Question

If the base sequence in DNA is 5’ AAAA 3’, then the base sequence in m-RNA
(a) 5’ UUUU 3’
(b) 3’ UUUU 5’
(c) 5’ AAAA 3’
(d) 3’ TTTT 5’

Answer 1

The formation of mRNA from DNA is known as transcription which is carried out by DNA-dependent RNA polymerase. This enzyme catalyzes transcription by arranging base pairs complementary to the template DNA strand.

Complete Answer:
The base sequence of mRNA 3’ UUUU 5’ is complementary to the DNA strand with the sequence 5’ AAAA 3’. In general, a purine pairs with a pyrimidine such as adenine (A) pairs with thymine (T) using 2 hydrogen bonds, and guanine (G) pairs with cytosine (C) using 3 hydrogen bonds. But in the case of RNA, thymine is replaced by uracil (U) to complement adenine.
And a complementary mRNA strand will have opposite polarity to that of the template DNA strand i.e. if the DNA strand is having 5’-3’ polarity then RNA will have 3’-5’ polarity.
Hence, the mRNA strand 3’ UUUU 5’ is complementary to 5’ AAAA 3’.

Additional information: Let us learn about transcription in detail.
Transcription takes place within a portion of DNA known as the transcription unit. This unit consists of:
(i) A Promoter : It is the region before the structural gene which provides the binding site for RNA polymerase.
(ii) The Structural gene : It is the region that has the genes to be transcribed into the mRNA.
(iii) A Terminator : It is the region located towards the end of the structural gene and has sequences to terminate transcription.
So, the correct option is ‘(b) 3’ UUUU 5’’.

Note:
- Replication occurs throughout the length of the DNA whereas transcription only takes place for a certain portion.
- In transcription, both the strands of DNA are not transcribed as it will lead to the formation of double-stranded RNA.
- Transcription is followed by the translation process in which amino acids are formed from mRNA.