BIOL 3301
Genetics
EXAM 3
Extra exam
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Name:
SSN:
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True-false statements (write True or False in front of the statement)
(2 points each).
1. The experiments by Griffith on Streptococcus pneumoniae were the first demonstration of
bacterial transformation. T
2. In a DNA molecule, nucleotides on complementary strands are held together by covalent
bonds. F
3. oriC is an enzyme involved in DNA replication. F
4. Telomerase is a reverse transcriptase that maintains chromosome ends. T
5. The mutation causing sickle-cell anemia leads to a single amino acid substitution in
hemoglobin. T
6. E. coli K is the permissive host and E. coli B is the restrictive host for rII mutants. F
7. Non-complementary rII mutants are allelic. T
8. Hot spots are sites that are more mutable than others. T
9. Protein synthesis is terminated by a rho-dependent mechanism. F
10. IF3 stimulates binding of the 30S ribosome subunit to mRNA. F
Multiple choice (circle the correct answer) (2 points each).
1. A sample of normal double-stranded DNA was found to have a guanine content of 18%. What
is the expected proportion of adenine ?
a. 9%
b. 32%
c. 36%
d. 68%
e. 82%
2. The figure shown below depicts the structure of:
a. dATP
b. dCTP
c. ddATP
d. UTP
e. ddTTP
3. Hershey and Chase proved that DNA is the transforming principle. They observed that the
isotope accumulated in the bacteria was:
a. 31P
b. 35S
c. 33P
d. 15N
e. none of the above
4. DNA polymerase I has three enzymatic activities. They are:
a. telomerase, RNA polymerase, helicase
b. gyrase, 3’->5’ exonuclease, 5’->3’ polymerase
c. helicase, 3’->5’ exonuclease, 5’->3’ polymerase
d. 5’->3’ exonuclease, 5’->3’ polymerase, 3’->5’ exonuclease
e. DNA polymerase I does not have 3 enzymatic activities.
5. DNA ligase catalyzes the formation of:
a. phosphodiester bonds
b. cAMP
c. ATP
d. hydrogen bonds
e. none of the above
6. B-DNA is the most common form of DNA. The spacing between basepairs is:
a. 10 A
b. 34 A
c. 3.4 A
d. 4 A
e. none of the above
7. A tRNA with the anticodon 3´AUG 5´would carry
a. phenylalanine
b. tyrosine
c. tryptophane
d. serine
e. threonine
8. Translation initiation in E. coli is mediated by basepairing of 16S rRNA to the:
a. promoter
b. origin of replication
c. Shine-Dalgarno sequence
d. transcription start site
e. none of the above
9. The following diagram shows a fragment of transcribed DNA, and the upper strand is the
template strand:
5´ CCGTTA 3´
3´ GGCAAT 5´
The transcribed RNA can be represented by:
a. 5´ AUUGCC 3´
b. 5´ TAACGG 3´
c. 5´ AUUGCC 3´
d. 5´ UAACGG 3´
e. 5´ GGCAAU 3´
10. EF-Ts is required for:
a. loading aminoacyl-tRNA into the A-site of the ribosome
b. regenerating EF-Tu
c. translocation of the peptidylchain
d. translocation of the ribosome
e. releasing empty tRNAs from the ribosome
11. Beta galactosidase converts
a. glucose to galactose
b. galactose to lactose
c. galactose to glucose and lactose
d. lactose to galactose and glucose
e. lactose to two glucose molecules
12. RNA polymerase binds to
a. repressor gene
b. promoter
c. operator
d. permease gene
e. lactose
13. A super-repressor mutation (IS) affects the Lac operon. It results in
a. no transcription
b. inducible transcription
c. transcription but no translation
d. no translation
e. constitutive transcription
14. A partial diploid of genotype I-P+O+Z-Y+/I+P-O+Z+Y+ will show
a. inducible production of beta galactosidase
b. inducible production of beta galactosidase and permease
c. constitutive production of beta galactosidase and permease
d. inducible production of permease
e. no beta galactosidase or acetylase production at all
15. [NOT SEEN] The trp operon has three levels of regulation. They are:
a. trpR, enzyme inhibition by Trp, attenuation
b. trpR, lacI, cAMP-CAP
c. RNA polymerase, cAMP-CAP, lacI
d. 30S subunit, IF3, amino-acyl tRNA
e. DNA polymerase, DNA ligase, enzyme inhibition
Fill in (2 points each).
