Question

The coding strand of DNA is: 5'-AATTCAAATAGG-3'
What is the sequence of mRNA?

• A. 3'-TTAAGTTTAATCC-5'
• B. 5'-AAUUCAAAUUAGG-3'
• C. 3'-AAUUCAAAUUAGG-5'
• D. 5'-TTAAGTTTAATCC-3'

Hint:

Remember that the mRNA sequence is almost identical to the DNA coding strand, with the only difference being that Thymine (T) in DNA is replaced by Uracil (U) in mRNA. Both strands are read in the 5' to 3' direction for this comparison.

Answer 1

The Correct Option is B

Step 1: Understand the relationship between the DNA coding strand and mRNA sequence.
In molecular biology, during the process of transcription, an mRNA molecule is synthesized from a DNA template. There are two strands in a DNA double helix:
Template strand (or Antisense strand): This strand serves as the direct template for mRNA synthesis. The mRNA sequence is complementary to this strand.
Coding strand (or Sense strand): This strand has a sequence that is nearly identical to the mRNA molecule that will be produced, except that DNA contains Thymine (T) where mRNA contains Uracil (U). It runs in the same 5' to 3' direction as the mRNA. Step 2: Apply the rule for converting the coding DNA strand to mRNA.
To derive the mRNA sequence from the coding DNA strand, you essentially copy the sequence, but substitute every Thymine (T) with Uracil (U). The directionality (5' to 3') remains the same. Given coding DNA strand: \text{5'-AATTCAAATAGG-3'} Let's convert each base:
A (DNA) $\rightarrow$ A (mRNA)
A (DNA) $\rightarrow$ A (mRNA)
T (DNA) $\rightarrow$ U (mRNA)
T (DNA) $\rightarrow$ U (mRNA)
C (DNA) $\rightarrow$ C (mRNA)
A (DNA) $\rightarrow$ A (mRNA)
A (DNA) $\rightarrow$ A (mRNA)
A (DNA) $\rightarrow$ A (mRNA)
T (DNA) $\rightarrow$ U (mRNA)
A (DNA) $\rightarrow$ A (mRNA)
G (DNA) $\rightarrow$ G (mRNA)
G (DNA) $\rightarrow$ G (mRNA) Step 3: Construct the final mRNA sequence.
Combining these conversions, the mRNA sequence will be: \text{5'-AAUUCAAAUUAGG-3'} Step 4: Compare with the given options.
(A) 3'-TTAAGTTTAATCC-5': This is the template DNA strand (complementary to the coding strand and in reverse direction).
(B) 5'-AAUUCAAAUUAGG-3': This matches our derived mRNA sequence.
(C) 3'-AAUUCAAAUUAGG-5': This sequence is in the incorrect 3' to 5' direction.
(D) 5'-TTAAGTTTAATCC-3': This is the template DNA strand, but in the 5' to 3' direction relative to itself (reverse of A), not an mRNA sequence. Therefore, option (B) is the correct sequence of mRNA.