AIPMT DISCUSSION QUESTIONS ON TRANSCRIPTION IN PROKARYOTES

Q1: What is the correct direction of transcription?

a)      5’à 3’

b)      3’à 5’

c)       5’à 5’

d)      3’à 3’

Explanation:

The correct answer is ‘a’ i.e., transcription always occurs in the 5’ to 3’ direction. 5’ is called the head and 3’ is called the tail of a nucleotide chain. Replication also occurs in the same direction.

 Q2. A transcription unit comprises of:

a)      A promotor, structural gene, and a terminator

b)      A promotor and structural genes

c)       Structural genes and a terminator

d)      A promotor and a terminator

Explanation: 

The correct answer is ‘a’ i.e., a transcription unit comprises of a promoter, the structural gene and a terminator. Promoter is responsible for binding the enzyme RNA polymerase, strcutural genes are expressed to form proteins and a terminator sequence is required to end the process of transcription and translation. 

Q3. Which of the following statement is correct of a Lac operon?

a)      There are three promotor regions present for each gene Z, Y, and A

b)      There is a single promotor region for each gene Z, Y, and A

c)       There could be more than one promotors for the three genes Z, Y, and A

d)      No promotor is required for the three genes Z, Y, and A

Explanation:

The correct answer is ‘b’ i.e., for the Lac operon, only one promotor is required to express the three genes Z, Y, and A. On the other hand, in eukaryotes, each gene has its own promotor region.

Q4. Coding strand in DNA is the one:

a)      Which does not take part in transcription, but its bases will be the same as a new transcript with T replaced by U

b)      Which takes part in transcription, and its bases will the same as a new trascript with T replaced by U

c)       Which does not take part in transcription, but its bases will be the opposite of the new transcript bases

d)      None of the above

Explanation:

The correct answer is ‘a’ because, a coding strand is one that does not take part in transcription. It can also be called as a non template strand. However, since RNA strand does not contain Thymine, it is replaced by Uracil. 

Q5. The transcription start site i.e., +1 region contains:

a)      Mostly Purine bases

b)      Mostly Pyrimidine bases

c)       50 percent purine and 50 percent pyrimidine

d)      75 percent purine and 25 percent pyrmidine

Explanation:

The correct answer is ‘a’. In 90 percent of the cases, the +1 transcription start site contains purine bases especially Adenine.

Q6. Which of the following conserved sequence is called Pribnow box?

a)      TATAAT

b)      TATATA

c)       TTGACG

d)      GCAGAT

Explanation:

The correct answer is ‘a’ i.e., TATAAT a conserved seequence, which is present -10 seequence upstream on the DNA. This conserved sequence is essential for replication as RNA polymerase recognises this conserved sequence to start replication at the transcription start site.

Q7. Which of the following is the function of a sigma factor in transcription?

a)      Sigma factor is responsible for termination

b)      Sigma factor is responsible for elongation of RNA strand

c)       Sigma factor is involved in transcription initiation

d)      Sigma factor is involved in inhibiting the transcription process

Explanation:

The correct answer is ‘c’ i.e., sigma factor is responsible for transcription initiation. Sigma factor when attaches itself to the core enzyme, the combination is called Holoenzyme i.e., RNA polymerase. Sigma factor helps recognize the RNA polymerase to the correct place on the template strand to initiate transcription. Once the transcription process starts, sigma factor is released and the core enzyme gets involved in the elongation of RNA strand.

Q8. What mechanisms are involved in the termination of transcription process?

a)      Rho dependent and rho independent mechanism

b)      Rho dependent mechanism only

c)       Rho independent mechanism only

d)      None

Explanation:

The correct answer is ‘a’ i.e., rho dependent and rho independent mechanisms. The former is called protein based and the latter is called RNA based. 

