The operon theory of gene regulation explains how the production of

Question

The operon theory of gene regulation explains how the production of proteins is regulated in bacteria. A complex of genes, the operon consists of two regions: a coding region, for enzymes involved in a metabolic pathway, and a regulatory region that determines whether the gene is switched “on” or “off.” There are two types of operons. In an inducible operon, protein production is “off” by default, and turns “on” when a sufficient quantity of inducer is present. In a repressible operon, protein production is “on” by default, and turns “off” when an active repressor complex is present.

The lac operon is an example of an inducible operon. Normally, the lac operon's protein production is inactive; since the lac repressor is bound to the regulatory region, the gene does not produce enzymes. However, when lactose, or milk sugar, is present in high concentrations, the lac repressor binds instead to the lactose. Since the regulatory region is now free of a repressor, the gene activates and begins to produce proteins.

The process is neatly self-regulating. The proteins produced by the now-activated genes’ lactose-digesting enzymes, which in turn down the lactose. Once all lactose is broken down, there is no remaining lactose that can bind to the Lac repressor. The repressor returns to the regulatory region, turning the operon “off” and arresting the production of enzymes and the digestion of lactose.    1 
 1. Which of the following best describes the structure of the passage?

A. explains a theory by describing its execution
B. hypothesizes a reason for a phenomenon
C. describes a theory in scientific research
D. explores the possibilities induced by a theory
E. demonstrates a concept with a specific example

2 
 2. Which of the following can be inferred from the passage?

A. All genes are either inducible or repressible
B. Lactose is the inducer of the /ac operon
C. The lac repressor prefers to bind to lactose rather than the regulatory region
D. The repressible operon does not normally have anything bound to its regulatory region
E. Lactose is the main source of energy for bacteria

3 
 3. According to the passage, what is the purpose of the lac repressor?

A. it prevents lactose from binding to the regulatory region
B. it forms the active repressor complex in a repressible operon 
C. when present in high concentrations, it inhibits the activation of lactose-digesting enzymes
D. when bound to enzymes, it prevents lactose digestion
E. when bound to the regulatory region, it turns the /ac operon "off," halting enzyme production

RC Butler Competition 2021  -  Participate and Win!   
Passage # 8 Date: 6-Jan-2021
 This question is a part of RC Butler Competition 2021 .  Click here for Details

Sajjad1994 · Accepted Answer

Official Explanation

Passage Map:

Topic:  An operon is a complex of genes and is the basis of the operon theory, which explains how the production of proteins in bacteria is regulated.

Scope:  Lac operon is one type of operon.

Purpose:  To provide an example of how the lac operon assists in the production of proteins when high levels of lactose exist.

Paragraph 1:  Introduces the concept of the operon theory

Paragraph 2:  Describes how regulation occurs with the /ac operon

Paragraph 3:  Describes how the gene turns off

1. Which of the following best describes the structure of the passage?

Difficulty Level:   Medium

Explanation

The key words  "structure of the passage"  signal that this is a Logic question, which asks how a passage is organized. Logic question require analysis and evaluation of the organization and logic of a passage.

Research Relevant Text in the Passage:

Utilize notes on the passage map to prephrase an answer, and be skeptical of answer choices that focus too heavily on specifics.

Make a Prediction:

Returning to the prediction from the passage map, we see that the passage aims to provide an example.

Evaluate the Answer Choices:

Answer  Choice (E)  matches our prediction. The word  “example”  is our clue.

Choice (A)  is a distortion. It is not the theory that needs to be explained; it is the theory that explains the phenomenon of gene regulation. The first few words of the passage,  "the operon theory explains,"  provide a clue for eliminating this answer choice. While  choice (B)  uses the scientific word "hypothesizes," this is misleading. The passage is not hypothesizing a reason for a phenomenon; it presents the operon theory as an accepted explanation.

Choice (C)  begins well, but is too vague and does not take into account the specific example. The passage does more than simply describe the theory; it demonstrates how it plays out in the lac operon. Correctly identifying the scope is important because it helps eliminate choices that are too broad.

Choice (D)  is out of scope. The author does not extrapolate possibilities suggested by the theory.

Answer: E

2. Which of the following can be inferred from the passage?

Difficulty Level:   Medium

Explanation

Analyze the Question Stem:

The key word  "inferred"  signals that this is an Inference question. An inference requires going beyond the material explicitly stated in the passage.

