Advanced Mechanisms Of Transcriptional Regulation And Epigenetic Control Practice Tests

Question 1

Consider a mutation in the -35 consensus sequence of a bacterial promoter that reduces the binding affinity of the $\sigma^{70}$ factor by a factor of $k = 10$. If the initial rate of transcription initiation is given by $v = k_{cat}[E\sigma][P]$, where $[E\sigma]$ is the concentration of the holoenzyme-promoter complex, determine the relative change in the rate of transcription initiation assuming the concentration of free holoenzyme remains constant.

Accepted Answer

The rate of transcription initiation decreases by a factor of 10.

Answer

The rate of transcription initiation increases by a factor of 10.

Answer

The rate of transcription initiation remains unchanged.

Answer

The rate of transcription initiation decreases by a factor of 100.

Question 2

In a hypothetical operon, the repressor protein binds to the operator only when the concentration of a metabolite $M$ satisfies $[M] \geq 10^{-6} M$. If the dissociation constant for the repressor-operator complex is $K_d = 10^{-8} M$, calculate the fraction of time the operator is occupied by the repressor when $[M] = 10^{-6} M$, assuming the repressor is fully activated by $M$.

Accepted Answer

0.99

Answer

0.50

Answer

0.90

Answer

0.01

Question 3

A Rho-independent terminator sequence contains a GC-rich region followed by a sequence of 8 adenines on the template strand. If the melting temperature $T_m$ of the hairpin loop is $75^{\circ}C$ and the $T_m$ of the $rU:dA$ hybrid is $45^{\circ}C$, explain why the polymerase pauses and dissociates based on the thermodynamic stability of these structures.

Accepted Answer

The formation of the stable hairpin loop ($T_m = 75^{\circ}C$) provides the energy to destabilize the weaker $rU:dA$ hybrid ($T_m = 45^{\circ}C$), forcing the RNA polymerase to pause and release the transcript.

Question 4

Which of the following scenarios would result in a constitutive expression (always ON) of the structural genes in a classic Lac operon model?

Answer

A mutation in the operator sequence ($O^c$) that prevents repressor binding.

Answer

A mutation in the repressor gene ($I^-$) that produces a non-functional repressor protein.

Answer

A mutation in the promoter sequence that prevents RNA polymerase binding.

Answer

A mutation in the structural genes that prevents translation.

Question 5

If the concentration of the $\sigma^{70}$ factor is $100$ nM and the concentration of the core RNA polymerase is $50$ nM, the total concentration of functional holoenzyme is limited by the concentration of the core enzyme.

Accepted Answer

True

Answer

False

Question 6

In a Rho-dependent termination system, the Rho protein acts as a helicase. If the rate of RNA polymerase elongation is $v_{pol} = 40$ nucleotides/second and the rate of Rho translocation along the nascent RNA is $v_{rho} = 50$ nucleotides/second, determine the time required for Rho to catch up to the polymerase if the Rho loading site is $L = 100$ nucleotides behind the polymerase.

Accepted Answer

10 seconds

Answer

2 seconds

Answer

5 seconds

Answer

20 seconds

Question 7

Calculate the probability $P$ that a promoter is occupied by RNA polymerase given the binding energy $\Delta G = -15$ kJ/mol at $T = 310$ K, using the Boltzmann distribution $P = \frac{e^{-\Delta G / RT}}{1 + e^{-\Delta G / RT}}$, where $R = 8.314$ J/(mol·K).

Accepted Answer

0.996

Question 8

In a hypothetical experiment, the rate of transcription $R$ is modeled by the concentration of the pre-initiation complex $[PIC]$ and the activity of TFIIH, where $R = k \cdot [PIC] \cdot [TFIIH]$. If a mutation reduces the helicase activity of TFIIH by $40\%$, what is the new rate $R'$ relative to the original rate $R$?

Accepted Answer

0.6R

Answer

0.4R

Answer

0.24R

Answer

1.4R

Question 9

Histone octamers have a net positive charge due to lysine and arginine residues, interacting with the negatively charged DNA backbone. If the charge density of a histone segment is $\rho_h = +q$ and the DNA segment is $\rho_d = -q$, calculate the net electrostatic force $F$ if the distance between them is $r$. Express the effect of Histone Acetyl Transferase (HAT) on the magnitude of the force $F$ if it reduces the effective charge of the histone to $\rho'_h = +0.2q$.

