The sequence of nitrogen bases of the template strand of DNA in a transcription unit is 3'-ATTGAACTG-5'. The sequence of nitrogen bases in its mRNA transcript would be:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 5'-UAACUUGAC-3'
mRNA is made complementary to the template strand and runs in the opposite direction, with U in place of T.
Template: 3'-A T T G A A C T G-5'
mRNA: 5'-U A A C U U G A C-3'
Option (1) 3'-AUUGAACAG-5' — Incorrect. It copies the template instead of pairing with it.
Option (2) 5'-UAACUUGAC-3' — Correct. mRNA is made complementary to the template strand and antiparallel to it, with U in place of T. Against 3′-ATTGAACTG-5′ this gives 5′-UAACUUGAC-3′.
Option (3) 3'-CAGUUCAAU-5' — Incorrect. This is the right sequence written backwards with the wrong ends.
Option (4) 5'-GUUAGUCCA-3' — Incorrect. It does not pair with the template.
NCERT Extract:
Transcription Unit and the Gene – “… Since the two strands have opposite polarity and the DNA-dependent RNA polymerase also catalyse the polymerisation in only one direction, that is, 5'→3', the strand that has the polarity 3'→5' acts as a template, and is also referred to as template strand. The other strand which has the polarity (5'→3') and the sequence same as RNA (except thymine at the place of uracil), is displaced during transcription. Strangely, this strand (which does not code for anything) is referred to as coding strand.”