Given the lac Operon (pictured above) and what you know about the regulation of
ID: 91188 • Letter: G
Question
Given the lac Operon (pictured above) and what you know about the regulation of gene expression in this operon, fill out the last column in the table below.
Genotype of mutant strain
Glucose present?
Lactose present?
Significant amounts of beta galactosidase produced in significant amounts? (yes/no)
Wild type
Yes
No
Wild type
No
No
Wild type
Yes
Yes
Wild type
No
Yes
LacZ+LacI+Lac P+LacO-
No
No
LacZ+LacI-Lac P+LacO+
No
No
LacZ+LacI+Lac P+LacO-
No
No
• Gene Regulation Details (1.5 pts)
• How are most genes regulated (at the level of transcriptional, translational, or replication)?
• Why is this form of gene regulation advantageous?
• Are genes regulated at the level of DNA replication (i.e. some genes are replicated while others are not)? Why or why not?
• Clarifying Terminology & processes (3 pts)
Template
Product
Enzyme Responsible
Transcription
Translation
PCR (Replication)
• Questions about Transcription and Translation ( 2 pts)
• How does a strong promoter differ from a weak promoter?
• If you delete the Shine Dalgarno sequence from a prokaryotic mRNA molecule, could the protein be translated? Why or why not?
• Explain how the genetic code is both universal and degenerate. What is the wobble position and does it contribute most directly to degeneracy or universality of the genetic code?
• Challenge Question Review (5 pts)
• If you wanted to measure the amount of new DNA made by a flask of E. coli in an hour, which nucleotide could you label to measure new DNA (not RNA)? If you wanted to measure the new RNA made, which nucleotide would you use? Why?
• Fledgling grad student, Joe Peon, is doing PCR and goes to the freezer to get the reagents needed to set up the reaction. This overworked graduate student grabs dNTPs, 2 forward primers, enzyme buffer, and restriction enzyme TaqI. out of storage and sets up his reaction tube. He sets up 2 tubes: 1) the positive control with target DNA added 2) the test sample + target DNA. He adds all the reagents listed, in the amounts given in the protocol and sets up the PCR cycler to cycle a heat phase for 30 seconds and extend for 1 minute. The next morning, he ran the samples out in an agarose gel.
• What, if anything will Joe Peon see on the gel?
• What mistakes were made (there are numerous errors – name 3 specific errors)?
• If you design a lysogenic bacteriophage to express a toxin, such as the diphtheria toxin, and use this toxin-bearing phage to infect E. coli, what would happen to the E.coli if this phage became a prophage?
• The leader sequence of the histidine operon has multiple histidine codons and the leader of the tryptophan operon has tryptophan codons. You place the histidine leader in front of the tryptophan structural genes and vice versa. If you grew the resulting cells in media with tryptophan but lacking histidine, would you expect histidine or tryptophan to be produced? Why or why not?
• You discover and describe an enzyme that works in the cytosol of E.coli to increase the stability of the mRNA of a structural gene,
• Would this affect the transcription or translation of a gene?
• Would you expect more or less protein produced?
Explanation / Answer
Gene regulation
The expression of genes is usually controlled at certain levels of transcription.
Control of gene expression is called gene regulation.
Certain molecules regulate the expression of genes and decide whether gene has to be transcribed to mRNA or not.
These regulatory molecules bind to the DNA and either allows or blocks the activity of the transcription enzyme RNA polymerase.
Closely relayted genes are usually present in clusters.
These cluster of genes are called operons.
Operons are common in prokaryotes.
Lac operon is the best studied example.
Lac operon contains a set of genes (lac A, lac Y and lacZ )that encode proteins involved in the metabolism of lactose. in a controlled way.
Along with these genes, operon also contains a sequnce called the promoter which is the site of binding of RNA polymerase.
In addition, represor molecules are also present. THese molecules when bound to the DNA, blocks nthe binding of RNA polymerase and thereby reduces transcription.
Inducer is another molecule that when bound to the DNA assists in the binding of RNA polymerase and increases transcription.
Operons are operated in a on / off mode.
The genes on the operon are responsible for the secretion of B galactosidase enzyme that breaks down lactose.
Expression of the genes in the operon is induced only in the presence of lactose and is turned off when lactose is absent.
Thus based on the environmental changes the expression of the genes are regulated.
Lac Z+ LacI+LacP+LacO- Significant amount of B galactosidase is produced
Lac Z+LacI-LacP+LacO+ No significant amount of B galactosidase is prodeced.
Muation in the operator makes operator incapable to bind to repressor and leadto transcription
Mutation in I cause no binding of repressor and promoter and decreases transcription.
I
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