Chapter 18. The Genetics of Viruses and Bacteria. 0.5 m. Figure 18.1. Overview: Microbial Model Systems Viruses called bacteriophages Can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli. E. coli and its viruses
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The Genetics of Virusesand Bacteria
Animal cell nucleus
80–200 nm (diameter)
(c) Influenza viruses
80 225 nm
(d) Bacteriophage T4
Entry into cell and
uncoating of DNA
Self-assembly of new virus particles and their exit from cell
Attachment. The T4 phage usesits tail fibers to bind to specificreceptor sites on the outer surface of an E. coli cell.
Entry of phage DNA and degradation of host DNA.The sheath of the tail contracts,injecting the phage DNA intothe cell and leaving an emptycapsid outside. The cell’sDNA is hydrolyzed.
Release. The phage directs productionof an enzyme that damages the bacterialcell wall, allowing fluid to enter. The cellswells and finally bursts, releasing 100 to 200 phage particles.
Synthesis of viral genomes and proteins. The phage DNAdirects production of phageproteins and copies of the phagegenome by host enzymes, usingcomponents within the cell.
Assembly. Three separate sets of proteinsself-assemble to form phage heads, tails,and tail fibers. The phage genome ispackaged inside the capsid as the head forms.
The phage attaches to a
host cell and injects its DNA.
Many cell divisions produce a large population of bacteria infected with the prophage.
Occasionally, a prophage exits the bacterial chromosome,
initiating a lytic cycle.
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
The cell lyses, releasing phages.
New phage DNA and proteins are synthesized and assembled into phages.
Phage DNA integrates into the bacterial chromosome,becoming a prophage.
Glycoproteins on the viral envelope bind to specific receptor molecules(not shown) on the host cell, promoting viral entry into the cell.
Capsid and viral genome
The viral genome (red)
functions as a template forsynthesis of complementary
RNA strands (pink) by a viral
Viral genome (RNA)
strands also function as mRNA,
which is translated into both
capsid proteins (in the cytosol)and glycoproteins for the viral
envelope (in the ER).
New copies of viral
genome RNA are made
using complementary RNA
strands as templates.
envelope glycoproteins to
the plasma membrane.
A capsid assembles
around each viral
catalyzes the synthesis of a
DNA strand complementary
to the viral RNA.
The virus fuses with the
cell’s plasma membrane.
The capsid proteins are
removed, releasing the viral proteins and RNA.
Membrane of white blood cell
catalyzes the synthesis ofa second DNA strand
complementary to the first.
The double-stranded DNA is incorporated
as a provirus into the cell’s DNA.
HIV entering a cell
Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral proteins.
RNA genomefor the nextviral generation
The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER).
viral genomes and
Vesicles transport the
glycoproteins from the ER to
the cell’s plasma membrane.
New viruses bud
off from the host cell.
New HIV leaving a cell
Origin of replication
Termination of replication
Researchers had two mutant strains, one that could make arginine but not tryptophan (arg+ trp–) and one that could make tryptophan but not arginine (arg trp+). Each mutant strain and a mixture of both strains were grown in a liquid medium containing all the required amino acids. Samples from each liquid culture were spread on plates containing a solution of glucose and inorganic salts (minimal medium), solidified with agar.
Only the samples from the mixed culture, contained cells that gave rise to colonies on minimal medium, which lacks amino acids.
Because only cells that can make both arginine and tryptophan (arg+ trp+ cells) can grow into colonies on minimal medium, the lack of colonies on the two control plates showed that no further mutations had occurred restoring this ability to cells of the mutant strains. Thus, each cell from the mixture that formed a colony on the minimal medium must have acquired one or more genes from a cell of the other strain by genetic recombination.
Phage infects bacterial cell that has alleles A+ and B+
Host DNA (brown) is fragmented, and phage DNA and proteins are made. This is the donor cell.
A bacterial DNA fragment (in this case a fragment withthe A+ allele) may be packaged in a phage capsid.
Phage with the A+ allele from the donor cell infects
a recipient A–B– cell, and crossing over (recombination)
between donor DNA (brown) and recipient DNA
(green) occurs at two places (dotted lines).
The genotype of the resulting recombinant cell (A+B–) differs from the genotypes of both the donor (A+B+) and the recipient (A–B–).
A cell carrying an F plasmid(an F+ cell) can form amating bridge with an F– celland transfer its F plasmid.
A single strand of the F plasmid breaks at a specific point (tip of blue arrowhead) and begins tomove into the recipient cell. As transfer continues, the donor plasmid rotates(red arrow).
DNA replication occurs inboth donor and recipientcells, using the single parental strands of the F plasmid as templates to synthesize complementary strands.
The plasmid in the recipient cell circularizes. Transfer and replication result in a compete F plasmid in each cell. Thus, both cells are now F+.
Conjugation and transfer of an F plasmid from an F+ donor to an F– recipient
A T C C G G T…
A C C G G A T…
T A G G C C A …
T G G C C T A …
(a) Insertion sequences, the simplest transposable elements in bacteria, contain a single gene that encodes transposase, which catalyzes movement within the genome. The inverted repeats are backward, upside-down versions of each other; only a portion is shown. The inverted repeat sequence varies from one type of insertion sequence to another.
Figure 18.19aInsertion Sequences
(b) Transposons contain one or more genes in addition to the transposase gene. In the transposon shown here, a gene for resistance to an antibiotic is located between twin insertion sequences. The gene for antibiotic resistance is carried along as part of the transposon when the transposon is inserted at a new site in the genome.
(b) Regulation of enzyme production
Figure 18.20a, b
Genes of operon
Start codon Stop codon
Polypeptides that make up
enzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the promoter and transcribes the operon’s genes.
No RNA made
Tryptophan present, repressor active, operon off. As tryptophan
accumulates, it inhibits its own production by activating the repressor protein.
Lactose absent, repressor active, operon off. The lac repressor is innately active, and inthe absence of lactose it switches off the operon by binding to the operator.
Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced.
can bindand transcribe
Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized.If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces large amounts of mRNA for the lactose pathway.
Lactose present, glucose present (cAMP level low): little lac mRNA synthesized.When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription.