetics of Viruses and Prokaryotes - How do viruses reproduce and
transmit genes?
Viruses that infect bacteria are called
bacteriophage or phage.
Phage binds to a receptor on the host cell wall,
injects the nucleic acid, then one of two things
happens:
1) phage reproduces immediately and kills the
host cell - lytic cycle - cell bursts and releases
progeny viruses;
2) postpones reproduction by integrating into the
host cell’s genome - lysogenic cycle.
The Genetics of Viruses and Prokaryotes - How do viruses reproduce and
transmit genes?
The lytic and lysogenic cycles of bacteriophage
The Genetics of Viruses and Prokaryotes - How do viruses reproduce and
transmit genes?
Animal viruses enter cells in several ways:
• a naked virion is taken up by endocytosis;
• the enveloped virus has glycoproteins that
bind to receptors on host cell, also taken in
by endocytosis (e.g., influenza);
• the membrane of the host cell and
enveloped virus fuse (e.g., HIV).
The Genetics of Viruses and Prokaryotes - How do viruses reproduce and
transmit genes?
The reproductive cycle of the
influenza virus
The Genetics of Viruses and Prokaryotes - How do viruses reproduce and
transmit genes?
HIV is a retrovirus, it has reverse
transcriptase, which facilitates RNAdirected DNA synthesis.
A DNA provirus is produced that is
integrated permanently into the host’s
genome.
When proviral DNA is activated, new
virions are produced.
The Genetics of Viruses and Prokaryotes - How do viruses reproduce and
transmit genes?
The reproductive cycle of the HIV
The Genetics of Viruses and Prokaryotes - How do viruses reproduce and
transmit genes?
Prokaryotes have several ways of
recombining genes:
• conjugation;
• transformation;
• transduction;
• plasmids;
• transposable elements.
The Genetics of Viruses and Prokaryotes - How do prokaryotes exchange
genes?
The Genetics of Viruses and Prokaryotes - How do prokaryotes exchange
genes?
Bacterial conjugation
The Genetics of Viruses and Prokaryotes - How do prokaryotes exchange
genes?
Recombination following conjugation
The Genetics of Viruses and Prokaryotes - How do prokaryotes exchange
genes?
Plasmids exist independently of the main
chromosome.
They can be transferred during
conjugation.
Plasmids don’t need to recombine with the
main chromosome.
The Genetics of Viruses and Prokaryotes - How do prokaryotes exchange
genes?
Gene transfer by plasmids
The Genetics of Viruses and Prokaryotes - How do prokaryotes exchange
genes?
Many plasmids have special genes.
Metabolic factors are plasmids with
genes for unusual metabolic functions
such as breaking down hydrocarbons.
Fertility factors (F factors) have genes
needed for conjugation; F factor can be
transferred during conjugation.
The Genetics of Viruses and Prokaryotes - How do prokaryotes exchange
genes?
Resistance factors (R factors or R
plasmids) code for proteins that modify
or destroy antibiotics.
Some provide resistance to heavy metals.
R factors have become more abundant in
modern times, possibly because of the
heavy use of antibiotics.
Antibiotic resistance is a serious threat to
human health.
The Genetics of Viruses and Prokaryotes - How do prokaryotes exchange
genes?
Transposable elements: DNA
sequences that are inserted into new
locations
Transposon is a longer transposable
element (5,000 base pairs) that carries
one or more additional genes.
Transposable elements have contributed
to the evolution of plasmids.
The Genetics of Viruses and Prokaryotes - How do prokaryotes exchange
genes?
Transposable elements and transposons
The Genetics of Viruses and Prokaryotes - How is gene expression regulated
in prokaryotes?
Prokaryotes shut down transcription by
placing an obstacle - the operator between the promoter and the structural
gene. The operator binds to a protein
called a repressor-blocks transcription of
mRNA.
Operon: the whole unit - promoter,
operator, and one or more structural
genes. Operon containing genes for
lactose metabolism: lac operon.
The Genetics of Viruses and Prokaryotes - How is gene expression regulated
in prokaryotes?
The lac Operon of E. coli
The Genetics of Viruses and Prokaryotes - How is gene expression regulated
in prokaryotes?
The lac Operon: an inducible system (part 1)
The Genetics of Viruses and Prokaryotes - How is gene expression regulated
in prokaryotes?
The lac Operon: an inducible system (part 2)
The Genetics of Viruses and Prokaryotes - How is gene expression regulated
in prokaryotes?
The trp Operon: a repressible system (part 1)
The Genetics of Viruses and Prokaryotes - How is gene expression regulated
in prokaryotes?
The trp Operon: a repressible system (part 2)