BCH5413 Exam 3 ACTUAL Exam Questions and
CORRECT ANSWERs
.
How does mRNA secondary structure effect translation from a
polycistronic message? - ANSWER -by blocking translation.
Ribosomes must bind to linear mRNA in order to proceed with
translation, however, if an ORF is in the shape of say, a double-stranded
helix, the ribosome will be unable to bind at this location. Therefore,
there may be more proteins synthesized from one ORF that is linear and
unblocked compared to another ORF on the same mRNA that is not in a
linear shape.
How do eukaryotic mRNAs become circular in the cytoplasm? Why is
this important for translation? - ANSWER -There are circular
polysomes in eukaryotes. This is accomplished by the help from eIFs.
eIF4E binds to cap and then binds eIF4G which then binds to PABPI
which is bound to the poly A tail. This is important because more than 1
ribosome can translate a mRNA at one time.
What is the advantage in regulating translation as well as transcription
in eukaryotes? - ANSWER -The main advantage of regulating
translation, in addition to all the regulations involved with transcription,
is SPEED. With the separation of transcription in the nucleus and
translation in the cytoplasm, regulation at both levels allows for new
gene products to be produced quickly and as needed during translation.
Most regulation of translation happens during initiation.
, How does Maskin (global control) normally block translation? -
ANSWER -Translation is initiated when the protein eIF4E binds eIF4G.
Maskin is a translation repressor protein. It binds CPEB and eIF4E
thereby blocking binding between eIF4E and eIF4G resulting in
repression of translation.
When CPEB is phosphorylated, Maskin falls off and then dissociates
from eIF4E too. eIF4E can then bind eIF4G to initiate translation.
If drug X which inhibits Maskin, explain how the addition of the drug X
would affect translation? - ANSWER -Maskin would not be able to bind
eIFG4 and CPEB so translation would not be inhibited by Maskin. If no
other inhibitor is present, eIF4E would be free to bind eIF4G and
translation would be initiated.
How does the phosphorylation of eIF2 lead to global inhibition of
translation? - ANSWER -When this protein is phosphorylated, it blocks
the way that the initiation factor can be recycled back onto the ribosome.
When it is phosphorylated, GTP hydrolysis occurs, and the GDP cannot
be replaced with a GTP causing everything to stop.
For each of the following conditions, explain how the Iron Response
Element (IRE) regulates translation:
a. IRE at the 5' end of the message, low iron:
b. IRE at the 5' end of the message, high iron:
c. IRE at the 3' end of the message, low iron:
d. IRE at the 3' end of the message, high iron: - ANSWER -a.IRE at the
5' end of the message, low iron: no ferritin is made, cytosolic
aconitase binds stem loop blocking translation of ferritin mRNA
, b.IRE at the 5' end of the message, high iron: ferritin is made, iron binds
cytosolic acontinase leading to conformational change that prevents it
from binding the stem loop, so ferritin mRNA is translated
c.IRE at the 3' end of the message, low iron: transferrin receptor is made,
cytosolic acontinase binds stem loop at 3' end so transferrin receptor
mRNA is stable and translated
d.IRE at the 3' end of the message, high iron: no transferrin receptor is
made, iron binds cytosolic acontinase leading to conformational change
that prevents it from binding the stem loop, so now stem loop is cleaved
and transferrin receptor mRNA is degraded.
What happens when Kozak sequence is not strong? - ANSWER -When
the sequence is weak, this results in defective/"leaky" scanning. For
optimal translation of an mRNA, a strong Kozak sequence is required.
What happens when there if a short open reading frames upstream of a
protein of interest? - ANSWER -Short open reading frames are at the
beginning of a message, they have a strong AUG and a stop, with the
protein of interest being further down in the message. If there is a short
open reading frame upstream of a protein of interest, ribosomes bind and
they make the previously mentioned short piece (AUG—stop), most of
them fall off, but since there is only one stop a few of the 40s subunits
continue to scan again until they find the second AUG and then they
build the ribosome translation system to then make the protein of
interest. This is a way to control how much of that protein is made.
How can viruses suppress termination of a translation to produce a
longer protein? - ANSWER -Viruses can suppress termination of
CORRECT ANSWERs
.