Q9. Which of the following is true of rho dependent mechanism of termination?

a)      Near the end of the gene, RNA polymerase encounters a run of ‘A’ nucleotides

b)      Near the end of the gene, RNA polymerase encounnters a run of ‘G’ nucleotides

c)       Near the end of the gene, RNA polymerase encounters a run of ‘C’ nucleotides

d)      Near the end of the gene, RNA polymerase encounters a run of ‘U’ nucleotides

Explanation:

The correct answer is ‘b’ i.e., at the end of the gene, RNA polymerase encounters a run of G nucleotides. As a result, the rho protein collides with the polymerase. The interaction with the rho protein releases the mRNA from the termination bubble.

Q10. In RNA based termination, the polymerase enzyme encounters a region rich in:

a)      C-G nucleotides

b)      A-G nucleotides

c)       A-T nucleotides

d)      A-U nucleotides

Explanation:

The correct answer is ‘a’ i.e., the C-G nucleotides. The mRNA folds back on itself and the complementary C-G nucleotides bind together. The result is a hairpin that causes the polymerase to stall as soon as it begins to transcribe a region rich in A-T nucleotide.

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Discussion on Gene Regulation Questions

Q1. Which of the following is true of an Operon?

a)      An operon consists of structural genes only

b)      An operon consists of Strcutural genes and promoter regions

c)       An operon consists of promoter and I gene

d)      None

Explanation:

The correct answer is ‘b’ because an operon is a complete entity that includes promoter and structural genes. Since promoter region is important to bind RNA polymerase, it must be included in an operon to allow structural genes to express themselves.

Q2. The control of gene expression in prokaryotes mostly occurs at which level?

a)      Transcriptional level

b)      Translational level

c)       Replication level

d)      All of the above

Explanation:

The correct answer is ‘a’ because in prokaryotes, transcription and translation occurs simultaneously.  So, if there is no transcription then, no translation as well.

Q3. Control of gene expression in eukaryotes occurs at which level?

a)      Transcriptional level

b)      Translational level

c)       Processing level

d)      All of the above

Explanation:

The correct answer is ‘d’ because in eukaryotes, transcription and translation processes are separated by the nuclear membrane. In other words, transcription and post transcriptional modifications are done in the nucleus and translation occurs in the cytoplasm.

Q4. Which of the following is a constitutive gene?

a)      I gene

b)      Z gene

c)       Y gene

d)      A gene

Explanation:

The correct answer is ‘a’ because I gene is always on. It is also called as housekeeping gene. On the other hand, Z, Y, and Z genes are expressed under certain conditions like the glucose and lactose level.

Q5. In Lac Operon, when the repressor gene binds to the operator, no transcription and translation process occurs. This type of regulation is called:

a)      Positive regulation

b)      Negative regulation

c)       Neutral regulation

d)      None

Explanation:

The correct answer is ‘b’ because the repressor protein, as the name suggests represses the process of transcription and translation. So, it is called negative regulation.

Q6. What happens when the concentration of lactose in the cell increases?

a)      The repressor protein attaches to the lactose and the combined structure binds to the operator to switch off the process of transcription and translation

b)      The repressor protein binds to the operator without binding to the lactose molecules

c)       Lactose molecules inhibit the repressor protein to bind to the operator sequence

d)      The repressor protein attaches to the lactose and the combined structure fails to bind to the operator to switch on the process of transcription and translation

Explanation:

The correct answer is ‘d’ because lactose acts as an inducer. When there is no lactose in the cell, no transcription and translation process occurs. In this situation, repressor protein binds to the operator sequence to switch off the process of transcription and translation. This is because, the cell already has sufficient amount of glucose. However, when the lactose concentration increases, it means, there is low amount of sugar in the cell and lactose needs to be converted to glucose and galactose to get energy. So, lactose induces the repressor protein and the two collectively fails to bind to the operator. So, when repressor is not attached to the operator, the process of transcription and translation occurs smoothly.  

 Q7. Tryptophan in Trp Operon is called as:

a)      Repressor

b)      Co-repressor

c)       Inhibitor

d)      Suppressor

Explanation:

The correct answer is ‘b’ because when the tryptophan concentration in the cell increases due to rapid transcription and translation process, further synthesis is not required. In that situation, tryptophan attaches itself to the repressor protein and both collectively attaches to the operator sequence to switch off the process.  Since tryptophan attaches to the repressor protein to inhibit the process of transcription and translation – it is called co-repressor.