Research Relevant Text in the Passage:

The question stem does not reference any particular detail, so there's nothing to research ahead of time.

Make a Prediction:

We'll have to research as we evaluate each answer, predicting only that four of these answers won't be necessarily true based on the passage, and one will be.

Evaluate the Answer Choices:

Choice (B)  is correct. While it is not explicitly stated that lactose is the inducer, this can be inferred from the fact that the concentration of lactose is what activates the gene. Sweeping statements such as  choice (A)  should be treated with caution. While only inducible and repressible genes are mentioned in this passage, this does not preclude other types.

Choice (C)  goes beyond what is stated in the passage. Although lactose in high concentrations binds to the /ac repressor and takes it away from the regulatory region, it cannot be inferred that the /ac repressor prefers lactose to the regulatory region.

Choice (D)  is incorrect. The repressible operon is merely mentioned at the end of the first paragraph, but it is not stated whether anything is normally bound to the regulatory region.

Choice (E)  is out of scope. The passage does not mention the significance of the role that lactose plays in bacterial metabolism.

Answer: B

3. According to the passage, what is the purpose of the lac repressor?

Difficulty Level:   Medium

Analyze the Question Stem:

The phrase  "According to"  in the stem signals a Detail question. So the right answer must be something that the author explicitly states.

Research Relevant Text in the Passage:

For questions about details in the passage, use your Passage Map to locate the relevant section of the passage to research. We are asked about the purpose of the lac repressor, which is mentioned in  paragraph 2 . In the second sentence we read "since the lac repressor is bound to the regulatory region, the gene does not produce enzymes.”

Make a Prediction:

The correct answer will touch on the role of the lac repressor in regards to enzyme production.

Evaluate the Answer Choices:

Answer  Choice (E)  matches our prediction.

Choice (A)  is a distortion. Lactose binds to the repressor, not the regulatory region.

Choice (B)  is a misuse of detail. The concepts of  "active repressor complex"  and  “repressible operon”  are found in  paragraph 1 , and do not apply to the lac operon, which is an inducible operon.

Choice (C)  is a distortion. It is the activation of the genes, and not the activation of the enzymes, that is of concern.

Choice (D)  is also a distortion. The passage clearly states that the /ac repressor binds to the regulatory region, not the enzymes.

Answer: E

RashmikaAnegama · Answer

The Passage goes like this:
 1 - It introduced Operon theory, explained what it contains, explained it's types
2 - Gives an example of a type of operon and explains how it works 
3 - The explanation of working of the example given in para 2 is continued

1. Which of the following best describes the structure of the passage?

A. explains a theory by describing its execution  
 It is not describing it's execution

B. hypothesizes a reason for a phenomenon  
 It's not hypothesizing any reason

C. describes a theory in scientific research

D. explores the possibilities induced by a theory  
 It's not exploring any possibilities

E. demonstrates a concept with a specific example  
 Yes, it's demonstrating operon theory with an example

2. Which of the following can be inferred from the passage?

It is stated in the first para :
 In an inducible operon, protein production is “off” by default, and turns “on” when a sufficient quantity of inducer is present. 
And as /ac operon is inactive and lactose helps it to become on as given in the second para, it can be inferred that lactose is it's inducer.
So, the answer here is B

A. All genes are either inducible or repressible  
 No, we can't say that. 'All' is extreme.

B. Lactose is the inducer of the /ac operon  
 As explained above, this can be inferred from the passage

C. The Jac repressor prefers to bind to lactose rather than the regulatory region 
It didn't talk about Jac anywhere

D. The repressible operon does not normally have anything bound to its regulatory region 
This line from the paragraph 2 helps us to say that D is incorrect Since the regulatory region is now free of a repressor, the gene activates and begins to produce proteins.

E. Lactose is the main source of energy for bacteria 
It can't be inferred

3. According to the passage, what is the purpose of the lac repressor?

A. it prevents lactose from binding to the regulatory region

B. it forms the active repressor complex in a repressible operon 
It doesn't form any complex

C. when present in high concentrations, it inhibits the activation of lactose-digesting enzymes

D. when bound to enzymes, it prevents lactose digestion 
It doesn't prevent the lactose digestion

E. when bound to the regulatory region, it turns the /ac operon "off," halting enzyme production 
It actually turns on the /ac operon

So, IMO, the answers are  E,B,C

Vie3nrose · Answer

Question 3. According to the passage, what is the purpose of the lac repressor?

