Question 1

Uses attenuation as a mechanism to control expression of its genes.

Accepted Answer

The trp operon uses attenuation to control expression of its genes. Attenuation is a mechanism by which gene expression is controlled by the premature termination of transcription in response to a particular regulatory molecule. It occurs in the trp operon when the presence of tryptophan activates the binding of a repressor protein to the operator region, preventing transcription of the full operon. However, the trp operon also contains a region called the leader sequence that can form different stem-loop structures, depending on whether or not tryptophan is present. The formation of the stem-loop structures in the leader sequence can either favor or interfere with the formation of a transcription termination hairpin in the downstream mRNA. This is called attenuation, and it allows the cell to fine-tune the expression of the trp operon in response to the availability of tryptophan.

Question 2

Adenylate cyclase is the enzyme which converts ATP to 5'-cAMP. What is the expected effect on the in vivo expression of the lactose operon of a nonsense mutation in the adenylate cyclase structural gene (which totally inactivates the enzyme)?

Accepted Answer

The lactose operon is regulated by cAMP levels in the cell. Adenylate cyclase converts ATP to cAMP. A mutation that inactivates adenylate cyclase would lead to low cAMP levels, which in turn would impair the efficient transcription of the lactose operon in the presence of lactose.

Question 3

Which of the following statements about regulation of the trp operon are true? When the Trp concentration in the cell is very low, 1. ribosomes stall at Trp codons in the attenuator. 2. RNA polymerase terminates transcription of the trp operon prematurely. 3. the secondary structure of the RNA transcribed from the leader region changes. 4. enzymes which catalyze the synthesis of Trp are made in greater abundance.

Accepted Answer

When the Trp concentration in the cell is very low, ribosomes stall at Trp codons in the attenuator (1), causing the formation of a secondary structure in the RNA transcribed from the leader region (3) that allows the transcription to proceed uninterrupted. This results in enzymes which catalyze the synthesis of Trp are made in greater abundance (4). RNA polymerase does not terminate transcription of the trp operon prematurely (2) in this scenario.

Question 4

Phenotypic repression can be mediated by:

Accepted Answer

All the options describe mechanisms by which phenotypic repression can be mediated. Repressor proteins can be activated or deactivated by small molecules, and activator proteins can also be regulated in a similar manner, either by the removal of coactivators or the addition of anti-activators.

Question 5

The SREBP-1a domain of SREBP (sterol responsive element-binding protein 1a) must be transported into the nucleus for LDL receptor synthesis to occur. From which membrane is this protein released for transfer to the nucleus?

Accepted Answer

The answer of The SREBP-1a domain of SREBP (sterol responsive...

Question 6

What family of genes are frequently carried by bacterial transposons?

Accepted Answer

Antibiotic resistance genes are frequently carried by bacterial transposons, allowing for the transfer of resistance between bacteria.

Question 7

Where do insulator sequences occur in a gene?

Accepted Answer

Insulator sequences are typically found between the start of a gene and an enhancer to block the enhancer's influence on the gene.

Question 8

The following protein motifs participate directly in DNA binding

Accepted Answer

Helix-turn-helix, zinc fingers, and homeodomain motifs directly participate in DNA binding. Helix-loop-helix and leucine zipper motifs are involved in protein-protein interactions but may also indirectly contact DNA. Therefore, only option C contains three motifs that directly participate in DNA binding.

Question 9

In the lac system, catabolite repressor protein reacts with

Accepted Answer

The catabolite repressor protein, also known as CAP (catabolite activator protein), binds to the lac promoter in the presence of cAMP to facilitate transcription of the lac operon.

Question 10

A protein motif found in several eukaryotic transcription factors with a function in protein dimerization is the

Accepted Answer

The leucine zipper motif is known to mediate protein dimerization and is found in several eukaryotic transcription factors. This motif consists of a repeating pattern of leucine residues which form an alpha helix, and two alpha helices from different proteins will come together to form a dimer.

Question 11

Mutation of the repressor protein to an inactive form leads to constitutive (always on) expression of the operon.

Accepted Answer

Both the lac and trp operons can exhibit constitutive expression if the repressor protein is mutated to an inactive form. In the lac operon, this would result in continuous expression of genes involved in lactose metabolism, while in the trp operon, it would lead to unregulated expression of genes involved in tryptophan synthesis.

Question 12

All of the following statements concerning the lactose operon are correct EXCEPT

Accepted Answer

The repressor binds specifically to the operator site on the DNA, not to the CAP protein.

Question 13

A mutation in the lac operator that blocks the binding of the lac repressor [all of the following EXCEPT]:

Accepted Answer

A mutation in the lac operator that blocks the binding of the lac repressor causes constitutive expression of the lac operon genes (including the z and y genes) because the repressor can no longer bind to the operator to inhibit transcription. This mutation does not lead to an inactivation of the lac repressor protein itself; the repressor is still functional but cannot bind to the mutated operator.

Question 14

Allolactose and cAMP function in the regulation of the lactose operon in

Accepted Answer

Allolactose and cAMP act as allosteric effectors for their respective regulatory proteins. Allolactose binds to the repressor protein, causing a conformational change that decreases its affinity for DNA and allows transcription of the lactose operon. cAMP binds to the activator protein, causing a conformational change that increases its affinity for DNA and promotes transcription of the operon.

Question 15

In the Holliday model for recombination, which is not true?

Accepted Answer

The strand exchange is not initiated by DNase I nicking, but rather by a double-strand break that is induced by the RecBCD complex.

Question 16

Which of the following substances binds to lac repressor?

Accepted Answer

Lac repressor binds to the lac operator DNA in the absence of lactose. However, in the presence of lactose, it binds to allolactose, a lactose isomer, causing it to release from the operator DNA and allow transcription of the lac operon. Beta-galactosidase is one of the enzymes encoded by the lac operon, but it does not directly interact with the lac repressor protein.

Question 17

Positive regulation of gene expression has a specific meaning relating to the effect of a regulatory protein on the gene in question. In which of the following systems does positive regulation occur?

Accepted Answer

Catabolite repression involves positive regulation because it involves the activation of gene expression in the presence of certain catabolites (e.g., cAMP levels increase in the absence of glucose, leading to the activation of CAP protein, which then enhances the transcription of operons involved in the metabolism of alternative sugars).

Question 18

Which of the following provides information for the structure of specific proteins?

Accepted Answer

Structural genes provide information for the structure of specific proteins. The other choices are involved in gene regulation and mutation.

Question 19

Polycistronic transcripts

Accepted Answer

Polycistronic transcripts are usually observed in prokaryotes and permit the coordinated production of a set of proteins by control of transcription from a single promoter region. They encode functionally related proteins rather than unrelated ones, and do not undergo splicing in spliceosomes as they are not present in eukaryotes.

Question 20

What is the most expected effect of a deletion mutation covering the promoter sequence of the lactose operon?

Accepted Answer

The promoter sequence is essential for the initiation of transcription of the lactose operon. A deletion mutation covering the promoter sequence would prevent RNA polymerase from binding and initiating transcription. Therefore, there would be a loss of transcription of the beta-galactosidase gene in the presence of lactose. The other options are not expected as they are not directly related to the promoter sequence.

Quiz 8: Regulation of Gene Expression