Essential Cell Biology 5th edition

Questions
293
• MicroRNAs (miRNAs) control gene expression by base-pairing with
specific mRNAs and inhibiting their stability and translation.
• Cells have a defense mechanism for destroying “foreign” doublestranded
RNAs, many of which are produced by viruses. It makes use
of small interfering RNAs (siRNAs) that are produced from the foreign
RNAs in a process called RNA interference (RNAi).
• The recent discovery of thousands of long noncoding RNAs in
mammals has revealed new roles for RNAs in assembling protein
complexes and regulating gene expression.
KEY TERMS
cell memory
combinatorial control
differentiation
DNA methylation
epigenetic inheritance
gene expression
induced pluripotent stem
(iPS) cells
long noncoding RNA
microRNA (miRNA)
positive feedback loop
post-transcriptional control
promoter
regulatory DNA sequence
regulatory RNA
reporter gene
RNA interference (RNAi)
small interfering RNA (siRNA)
transcription regulator
transcriptional activator
transcriptional repressor
QUESTIONS
QUESTION 8–4
A virus that grows in bacteria (bacterial viruses are called
bacteriophages) can replicate in one of two ways. In the
prophage state, the viral DNA is inserted into the bacterial
chromosome and is copied along with the bacterial genome
each time the cell divides. In the lytic state, the viral DNA
is released from the bacterial chromosome and replicates
many times in the cell. This viral DNA then produces viral
coat proteins that together with the replicated viral DNA
form many new virus particles that burst out of the bacterial
cell. These two forms of growth are controlled by two
transcription regulators, the repressor (product of the cI
gene) and Cro, both of which are encoded by the virus. In
the prophage state, cI is expressed; in the lytic state, Cro is
expressed. In addition to regulating the expression of other
genes, cI represses the Cro gene, and Cro represses the cI
gene (Figure Q8–4). When bacteria containing a phage in
the prophage state are briefly irradiated with UV light, cI
protein is degraded.
A. What will happen next?
B. Will the change in (A) be reversed when the UV light is
switched off?
C. What advantage might this response to UV light provide
to the virus?
Figure Q8–4
cI gene
PROPHAGE
STATE
cI gene
Cro protein
NO cI GENE
TRANSCRIPTION
cI protein
Cro gene
NO Cro GENE
TRANSCRIPTION
Cro gene
LYTIC
STATE