282 MHR • Unit 3 Molecular Genetics
26.Suppose you are provided with a sample
of DNA. After you have analyzed its base
composition, you divide the sample into
three separate reaction mixtures in order to
transcribe mRNA. Once the transcription is
complete, you analyze the base composition
of the mRNA from each reaction mixture. You
obtain the following results. Based on these
findings, answer the questions below.(a)Which strand of the DNA is the sense
strand that serves as the template for the
synthesis of mRNA? Explain your
reasoning.
(b)How can you explain the composition of
mRNA type B?
(c)What substance(s) might have been added
to reaction mixture B in order to obtain
mRNA type B?(d)What might have happened in reaction
mixture C in order to produce mRNA
type C?
27.A researcher studying protein synthesis in
E. coliuses an electron microscope to record
the image below.(a)What structure is shown in this image?
What is its significance?
(b)The same researcher puts a new sample of
E. coliinto a medium containing a substance
that destroys activating enzymes. She then
records another image using the electron
microscope. Assuming that this substance
has no other effect on the cell, how would
you expect the image to differ? Explain.
28.In a fictional species of bacterium, the caf
operon contains genes that code for an enzyme
that breaks down caffeine into its component
molecules. The cell uses one of these
molecules as a source of energy.A G
DNA strand I
DNA strand II
mRNA type A
(reaction mixture A)
mRNA type B
(reaction mixture B)
mRNA type C
(reaction mixture C)19.1
24.2
19.0
23.2
36.0
26.0
30.8
25.9
27.6
23.0
C T
31.0
25.7
30.8
22.9
19.1
23.9
19.3
0
0
0
U
0
0
24.3
26.3
21.9
17.Our bodies often benefit from antibiotics when
fighting bacterial infections. Explain what this
might have to do with ribosomes.
18.What functions are performed
(a)by the polypeptide component of a
ribosome?
(b)by the rRNA component of a ribosome?
19.What is an operator, and how does it relate to
operons? In what kind of cells are operons
found?
20.Prokaryotic promoter sequences are sometimes
described as being asymmetrical.
(a)To what property of the promoter
sequences does this term refer?
(b)What is the significance of this feature?
21.List the main control steps in the protein
synthesis pathway of prokaryotes, and briefly
describe why each step is important.22.Name the five control points available to
eukaryotic cells and briefly describe each one.
23.How does the regulation of gene expression
enable a cell to conserve energy? Give an
example of a case in which another advantage
might outweigh the need to conserve energy.
24.Explain what will happen to a pre-mRNA
strand that lacks a 3 ′poly-A tail or 5 ′cap.
How will this affect translation?
25.Choose the correct completion for the
following statement: In a eukaryotic cell,
most of the genome
(a)remains unexpressed most of the time.
(b)is expressed most of the time.
(c)is repressed by a repressor most of the time.
(d)is expressed by a promoter most of the
time.INQUIRY