A. it prevents lactose from binding to the regulatory region
 INCORRECT   -- We're not given information on whether lactose would bind to the regulatory region even if the /ac repressor were not present. We're just told that the /ac repressor likes to bind with both lactose and the reg region (altho it prefers lactose).
 
B. it forms the active repressor complex in a repressible operon
 INCORRECT   -- the /ac repressor was discussed in relation to the /ac operon, which is an example of an  inducible  operon, not a repressible operon.
 
C. when present in high concentrations, it inhibits the activation of lactose-digesting enzymes
 INCORRECT   -- We're given information on what happens when lactose is present in high concentrations, but not what happens when /ac repressors are present in high concentrations. Maybe it will inhibit the activation of lactose-digesting enzymes even in low concentrations?
 
D. when bound to enzymes, it prevents lactose digestion
 INCORRECT   -- in the given example, /ac repressors prevent the operon from producing lactose-digesting enzymes (thus preventing lactose digestion) when bound to the  regulatory region , not when bound to enzymes themselves. 
 
E. when bound to the regulatory region, it turns the /ac operon "off," halting enzyme production
 CORRECT   -- PG2 "Normally, the /ac operon's protein production is inactive  ;  since the /ac repressor is bound to the regulatory region, the gene does not produce enzymes ."

ikatke · Answer

1. Which of the following best describes the structure of the passage?

A. explains a theory by describing its execution
B. hypothesizes a reason for a phenomenon
C. describes a theory in scientific research
D. explores the possibilities induced by a theory
E. demonstrates a concept with a specific example

Ans: A (Not sure: Close call among A, C and E)

P-1 introduce a theory
P-2 Example to explain the theory
P3-Further Description of theory. 
B and D can be rejected easily. C seems incomplete because passage not only describes theory but also gives example. A guessed because 2nd & 3rd para shows execution of concept explained in para 1

2. Which of the following can be inferred from the passage?

A. All genes are either inducible or repressible: Word All seems not good.
B. Lactose is the inducer of the /ac operon: Can be inferred
C. The Jac repressor prefers to bind to lactose rather than the regulatory region: Even though /ac bind to lactose, when lactose present in high concentration, preference at normal concentration can not be inferred.
D. The repressible operon does not normally have anything bound to its regulatory region.: Seems extreme
E. Lactose is the main source of energy for bacteria: Nowhere it is mentioned that lactose
Ans: B
As per first para,  There are two types ---, and turns “on” when a sufficient quantity of inducer is present.  
As per second para,  The /ac operon is an example of an inducible operon. when lactose, or milk sugar, is present in high concentrations, the /ac repressor binds instead to the lactose. 
Hence it can be conclude that Lactose is the inducer of the /ac operon.

3. According to the passage, what is the purpose of the lac repressor?

A. it prevents lactose from binding to the regulatory region- It does not prevent
B. it forms the active repressor complex in a repressible operon - Not supported by passage
C. when present in high concentrations, it inhibits the activation of lactose-digesting enzymes -Not supported by passage
D. when bound to enzymes, it prevents lactose digestion - Not mentioned. Seems extreme. 
E. when bound to the regulatory region, it turns the /ac operon "off," halting enzyme production- Correct
Ans: E
As per para 2,  when /ac repressor is bound to the regulatory region, the gene does not produce enzymes. --
when lactose, or milk sugar, is present in high concentrations, the /ac repressor binds instead to the lactose. Since the regulatory region is now free of a repressor. .

Tahamohsin · Answer

1. Which of the following best describes the structure of the passage?

1=E

A. explains a theory by describing its execution
  The passage does explain the theory in the first para, but it doesn't explain its execution.

B. hypothesizes a reason for a phenomenon
  The passage never stated the reason for the production of proteins is regulated in bacteria.

C. describes a theory in scientific research
  This choice seems good, but I like E better.

D. explores the possibilities induced by a theory
  The passage does not explore the possibilities induced by a theory.

E. demonstrates a concept with a specific example
  YEs , the passage demonstrates a concept of how the production of proteins is regulated with a specific example

2. Which of the following can be inferred from the passage? 
 
  2=B

A. All genes are either inducible or repressible
  From the passage, we can not surely conclude that all genes are either inducible or repressible

B. Lactose is the inducer of the /ac operon
  Yes, the second para stated that, normally, the /ac operon's protein production is inactive; since the /ac repressor is bound to the regulatory region, the gene does not produce enzymes. However, when lactose, or milk sugar, is present in high concentrations, the /ac repressor binds instead to the lactose.