1. Two of the bases, adenine and guanine, are similar in structure and are called purines .
The other two bases, cytosine and thymine are called pyrimidines.
2. Meselson and Stahl demonstrated that DNA replication is semi-conservative .
3. Yanofsky studied the trp synthetase gene and demonstrated the colinearity of gene and protein.
4. Stop codons are not recognized by aminoacyl-tRNAs, but by release factors.
5. The trp operon has a level of regulation called attenuation , which is only possible because
prokaryotes have .............................. translation. [NOT SEEN]
Homework (10 points each).
1. We are studying some of the Neurospora mutants described by Beadle and Tatum. These
mutants (numbered 1 to 7) are all involved in the same biosynthetic pathway which yield two
different end products. The different compounds generated in the different steps are A, B, C, D,
F, G, H, I. Give the order in which the compounds are made and indicate which enzymes
(corresponding to the mutants) are involved in the different biosynthetic steps.
Compound tested (+= growth; -+ no growth)
1
2
3
4
5
6
7
A
B
C
D
F
G
H
I
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
-
-
+
+
H -> G -> I -> A -> D -> F -> C -> B
4
7
3
2
5
1
6
(B) In sweet peas, the synthesis of purple anthocyanin pigment in the petals is controlled by two
genes, B and D. The pathway is
white intermediate ---------------> blue intermediate ---------------> anthocyanin (purple)
gene B
gene D
enzyme
enzyme
a. What color petals would you expect in a pure-breeding plant unable to catalyze the first
reaction ? White
b. What color petals would you expect in a pure-breeding plant unable to catalyze the second
reaction ? Blue
c. If the plants in parts a and b are crossed, what color petals will the F1 plants have? Purple
d. What ratio of purple:blue:white plants would you expect in the F2 ?
9:3:4
2. You are studying a gene in E. coli that specifies a protein. A part of its sequence is:
-Phe-Tyr-Ser-Trp-Glu-Ala-Met-ThrYou recover a series of mutants for this gene that show no enzymatic activity. By isolating the
mutant enzyme products, you find the following sequences:
Mutant 1:
-Phe-Asn-Ser-Trp-Glu-Ala-Met-Thr-
Mutant 2:
-Phe-Tyr-Ser-Trp
Mutant 3:
-Phe-His-Cys-Leu-Pro-Ala-Val-Thr-
Mutant 4:
-Phe-Tyr-Met-Leu-Gly-Ala-Met-Thr-
What is the DNA sequence that specifies this part of the protein ? What is the molecular nature
for each mutation?
Wildtype
TTT TAT TCN TGG GAA GCN ATG ACN
T
C
G
Mutant 1
TTT AAT TCN TGG GAA GCN ATG ACN
T
C
G
T-> A point mutation leads to amino substitution
Mutant 2
TTT TAT TCN TGG UAA
T
C
G
G->U point mutation leads to stop codon
Mutant 3
TTT CAT TGT CTN CCN GCN GCN GTN ACN
C
C
C TTA
G
inversion of the sequence corresponding to the sequence Tyr through Met in wild-type
Mutant 4
TTT TAT ATG CTN GGN GCN ATG ACN
T
C
TTA
G
insertion of T in Ser codon and futher down loss of G in Glu codon
Open questions (20 points).
NOTE: If you would run out of space, you can also use the back of this page to continue your
answer.
Describe all components necessary for and explain RNA transcription (initiation, elongation,
termination) and protein translation initiation in prokaryotes.