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The Concept of Lac Operon

In 1961, Jacob and Monad performed a number of experiments to find out the nature of induction of enzyme synthesis in E.coli. Of the 800 enzymes thought to be synthesized by the bacteria, some enzymes are synthesized continuously and are called CONSTITUTIVE ENZYMES; others are synthesized only in the presence of an inducer compound, which may not be the substrate and are called INDUCIBLE ENZYMES.

E.coli can grow tremendously on a culture medium containing glucose. However, when bacteria transferred to a medium containing lactose and not glucose, it will not grow immediately, but after a short span of time, it begins to show the same growth rate as seen on a glucose medium. Investigators have revealed that growth on the lactose medium required the presence of two substances not normally synthesized: Beta-galactosidase (which hydrolyses lactose to glucose and galactose), and lactose permease (which enables the cell to take up lactose).

LAC OPERON

The lac operon has to do with the ability of E.coli to utilize lactose sugar. Lactose is a disaccharide, which is made up of glucose and galactose. 

The enzyme present in E.coli that breaks down lactose is Beta-galactosidase

The lac operon is actively a series of adjacent genes and regulatory elements in one small part of the E.coli circular chromosome. 

Lac Operon Component:

 

I = Inducer/regulator gene – this gene is transcribed to make mRNA and then translated to a repressor protein or allosteric protein, which can either activate or suppress the operator gene. 

O = Operator – a short sequence of bases that acts like a switch that can be recognized by repressor protein.

Operon = a functioning unit of genomic DNA containing a cluster of genes under the control of a single regulatory signal or promoter. 

ZYA = structural genes

Z – Codes for β-galactosidase

Y –Codes for lactose permease, a protein that functions to actively bring lactose from outside the cell to the inside, even against a concentration gradient.

A – Codes for transacetylase, an enzyme that is also needed to breakdown many sugars related to lactose.

ð  All three genes that code for enzymes needed to use β-galactosidase molecules as a source of carbon and energy are adjacent to each other and are co-ordinately turned on or off. 

ð  Operons are found only in prokaryotes. In eukaryote, each gene has its own promoter and regulatory elements.

Mechanism of Lac Operon:-

Step: The promoter for the I gene is always ON, but is very weak so it is transcribed only rarely.

-          The I-mRNA is translated into a polypeptide -- 4 copies make one repressor protein. A typical cell will have only about 10 copies of this protein.

-          In the absence of lactose, the repressor protein binds to the operator preventing transcription from the second promoter. Almost no ZYA mRNA is made.

CASE I: When only lactose is present, the model works as follows –

When the concentration of lactose in the cell increases, it means, there is shortage of glucose. In such a condition, lactose binds to the repressor protein and inactive it so that the protein no longer could bind to the operator. When the repressor protein is not bound to the operator, Z, Y, and A genes are transcribed and translated. 

CASE II: When lactose is not present, the model works as follows:

When lactose is not present in the cell, it means, there is sufficient quantity of glucose in the cell. In such a condition, no further glucose synthesis is required. So, the repressor protein binds to the operator and blocks it. Therefore, no ZYA genes are transcribed or translated.

Note: In the first case when lactose is high in the cell, it promotes the genes to transcribe and translate. Therefore, lactose acts as an inducer. 

CASE III: When both glucose and lactose are present

If both glucose and lactose are present, cells use up the glucose before turning on the Lac Operon. When energy begins to become limiting, a signal moledule, cyclic adenosine mono phosphate (cAMP) builds up; it binds to a catabolic activating protein (CAP) and the complex in turn binds to a site between the promoter segments of the Lac Operon.

Binding of the cAMP/CAP complex opens the promoter for RNA polymerase binding.

Note:

-          When glucose level is high – cAMP is low

-          When glucose level is low – cAMP is high

 

 

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