C. The Jac repressor prefers to bind to lactose rather than the regulatory region
   What is the Jac repressor ? We don't know.

D. The repressible operon does not normally have anything bound to its regulatory region
  The passage never stated that a repressible operon does not normally have anything bound to its regulatory region.

E. Lactose is the main source of energy for bacteria
  We cannot conclude from the passage that lactose is the main source of energy for bacteria.

3. According to the passage, what is the purpose of the lac repressor? 
  3=E........ Although I am assuming "lac repressor" is "/ac repressor", which is used in the passage.

A. it prevents lactose from binding to the regulatory region
  The passage stated the opposite that lac repressor binds the lactose to the regulatory region.

B. it forms the active repressor complex in a repressible operon 
  No such information in the passage was given that lac repressor forms the active repressor complex in a repressible operon.

C. when present in high concentrations, it inhibits the activation of lactose-digesting enzymes
  Same no such information was given.

D. when bound to enzymes, it prevents lactose digestion
  Same no such information was given in the passage that lac repressor when bound to enzymes, it prevents lactose digestion.

E. when bound to the regulatory region, it turns the /ac operon "off," halting enzyme production
  Correct, this is on the last sentence of the passage .The repressor returns to the regulatory region, turning the operon “off” and arresting the production of enzymes and the digestion of lactose.

TarunKumar1234 · Answer

Summary:  Operon theory- Production of protein and regulated in Bacteria, 1) a coding 2) regulatory
Type 1)inducible (default-OFF) 2) Repressible (default ON)
Then example, how Lactose milk works and how regulatory works making ON and OFF for /ac operon.

1. Which of the following best describes the structure of the passage?

A. explains a theory by describing its execution -> Incorrect. It is not an execution in para 2 and 3.
B. hypothesizes a reason for a phenomenon -> No. Incorrect.
C. describes a theory in scientific research -> There is no scientific research talked about
D. explores the possibilities induced by a theory -> No discussion about possibilities
E. demonstrates a concept with a specific example ->  Correct  /ac is an Operon example taken.

2. Which of the following can be inferred from the passage?

A. All genes are either inducible or repressible -> All genes are either coding or regulatory. Incorrect.
B. Lactose is the inducer of the /ac operon -> Yes.  Correct.  
C. The /ac repressor prefers to bind to lactose rather than the regulatory region -> Incorrect.
D. The repressible operon does not normally have anything bound to its regulatory region-> Incorrect.
E. Lactose is the main source of energy for bacteria-> Not sure.

3. According to the passage, what is the purpose of the lac repressor?

A. it prevents lactose from binding to the regulatory region- Incorrect.
B. it forms the active repressor complex in a repressible operon- Incorrect.
C. when present in high concentrations, it inhibits the activation of lactose-digesting enzymes- Incorrect. "high concentration"
D. when bound to enzymes, it prevents lactose digestion -> Incorrect, It triggers further action.
E. when bound to the regulatory region, it turns the /ac operon "off," halting enzyme production->  Correct.

Sumi1010 · Answer

Short note on the passage:
P1 - Operon theory: Regulation of proteins. Types of operons - inducible (protein off) and repressible (protein on).
P2 - Inducible /ac : When repressor binds to lactose , protein off --> On.
P3 - Self Regulate: Protein on --> lactose digestion; protein On--> Off.

1. Which of the following best describes the structure of the passage?

A. explains a theory by describing its execution
 The passage talks about the theory and then takes the  assistance of an example  (/ac operon) to demonstrate the self regulatory mechanism of protein control. The passage does not keep describing the theory it introduced for its  in general  execution. 
B. hypothesizes a reason for a phenomenon
 There is no hypothesis discussed in the passage. 
C. describes a theory in scientific research
 There is no scientific research discussed.  
D. explores the possibilities induced by a theory
 The passage does not talk about the possibilities on the self regulated protein concept discussed. 
E. demonstrates a concept with a specific example
 The passage talks about the theory and then takes the  assistance of an example  (/ac operon) to demonstrate the self regulatory mechanism of protein control.

