Chapter 18. Viruses and Gene regulation. Concept 18.1: A virus has a genome but can reproduce only within a host cell Scientists were able to detect viruses indirectly Long before they were actually able to see them. Figure 18.3. The Discovery of Viruses: Scientific Inquiry.
Viruses and Gene regulation
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
(b) The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glycoprotein spikes protruding from the envelope.
(a) Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS.
Figure 18.11 A, B
(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.