Question

A geneticist finds the sequences of the coding strands in transcription units as follows:
(a) 5'-TGAACTGTAGCATGC-3'
(b) 5'-TGATACTGGTACATTC-3'
Find out the correct sequences of the mRNAs transcribed from them.

• A. (a) 5'-UGAACUGUAGCAUGC-3'
  (b) 5'-UGAUACUGGUACAUUC-3'
• B. (a) 3'-CGUACGAUGACAAGU-5'
  (b) 3'-TGATACTGGTACATTC-5'
• C. (a) 3'-CGUACGAUGUCAAGU-5'
  (b) 3’-TGATACUGGTACAUTC-5'
• D. (a) 5'-UGACUGUAGCUUGCC-3'
  (b) 5'-TGATACTGGTACATTC-3'

Answer 1

The correct option is A (a) 5'-UGAACUGUAGCAUGC-3'

(b) 5'-UGAUACUGGUACAUUC-3'
The genetic information from one strand of the DNA is copied into RNA during transcription. Hence, synthesis of RNA occurs during transcription.

A transcription unit in DNA consists of three regions: promoter, structural gene and terminator.

The promoter located towards 5'-end (upstream) and terminator located towards 3'-end (downstream) of the coding strand, flank the structural gene in a transcription unit.

In the transcription unit, the strand that has the polarity 3'→5' acts as a template for the synthesis of mRNA, and it is also referred to as a template strand.

The other strand that has the polarity 5'→3' is called a coding strand.

If the coding strand in a transcription unit is having the following sequence:
5'-TGAACTGTAGCATGC-3', then, it is known that the sequence of mRNA transcribed is the same as the coding strand of DNA.

However, in RNA, thymine is replaced by uracil.
Hence, the sequence of mRNA will be
5'-UGAACUGUAGCAUGC-3'.

Similarly, if the coding strand in a transcription unit is having the following sequence:
5'-TGATACTGGTACATTC-3', then the sequence of mRNA will be
5'-UGAUACUGGUACAUUC-3'.