SCOTT FREEMAN
Biological Science 2e
PRENTICE HALL 2005
ISBN: 0-13-140941-7
Classroom Response System - Questions
Using the information below, answer the following two questions.
It has been estimated that approximately half of human genes yield mRNAs of different
sequences because the splicing of introns can vary among different tissues and cellular
circumstances. Consider what would be the consequence of splicing RNAs in different
ways; different proteins would be produced from the same gene. Such a condition would
be a direct challenge to Beadle and Tatum’s one gene one enzyme hypothesis.
Interestingly, the condition in mice called “quaking”, is caused by a recessive
dysmyelination mutation (qk) which appears to involve defects in the way in which initial
mRNAs are spliced. Homozygotes (qk/qk) suffer tremors during exertion. Below is a
figure that describes the splicing patterns in various genotypes of quaking and normal
mice. Portions of brain MAG (myelin associated glycoprotein) mRNA (including exons
11, 12 and 13) from young (14 day) and adult (2 month) mice are represented. The
relative concentration of each RNA is indicated in the gel diagram by the density of the
bands. Figure and data are modified from Wu, J. et al. 2002. PNAS 99:4233-4238. Exons
are represented by boxes.
A
B
C
D
11, 12, 13
11
12
13
AAAA..
.
11, 13
A = +/qk young
C = +/qk adult
B = qk/qk young
D = qk/qk adult
1) With respect to age (young vs. adult), what two general patterns of RNA splicing are
apparent?
A. Young mice appear to generate mRNAs with and without exon 12 while adult mice
preferentially generate mRNAs which contain exon 12.
B. Young mice appear to generate mRNAs without exon 12 while adult mice
preferentially generate mRNAs which contain exons 12 and 13.
C. Young and old mice generate mRNAs in identical fashion.
Question Type: Conceptual/Applied
Question Skill/Objective: Data interpretation
Difficulty level: High
Section Reference: 16.2
Source/Suggested Use: In Class
Notes: It might be necessary to review the technique of gel electrophoresis, then go over the
results of this type of experiment.
2) With respect to genotype (+/qk vs. qk/qk), what two general patterns of RNA splicing
are apparent?
A. Young mice which are +/qk produce more mRNAs which contain exon 12 than do
young mice which are qk/qk. Genotype does not appear to influence the pattern of
splicing in adult mice.
B. Adult mice which are qk/qk produce more mRNAs which contain exon 12 than do
adult mice which are +/qk. Genotype does not appear to influence the pattern of splicing
in young mice.
C. Young mice which are qk/qk produce more mRNAs which contain exon 12 than do
young mice which are +/qk. Genotype does not appear to influence the pattern of splicing
in adult mice.
Question Type: Conceptual/Applied
Question Skill/Objective: Data interpretation
Difficulty level: High
Section Reference: 16.2
Source/Suggested Use: In Class
Notes: It might be necessary to review the technique of gel electrophoresis, then go over the
results of this type of experiment.
3) It has been estimated that approximately half of human genes yield mRNAs of
different sequences because the splicing of introns can vary among different tissues and
cellular circumstances. Consider what would be the consequence of splicing RNAs in
different ways; different proteins would be produced from the same gene. Such a
condition would be a direct challenge to Beadle and Tatum’s one gene one enzyme
hypothesis. In addition to alternative splicing as described above, incorrectly spliced
RNAs lead to human pathologies. Xu and Lee (2003. Nucleic Acids Research 31:56355643) examined human cancers for splice-specific changes and found that many of the
changes disrupted tumor suppressor function, that is, gene products which control the
mitotic cell cycle. What types of RNAs would be probably not be expected when there is
a loss of splicing specificity?
A. a variety of non-specific variants producing RNA pools with many lengths and
combinations of exons and introns
B. incomplete splicing where introns and exons are erroneously included or excluded in
the mRNA product
C. a variety of nonsense products which result in premature RNA decay or truncated
protein (shorter than usual) products
D. RNA pools which are extremely homogeneous and produce only a single wild type
protein.
Question Type: Conceptual/Applied
Question Skill/Objective: Data interpretation
Difficulty level: High
Section Reference: 16.2
Source/Suggested Use: In Class
Notes: This question should be discussed at some length. It is important that students understand
that various splicing possibilities occur naturally and that sometimes improper splicing leads to
abnormal phenotypes. Notice that the introduction to this question is the same as in the previous
question so that either question can stand alone.
4) If one compares the base sequences of related genes from different but closely related
species one is likely to find that corresponding _______ are usually conserved but the
sequences of _________ are much less well conserved.
A. exons; introns
B. introns; exons
C. introns; chaperons
D. chaperons; exons
E. introns; proteins
Question Type: Conceptual/Applied
Question Skill/Objective: General
Difficulty level: Low
Section Reference: 16.2
Source/Suggested Use: Exam or in class
Notes: This question would be useful in a class discussion. It would help students see that introns
are much less subject to selective pressure than exons generally.
5) Select three posttranscriptional modifications often seen in the maturation of mRNA
in eukaryotes.
A.
B.
C.
D.
E.
5'-capping, 3'-poly(A) tail addition, splicing
3'-capping, 5'-poly(A) tail addition, splicing
removal of exons, insertion of introns, capping
5’-poly(A) tail addition, insertion of introns, capping
heteroduplex formation, base modification, capping
Question Type: Conceptual/Applied
Question Skill/Objective: General
Difficulty level: Moderate
Section Reference: 16.2
Source/Suggested Use: Exam or in class
Notes: It might be helpful to summarize this information for the students in classroom discussion.