E

2. Which of the following can be inferred from the passage?

A. All genes are either inducible or repressible
 Operons (a type of complex genes) are inducible or repressible. 
B. Lactose is the inducer of the /ac operon.
 The /ac repressor binds to lactose & protein production triggers. That means, Protein production turns “on” when a sufficient quantity of inducer is present. This is what called the inducer. 
C. The /ac repressor  prefers  to bind to lactose rather than the regulatory region
 This happens only in high concentrations of lactose. ( However, when lactose, or milk sugar, is present in  high concentrations , the /ac repressor binds instead to the lactose ). 
D. The repressible operon does not normally have anything bound to its regulatory region
 No such information is provided about repressible operon. 
E. Lactose is the  main source of energy  for bacteria
 Cannot be inferred from the passage. The passage does not talk about the source of energy.

B

3. According to the passage, what is the purpose of the /ac repressor?

A. it prevents lactose from binding to the regulatory region
 The /ac repressor binds to the lactose at high concentration. The regulatory region stays free in this case, but we don't know if lactose could have bound to the region if it were not free. 
B. it forms the active repressor complex in a repressible operon
 /ac is an example of inducible operon. 
C. when present in high concentrations, it  inhibits  the activation of lactose-digesting enzymes
 The concentration of the repressor does not control anything . Concentration of lactose does and lactose is not a repressor. 
D. when bound to enzymes, it prevents lactose digestion
 Repressor does not prevent the lactose digestion.  
E. when bound to the regulatory region, it turns the /ac operon "off," halting enzyme production
 Since the /ac repressor is bound to the regulatory region, the gene does not produce enzymes.

E

Pankaj0901 · Answer

Answer IMO 
1. A
2. E
3. E

1. Which of the following best describes the structure of the passage?

A. explains a theory by describing its execution
B. hypothesizes a reason for a phenomenon
C. describes a theory in scientific research
D. explores the possibilities induced by a theory
E. demonstrates a concept with a specific example
 Theory is explained and also the execution is described. A is covering this correctly.
- There is no hypothesis or assumptions- B Eliminate
- In Option C, "scientific research" is not correct.
- This is not about the possibilities but rather the fact. D is out.

2. Which of the following can be inferred from the passage?

A. All genes are either inducible or repressible
B. Lactose is the inducer of the /ac operon
C. The Jac repressor prefers to bind to lactose rather than the regulatory region
D. The repressible operon does not normally have anything bound to its regulatory region
E. Lactose is the main source of energy for bacteria
 IMO correct answer E. As mentioned in the paragraph: "Once all lactose is broken down, there is no remaining lactose that can bind to the /ac repressor. The repressor returns to the regulatory region, turning the operon “off” and arresting the production of enzymes and the digestion of lactose."

3. According to the passage, what is the purpose of the lac repressor?

A. it prevents lactose from binding to the regulatory region
B. it forms the active repressor complex in a repressible operon
C. when present in high concentrations, it inhibits the activation of lactose-digesting enzymes
D. when bound to enzymes, it prevents lactose digestion
E. when bound to the regulatory region, it turns the /ac operon "off," halting enzyme production
 Correct Answer E. As mentioned in the paragraph- "Once all lactose is broken down, there is no remaining lactose that can bind to the /ac repressor. The repressor returns to the regulatory region, turning the operon “off” and arresting the production of enzymes and the digestion of lactose."

Deepakjhamb · Answer

1. Which of the following best describes the structure of the passage?

answer is  e)demonstrates a concept with a specific example

A. explains a theory by describing its execution :  explaining a theory is fine by describing the execution , but example part which is very imp part of structure of passage is the one that is ignored in this  
B. hypothesizes a reason for a phenomenon :  in first para hypothesis is stated for a phenomenon , but that hypothesis is also demonstrated through example as part of passage structure ,so its out  
C. describes a theory in scientific research :  example is not taken care ,also hypothesis is demonstrated , not a theory  
D. explores the possibilities induced by a theory :  possibilities induced by theory are not discussed , so out  
E. demonstrates a concept with a specific example -  this is right as firstly a concept is introduced -the operon theory of gene regulation explains how the production of proteins is regulated in bacteria and then explained through a specific example