Accepted Answer

The force decreases to 20% of the original magnitude.

Question 10

Which of the following scenarios would mathematically result in a decrease in the probability $P$ of transcription initiation, where $P$ is dependent on the successful recruitment of TFIID, TFIIB, and TFIIH?

Answer

A decrease in the binding affinity constant $K_a$ for TBP to the TATA box

Answer

A reduction in the concentration of available TFIIH helicase subunits

Answer

An increase in the concentration of RNA Polymerase II

Answer

The addition of an activator protein that stabilizes the TFIID-DNA interaction

Question 11

If an enhancer is located at a distance $d = 1000$ base pairs from the promoter, and the DNA bending protein reduces the effective distance to $d_{eff} = \frac{d}{10}$, the probability of interaction between regulatory factors and the basal machinery increases because the spatial volume $V$ for collision decreases according to $V \propto d_{eff}^3$.

Accepted Answer

True

Answer

False

Question 12

Consider a system where the transcription rate $T$ is governed by the equation $T = \alpha \cdot [E] + \beta \cdot [B]$, where $[E]$ is the concentration of regulatory factors at the enhancer and $[B]$ is the concentration of basal factors. If $[E]$ increases by a factor of $3$ and $[B]$ remains constant, which expression represents the new rate $T'$?

Accepted Answer

Option A

Answer

Option B

Answer

Option C

Answer

Option D

Question 13

If the energy required to unwind a DNA segment is $E_{unwind} = 50$ kcal/mol and the energy provided by the ATP-dependent helicase activity of TFIIH is $E_{TFIIH} = 10$ kcal/mol per ATP molecule, calculate the minimum number of ATP molecules required to initiate transcription for this segment.

Accepted Answer

5

Question 14

In a cell line, the promoter of a gene is 80% methylated at CG sites. If the recruitment of histone deacetylases (HDACs) is proportional to the methylation density $M$ where $HDAC_{activity} = k \cdot M^2$, and $k = 0.5$, calculate the relative HDAC activity level.

Accepted Answer

0.32

Answer

0.40

Answer

0.64

Answer

0.80

Question 15

A patient exhibits symptoms consistent with Prader-Willi syndrome due to a deletion on the paternal chromosome 15. If the maternal copy is normally silenced by methylation, calculate the total functional expression level $E_{total}$ of the gene, assuming the paternal copy normally provides $100$ units of expression and the maternal copy provides $0$ units due to imprinting.

Accepted Answer

0

Question 16

In a study of Fragile X syndrome, the number of CGG repeats $n$ correlates with the methylation probability $P$ according to the function $P(n) = 1 - e^{-\lambda(n-n_0)}$, where $\lambda = 0.05$ and $n_0 = 200$. If a patient has $n = 260$ repeats, determine the probability of promoter methylation.

Accepted Answer

0.95

Answer

0.05

Answer

0.50

Answer

0.99

Question 17

In a microarray experiment, the fluorescence intensity of a target gene in a treated sample is $I_t = 1200$ and in the control sample is $I_c = 300$. Calculate the fold change $FC = \log_2(\frac{I_t}{I_c})$.

Accepted Answer

2

Answer

1

Answer

4

Answer

8

Question 18

A researcher uses Northern blotting to compare mRNA abundance. If the band intensity for Sample A is $10$ units and Sample B is $2.5$ units, determine the ratio of mRNA concentration $\frac{[A]}{[B]}$ assuming linear proportionality between intensity and concentration.

Accepted Answer

4

Question 19

Which of the following scenarios would theoretically result in a decrease in gene expression based on the mechanisms of DNA methylation and histone modification?

Answer

Increased recruitment of histone deacetylases to the promoter region

Answer

Methylation of cytosine residues within the promoter CG islands

Answer

Increased histone acetylation at the promoter

Answer

Demethylation of the gene body

Question 20

If a microarray experiment shows a color intensity ratio of $1:1$ between a cancer cell line and a normal cell line, the gene is differentially expressed.

Accepted Answer

False

Answer

True

Advanced Mechanisms Of Transcriptional Regulation And Epigenetic Control Practice Tests

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Advanced Mechanisms of Transcriptional Regulation and Epigenetic Control

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