Download Lecture 6, Exam III Worksheet Answers

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Lecture 6, Worksheet Answers
1. What is special about the sequence of a protein destined to go to the ER or outside of the cell?
8-10 amino acids synthesized at the beginning; at the amino terminal end, called
the signal peptide.
2. What attaches to this marker in number 1? What special complex does it bring the polypeptide
to? What component in this complex allows the polypeptide to enter the ER lumen? What two
components do NOT go into the ER lumen?
The Signal Recognition particle, (SRP). The translocation complex. The translocation
complex includes a protein pore that allows the polypeptide to be synthesized and threaded
into the inside of the ER lumen. As soon as the SRP attaches to the signal peptide and carries
it and the ribosome to the ER membrane, it leaves and goes back to the cytosol. After the
entire polypeptide, (protein), is synthesized and threaded into the ER lumen; the signal
peptide is cleaved off by an enzyme and isn’t allowed to go into the ER lumen.
3. What are the two types of mutations that cause frameshift mutations? Do they always cause
frameshift mutations?
Insertions and deletions; no they do not. An insertion does not cause this if it is in a multiple
of three and doesn’t insert between other triple codons. A deletion doesn’t cause this if it is
in a multiple of three and specifically deletes triplet codons; not two nucleotides of one
codon and one nucleotide of another codon.
4. What are the three types of point mutations? Which is often lethal? Which causes no change
within the protein? Which causes only minimal damage usually?
1. Silent mutation- causes no change within the protein. A change in a base pair may
make one codon into another codon that codes for the exact same amino acid as the
first one.
2. Missense mutation- usually causes only minimal damage. These usually change one
amino acid into another amino acid; the new a.a. may have properties similar to the
first or it may not affect the total protein structure.
3. Nonsense mutation- often lethal. This mutation changes a base pair that results in
the creation of a stop codon. Stops protein synthesis before it is finished.
5. How does translation differ between eukaryotes and prokaryotes? What is an operon? What
access does it control? Are one or multiple genes located within one operon?
Eukaryotes have a specific promoter for each individual gene, and do not have
operons. Prokaryotes can group genes together and attach a single promoter that regulates
all of the genes together. Prokaryotes can also have an operator which is usually located in
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the promoter or close to the promoter and controls the access of the RNA polymerase to the
promoter and genes. Is like an on/off switch. An operon is the operator, promoter, and all of
the genes they control.
6. Are the trp and lac operon found within eukaryotes or prokaryotes? What kind of operons are
they?
Prokaryotes; E.coli specifically. Trp: repressible; Lac-inducible.
7. Within the trp operon, is it usually turned on or off? ON, coding and creating for mRNA and
proteins. So what is usually bound? RNA polymerase is usually bound to the promoter and is
creating the gene products from the genes of the trp operon.
Is the trp repressor usually activated or inactive? Inactive, ( remember it is an allosteric
molecule).
What activates it? Tryptophan can bind to the allosteric site of the repressor and cause its
shape to change to its active form. It then binds to the promoter and stops transcription by
the RNA polymerase.
What codes for the trp repressor? The gene TrpR, which is a regulatory gene, which has its own
separate promoter and is located upstream from the trp operon.
8. Is the lac operon usually on or off? OFF, not coding for gene products, (mRNA and proteins).
So what is usually bound? The lac repressor is usually bound to the promoter under normal
conditions, (when lactose isn’t present). It is preventing the RNA polymerase from binding
and carrying out transcription.
Is the lac repressor usually activated or inactive? Activated. It is already active, carrying out its
role of repression, without any molecules attaching to it.
What deactivates it? The isomer of lactose called allolactose, (think of them as the same, if
one is present the other is present). Allolactose binds to the allosteric site of the repressor
and causes it to leave the promoter. Now transcription can happen once the RNA polymerase
binds.
What codes for the lac repressor? The gene lacI, which has its own promoter and is located
upstream from the trp operon.
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9. When glucose is scarce and lactose is present are cAMP levels high or low? cAMP levels are
high. How does cAMP interact with the catabolite activator protein? cAMP is an allosteric
regulator protein that when present binds to the activator the catabolite activator protein
(CAP). CAP is only active whenever cAMP is bound to it. What do they bind to together? After
cAMP binds to the CAP, they travel together to the promoter where they signal for the RNA
polymerase to come in and start transcription. What response does this illicit? The start of
transcription and the making of proteins of the lac operon. What happens when glucose is
present, even if lactose is still present? Glucose is used and preferred over lactose; so no cAMP
is made, the CAP cannot become activated, and transcription often doesn’t start for the lac
operon, (but can go at very low levels). For both situations is the lac repressor inactive or active?
Both situations are: 1. When lactose is present and glucose isn’t present- the lac repressor is
inactivated, because allolactose-the form of lactose- binds to the repressor and opens up the
promoter for RNA polymerase to be able to come in. 2. When lactose AND glucose are
present: Again, allolactose will still bind to the repressor and inactivate it opening up the
area for RNA polymerase; however, since glucose is preferred no cAMP is created, so the CAP
does not signal for the RNA polymerase to come in at the open area of the promoter.
cAMP and the catabolite activator protein are needed to start transcription of the lac
operon, because the RNA polymerase has a hard time binding to the promoter without them.
These molecules also sense the relative amounts of glucose and lactose.