2. Which of the following can be inferred from the passage?

answer is b)Lactose is the inducer of the /ac operon 
A. All genes are either inducible or repressible :  actually operons are either inducible or repressible and not genes ,so out  
B. Lactose is the inducer of the /ac operon :  this is correct as in an inducible operon, protein production is “off” by default, and turns “on” when a sufficient quantity of inducer is present , also second para statest that /ac operon is inducible operon and that lactose is present in high concentration . 
C. The /ac repressor prefers to bind to lactose rather than the regulatory region :  it is not about preference or choice , it is a fact that /ac repressor binds to lactose instead of the regulatory region . 
D. The repressible operon does not normally have anything bound to its regulatory region -  reperessible operon is not detailed abt , only inducible operon is taken in example  
E. Lactose is the main source of energy for bacteria :  this is outside info , not in the scope of the passage

3. According to the passage, what is the purpose of the /ac repressor?
 answer is e)when bound to the regulatory region, it turns the /ac operon "off," halting enzyme production

A. it prevents lactose from binding to the regulatory region :  actually lactose prevents /ac repressor from binding to regulatory region , so its not true facts as per passage  
B. it forms the active repressor complex in a repressible operon :  reperessible operon is not discussed , only induced operon is discussed , so its out  
C. when present in high concentrations, it inhibits the activation of lactose-digesting enzymes :  this is not given in passage , actually the trigger is that when /ac repressor returns to regulatory region enzyme production is arrested . 
D. when bound to enzymes, it prevents lactose digestion :  /ac repressor does not bind to enzymes to prevent lactose digestion , this is wrong  
E. when bound to the regulatory region, it turns the /ac operon "off," halting enzyme production :  this is correct as per last line of para 3 -the repressor returns to the regulatory region, turning the operon “off” and arresting the production of enzymes

Neha2404 · Answer

1-E
2-B
3-E

IMO-E
1. Which of the following best describes the structure of the passage?

A. explains a theory by describing its execution
INCORRECT:- in 1st para about theory is mention but in 2nd and 3rd para only inducible operon is dicussed with example

B. hypothesizes a reason for a phenomenon
INCORRECT:-NO phenomena is mentioned

C. describes a theory in scientific research
INCORRECT:- NO operon theory in scientific research is discribed in pasage

D. explores the possibilities induced by a theory
INCORRECT:-NO explores the possibilities induced by a theory IN PASSAGE

E. demonstrates a concept with a specific example
CORRECT:- in 1st para about theory is mention but in 2nd and 3rd para only inducible operon is dicussed with example

IMO-B

2. Which of the following can be inferred from the passage?

A. All genes are either inducible or repressible:-
INCORRECT:- only Inducible and repressible gene are give in passage we cant conclude about all gene

B. Lactose is the inducer of the /ac operon
Correct:- when Lactose introdue /ac operon active

C. The Jac repressor prefers to bind to lactose rather than the regulatory region
:-INCORRECT:-NOT MENIONED

D. The repressible operon does not normally have anything bound to its regulatory region
INCORRECT:-not mentioned in para

E. Lactose is the main source of energy for bacteria
INCORRECT:- Extreme reply it is not mentioned in passage

IMO-E

3. According to the passage, what is the purpose of the lac repressor?

A. it prevents lactose from binding to the regulatory region:-
INCORRECT:- lac repressor NOT prevents lactose from binding to the regulatory region

B. it forms the active repressor complex in a repressible operon
INCORRECT:-Not mentioned in passage

C. when present in high concentrations, it inhibits the activation of lactose-digesting enzymes
INCORRECT:- IT IS 180 DEGREE REPLY

D. when bound to enzymes, it prevents lactose digestion
INCORRECT:-IT not bound with enzymes

E. when bound to the regulatory region, it turns the /ac operon "off," halting enzyme production
CORRECT:- When bound to the regulatory region it turns the /ac operon "off," halting enzyme production
supporting lines :-Normally, the /ac operon's protein production is inactive; since the /ac repressor is bound to the regulatory region, the gene does not produce enzymes

rocky620 · Answer

1. Which of the following best describes the structure of the passage?

A. explains a theory by describing its execution...... Yes. First para introduces the theory and. second and third para describes how it happens through an example. Keep it. 
B. hypothesizes a reason for a phenomenon...... No reason (as in why is the generation important, what will happen if it does not happens etc.) is mentioned. Eliminate. 
C. describes a theory in scientific research....... There is a theory, but not in scientific research. The theory is a part of biological functioning. Eliminate. 
D. explores the possibilities induced by a theory...... No possibilities. Eliminate. 
E. demonstrates a concept with a specific example......The specific example is not to demonstrate, but to explain a theory. Eliminate.

