When the coding strand of DNA has the sequence of
5′-TACGTACGT-3′
then what will be the nucleotide sequence of transcribed mRNA?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 5′-UACGUACGU-3′
The strand of DNA that has the polarity 5′ to 3′ and the same sequence as the RNA (with thymine in place of uracil) is the coding strand. The other strand, the template strand, is the one that RNA polymerase actually reads. The mRNA is made 5′ to 3′, complementary to the template.
Coding strand 5′-TACGTACGT-3′
Template strand 3′-ATGCATGCA-5′
mRNA made on the template 5′-UACGUACGU-3′
So the mRNA is the coding strand with every T changed to U.
(1) 5′-ATGCATATC-3′ — Incorrect. It contains T, and RNA never has thymine. It does not match the coding strand either.
(2) 5′-UACGUACGU-3′ — Correct. It is the coding strand with U in place of T, written 5′ to 3′.
(3) 3′-TACGTATAU-5′ — Incorrect. One RNA cannot carry both T and U. It is also written 3′ to 5′, but mRNA is made in the 5′ to 3′ direction.
(4) 3′-UACGTATAG-5′ — Incorrect. It has both U and T, and its last bases do not match the coding strand.
NCERT Extract:
Transcription Unit – …