How does mRNA secondary structure effect translation from a
polycistronic message? - ANSWER -by blocking translation.
Ribosomes must bind to linear mRNA in order to proceed with
translation, however, if an ORF is in the shape of say, a double-stranded
helix, the ribosome will be unable to bind at this location. Therefore,
there may be more proteins synthesized from one ORF that is linear and
unblocked compared to another ORF on the same mRNA that is not in a
linear shape.
How do eukaryotic mRNAs become circular in the cytoplasm? Why is
this important for translation? - ANSWER -There are circular
polysomes in eukaryotes. This is accomplished by the help from eIFs.
eIF4E binds to cap and then binds eIF4G which then binds to PABPI
which is bound to the poly A tail. This is important because more than 1
ribosome can translate a mRNA at one time.
What is the advantage in regulating translation as well as transcription
in eukaryotes? - ANSWER -The main advantage of regulating
translation, in addition to all the regulations involved with transcription,
is SPEED. With the separation of transcription in the nucleus and
translation in the cytoplasm, regulation at both levels allows for new
gene products to be produced quickly and as needed during translation.
Most regulation of translation happens during initiation.
, How does Maskin (global control) normally block translation? -
ANSWER -Translation is initiated when the protein eIF4E binds eIF4G.
Maskin is a translation repressor protein. It binds CPEB and eIF4E
thereby blocking binding between eIF4E and eIF4G resulting in
repression of translation.
When CPEB is phosphorylated, Maskin falls off and then dissociates
from eIF4E too. eIF4E can then bind eIF4G to initiate translation.
If drug X which inhibits Maskin, explain how the addition of the drug X
would affect translation? - ANSWER -Maskin would not be able to bind
eIFG4 and CPEB so translation would not be inhibited by Maskin. If no
other inhibitor is present, eIF4E would be free to bind eIF4G and
translation would be initiated.
How does the phosphorylation of eIF2 lead to global inhibition of
translation? - ANSWER -When this protein is phosphorylated, it blocks
the way that the initiation factor can be recycled back onto the ribosome.
When it is phosphorylated, GTP hydrolysis occurs, and the GDP cannot
be replaced with a GTP causing everything to stop.
For each of the following conditions, explain how the Iron Response
Element (IRE) regulates translation:
a. IRE at the 5' end of the message, low iron:
b. IRE at the 5' end of the message, high iron:
c. IRE at the 3' end of the message, low iron:
d. IRE at the 3' end of the message, high iron: - ANSWER -a.IRE at the
5' end of the message, low iron: no ferritin is made, cytosolic
aconitase binds stem loop blocking translation of ferritin mRNA
, b.IRE at the 5' end of the message, high iron: ferritin is made, iron binds
cytosolic acontinase leading to conformational change that prevents it
from binding the stem loop, so ferritin mRNA is translated
c.IRE at the 3' end of the message, low iron: transferrin receptor is made,
cytosolic acontinase binds stem loop at 3' end so transferrin receptor
mRNA is stable and translated
d.IRE at the 3' end of the message, high iron: no transferrin receptor is
made, iron binds cytosolic acontinase leading to conformational change
that prevents it from binding the stem loop, so now stem loop is cleaved
and transferrin receptor mRNA is degraded.
What happens when Kozak sequence is not strong? - ANSWER -When
the sequence is weak, this results in defective/"leaky" scanning. For
optimal translation of an mRNA, a strong Kozak sequence is required.
What happens when there if a short open reading frames upstream of a
protein of interest? - ANSWER -Short open reading frames are at the
beginning of a message, they have a strong AUG and a stop, with the
protein of interest being further down in the message. If there is a short
open reading frame upstream of a protein of interest, ribosomes bind and
they make the previously mentioned short piece (AUG—stop), most of
them fall off, but since there is only one stop a few of the 40s subunits
continue to scan again until they find the second AUG and then they
build the ribosome translation system to then make the protein of
interest. This is a way to control how much of that protein is made.
How can viruses suppress termination of a translation to produce a
longer protein? - ANSWER -Viruses can suppress termination of
