Agro/ANSC/Gene/Hort/Bio 305
Fall, 2016
HW#3 (Due back on Nov 9)
1. In which of the ribosomal sites, the A site, P site, and/or E site, could the following be
found?
A. A tRNA without an amino acid: E site, (P site, transient)
B. A tRNA with a polypeptide attached: P site (A site, transient)
C. A tRNA with a single amino acid attached: A site
D. A tRNA with fMet: P site
Q2. The following DNA sequence occurs in the nontemplate strand of a structural gene in
bacterium (the promoter sequence is located to the left but is not shown):
5’-GAATGTCAGAACTGCCATGCTTCATATGAATAGACCTCTAG-3’
a) What is the ribonucleotide sequence of mRNA molecule that is transcribed from
this piece of DNA?
5’-GAAUGTCAGAACUGCCAUGCUUCAUAUGAAUAGACCUCUAG-3
b) What is the amino acid sequence encoded by this mRNA? Indicate both the amino
terminal and carboxy terminal ends.
+
NH2- Met-Ser-Glu-Leu-Pro-Cys-Phe-Ile-COO-
3. Transcriptional regulation often involves regulatory proteins that bind to a segment of
DNA and small effector molecule that binds to a regulatory protein. Do the following
terms apply to a regulatory protein, a segment of DNA or a small effector molecule?
a. Repressor
RP
b. Inducer
E
c. Operator
D
d. Corepressor
E
e. Activator
RP
f. Attenuator
D
g. Inhibitor
E
h. CAP site
D
i. lacZ
D
j. cAMP
E
k. Allolactose
E
l. Tryptophan
E
m. Enhancer
D
n. Mediator
RP
Q4. Drawn below is an outline of the Lac operon. Label the lac operon and describe the
function of each element on the operon. The arrows point to the different elements/genes
in the operon.
_______________________________________________________________________
_______________________________________________________________________
1
2
3
4
5
6
7
1.
CAP site: Site for binding of activator
2.
lacP: Promoter for lac operon
3.
lacO: Site for Repressor binding
4.
lacZ: -galactosidase 
Enzyme catalyzing the breakdown of lactose to glucose and galactose;
Conversion of lactose into allolactose
5.
lacY:
6.
lacA: Galactosidase acetylase
Modifies lactose by acetylation
7
lac terminator
Lactose permease
Uptake of lactose
Q5. How is the lac operon regulated when E. coli is grown both in the presence of
glucose and lactose?
-
CAP is an activator protein of the lac operon. It binds to the activator site
when cAMP levels are high. Glucose inhibits cAMP levels, thereby inhibiting
transcription of the lac operon.
Q6. What is the status of the trp operon in the following figure? Give an explanation for
your answer?
The ribosome at the stop codons stalls, with the result that regions 1 and 2
are blocked and are not able H-bond with each and neither can region 2 H-bond
with region 3. Thus regions 3 and 4 H-bond and form the terminator. The RNA
polymerase is released and the transcription of the whole operon is terminated.
This is the status of the operon under conditions of high Trp in the cells.
Q7. Gene expression in eukaryotes is regulated at steps beyond transcription.
Briefly, describe these steps and give the mechanism of regulation.
Posttranscriptional regulation:
i.
Alternative splicing involving splicing factors that regulate which exons
are involved in determining which exons are included in the resulting
mRNA. The splicing factors may be regulated by binding to effector
molecules, covalent modificator or protein-protein interaction
ii.
RNA stability mediated by sequences in the 3’UTR like the AU-rich
region involving some kind of RNA binding proteins.
iii.
Antisense RNA
iv.
RNA interference involving microRNA (miRNA) or short-interfering
RNAs (siRNA). Inhibit translation of RNA or causes mRNA
degradation
v.
Phosphorylation of ribosomal initiation factors.
vi.
Secondary structures in the 5’UTR along with specific RNA binding
proteins that can regulate translation