2. Which of the following can be inferred from the passage?

A. All genes are either inducible or repressible..... All operons are either inducible or repressible. Not all genes, as operon is only a complex of genes/type/element. Eliminate. 
B. Lactose is the inducer of the /ac operon..... Yes it can be inferred.Since operon is inducible. It needs an inducer in a sufficient qty to start the protein production. Lactose is that inducer as mentioned in the second para. Keep It.  
C. The Jac repressor prefers to bind to lactose rather than the regulatory region..... Yes it does, but only when Lactose is present in higher concentrations, not in normal concentrations. Eliminate. 
D. The repressible operon does not normally have anything bound to its regulatory region..... It do have repressors (mentioned in second para). Eliminate. 
E. Lactose is the main source of energy for bacteria..... It is a source of energy, but not the main source. ELiminate.

3. According to the passage, what is the purpose of the lac repressor?

This is mentioned in the last sentence of the passage. " The repressor returns to the regulatory region, turning the operon “off” and arresting the production of enzymes and the digestion of lactose ."

On these lines only Option E Matches.

A. it prevents lactose from binding to the regulatory region.
B. it forms the active repressor complex in a repressible operon
C. when present in high concentrations, it inhibits the activation of lactose-digesting enzymes
D. when bound to enzymes, it prevents lactose digestion
E. when bound to the regulatory region, it turns the /ac operon "off," halting enzyme production.  Correct

IMO Option A, B, E

Sajjad1994 · Answer

Question 1 is hard but rest are medium level, OAs has been revealed you may check your answers

Vie3nrose  hard luck on this one, let me know if anyone needs OE.

Tahamohsin  Excellent work!

Tier 1

Deepakjhamb
Tahamohsin
Sumi1010
Neha2404

Tier 2

RashmikaAnegama

Rest were OK, All those got the last one incorrect, You can read the passage and attempt the question again, this was not a question to be missed.

Point table will be updated soon, Good luck for today's questions.

Thank you

Vie3nrose · Answer

Hi Sajjad1994,

I have a question for Question 2 of this passage: why are we not able to infer that the /ac repressor prefers lactose to the regulatory region?
My logic on this answer choice: 
C. The Jac repressor prefers to bind to lactose rather than the regulatory region
CORRECT -- Assuming that "Jac" is a typo, this answer choice is the safest to infer since if the /ac repressor did not prefer binding to lactose over the regulatory region, then the /ac repressor might stay bound to the regulatory region despite the presence of lactose and the /ac operon would remain inactive (negating the entire gene regulation process explained in PG2 and 3).

Thanks for your help!

Sajjad1994 · Answer

Thank you! I have removed the typo.

Well, Keep one thing in mind, in the Hard RC questions (Especially Inference questions) try to find the explicit knowledge which can be validated by the text of the passage. There could be trap answers in the form of something implicitly given in the passage with  slight deviation , in the question #2 the same happens, Keep one more thing, in the GMAT we have to try to pick the answer choice which is best among the available five and  "Best among the available 5"  should not necessarily be the  perfect  one. I have highlighted one word in your query above, have a look at that. We have to keep a sharp eye on the strong and light words/languages  C is too strong to infer  while B is safe to infer. In the  second paragraph:

"However, when lactose, or milk sugar, is present in high concentrations, the /ac repressor binds instead to the lactose. Since the regulatory region is now free of a repressor, the gene activates and begins to produce proteins."

These lines clearly supports  answer choice B

Regards

eeshajain · Answer

For Q1, can someone explain why E is correct and not A?

A. explains a theory by describing its execution
vs
E. demonstrates a concept with a specific example

definition of a concept is an abstract idea whereas theory is a defined set of ideas that explain how something will work.
The Operon theory discussed is not an abstract idea it is scientific in nature, and the passage explains how it works. So shouldn't A be the correct answer choice?

xxdw0007 · Answer

how could you print lac to /ac? It's a silly error for misunderstanding.

The operon theory of gene regulation explains how the production of

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The operon theory of gene regulation explains how the production of

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