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.