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C H A P T E R
8
R E V I E W
Summary of Expectations
Language of Biology
Briefly explain each of the following points.
Write a sentence including each of the following
words or terms. Use any six terms in a concept map
to show your understanding of how they are related.
• The four different nucleotides found in DNA
make up the genetic “words” that code for all
20 different amino acids. (8.1)
• The two-step process of gene expression has
both costs and benefits for cells. (8.1, 8.4)
• The genetic code is redundant but not
ambiguous. (8.1)
• Many transcripts can be simultaneously
produced from a single gene. (8.2)
• Several types of processing are required before
a pre-mRNA transcript can meet the needs of a
eukaryotic cell. (8.2, 8.4)
• Four different types of RNA are involved in
the main steps of protein synthesis. (8.2, 8.3)
• In prokaryotes, translation can begin before
transcription is done. (8.2)
• The process of translation involves enzymes
both inside and outside the ribosomes. (8.3)
• An active ribosome complex has at least four
RNA binding sites. (8.3)
• An E. coli cell will only produce lactosemetabolizing enzymes if lactose is present in
the cell’s environment. (8.4)
• Crick’s “central dogma” does not fully capture
the process of gene expression. (8.1, 8.4)
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codon
triplet hypothesis
transcription
translation
reading frame
initiation
elongation
messenger RNA (mRNA)
termination
processing
sense strand
anti-sense strand
promoter sequence
RNA polymerase
precursor mRNA
(pre-mRNA)
5′ cap
poly-A tail
spliceosome
small nuclear RNA
(snRNA)
transfer RNA (tRNA)
• anticodon
• amino acid attachment
site
• amino-acyl tRNA
(aa-tRNA)
• activating enzyme
• ribosome
• ribosomal RNA (rRNA)
• translocation
• release factor
• polyribosome
• operon
• operator
• repressor
• negative gene
regulation
• inducer
• activator
• positive gene
regulation
• co-repressor
• transcription factors
UNDE RSTAN DIN G CON CEPTS
1. A given organism has many different tissues,
yet its cells all carry the same genetic
information. Explain how this is possible.
2. Identify the significance of the discovery that
the specific arrangement of amino acids is
directly related to the structure of specific
proteins.
3. Describe how Crick tested the triplet hypothesis.
4. What is the codon concept?
5. What was the term Crick coined to describe
the process by which he believed genes were
expressed? Why did he choose this term?
6. How is it possible for information on DNA that
is confined to the nucleus of a eukaryotic cell
to be expressed as protein products outside the
nucleus?
7. How did Nirenberg and Matthaei begin
working out the genetic code?
8. Identify the amino acids coded for by the
following codons: AGC, GUU, UAU, AUG.
9. Name three characteristics of the genetic code
and explain why they are important.
10. One species of bacteria manufactures 37
different tRNA molecules. Explain how these
bacteria can still match anticodons to all of
the 64 different potential mRNA codons.
11. RNA transcription in eukaryotes consists of
four steps. Identify and describe each.
12. Define the term “sense strand.” What is its
counterpart called, and why?
13. Explain how activating enzymes work.
14. What causes RNA polymerase to cease
transcription?
15. Draw a diagram that illustrates the main steps
in the elongation cycle of translation. What
enzymes play a role in maintaining this cycle?
16. Identify two different mechanisms that help
a single gene produce very high levels of a
polypeptide product in a short time.
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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.
IN QU IRY
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
G
C
T
U
DNA strand I
19.1
26.0
31.0
23.9
0
DNA strand II
24.2
30.8
25.7
19.3
0
mRNA type A
(reaction mixture A)
19.0
25.9
30.8
0
24.3
mRNA type B
(reaction mixture B)
23.2
27.6
22.9
0
26.3
mRNA type C
(reaction mixture C)
36.0
19.1
0
21.9
23.0
(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?
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MHR • Unit 3 Molecular Genetics
(d) What might have happened in reaction
mixture C in order to produce mRNA
type C?
27. A researcher studying protein synthesis in
E. coli uses 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. coli into 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) Would you expect this operon to be
repressible or inducible? Explain.
(b) Draw a diagram that illustrates how the
presence of caffeine in the medium in
which the cell is kept regulates the
expression of the caf genes.
29. A particular yeast cell reacts to an increase in
glucose in its medium by producing more of
enzyme X.
(a) Design an experiment to determine whether
the main control point for the synthesis of
enzyme X is found during transcription or
translation.
(b) Would the same experiment work if you
were studying a bacterial cell? Explain.
COMMU N ICATIN G
30. “Proteins, not DNA, are the key to cell
specialization.” In a report, explain what is
meant by this statement.
31. Write a short essay that compares and contrasts
the roles of DNA polymerase and RNA
polymerase.
32. Create a table that could be used by others to
distinguish between the various types of RNA
and their roles in transcription and translation.
33. Assume for a moment that you want to author
a paper on transfer RNA. With pen and paper,
illustrate the basic structure of a tRNA
molecule and label it accordingly. Write a
caption to go with it that explains its functions.
34. With an opponent, debate ways in which the
mechanisms involved in the control of the lac
operon can be compared to traffic signals, and
ways in which this analogy is not helpful.
35. When Crick first described the “central
dogma”, he had little experimental evidence to
support his theory. In groups of three or four,
brainstorm ways in which more recent
evidence both supports and challenges the
central dogma.
36. Form a debate team to discuss whether
prokaryotes or eukaryotes, based on their
methods of gene expression, have the
evolutionary advantage.
37. Gene regulation is an important part of the
growth and development of living organisms.
Explain this statement in a short essay.
M A KIN G CON N ECTION S
38. Researchers have the ability to add a number of
A nucleotides to the end of a strand of mRNA.
What could be some practical applications of
this procedure?
39. A molecular biologist creates a form of RNA
polymerase that has the same proofreading
ability as DNA polymerase. Explain what some
of the advantages and disadvantages of this
form of RNA polymerase could be:
(a) for researchers in a laboratory setting
(b) for living organisms
40. In small groups, interview people in your
community who work in science, medicine,
technology, or the environment to discover
their thoughts on dogma (either current or
historical) in their field. Do they believe this
dogma has been largely helpful or misleading?
Based on their answers, evaluate what role
dogma should play in scientific discovery.
41. How could you use the information from
this chapter to find ways to fight bacterial
infections in humans? Write a short report (up
to one page) identifying some processes that
might be significant in the development of
treatments. Could the same processes be
applied to fight infections by eukaryotic cells
such as yeast? Why or why not?
42. The study of the structure of genomes,
including projects such as the sequencing of
the human genome, is often referred to as
“genomics.” Many researchers claim that
genomics is not nearly as significant as
“proteonomics,” the study of protein structure
and function. Based on the information in this
chapter, which field would you argue holds the
greatest promise for advances in medicine and
in understanding human development?
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