Strand 1 is the template for mRNA synthesis in this question.

Understanding the question

The table shows base composition of two DNA strands and the mRNA:

• DNA strand 1: A 19.1%, G 26.0%, C 31.0%, T 23.9%

• DNA strand 2: A 24.2%, G 30.8%, C 25.7%, T 19.3%

• mRNA: A 19.0%, G 25.9%, C 30.8%, U 24.3%

In transcription, mRNA is complementary to the template strand and therefore has a sequence that matches the non‑template (coding) strand, except that mRNA contains U where DNA has T.​

Here, the mRNA base percentages match DNA strand 1 almost exactly for A, G and C, and T in strand 1 is replaced by nearly equal U in mRNA (23.9 vs 24.3). This means strand 1 has the same sequence as mRNA (except T/U), so strand 1 is the coding (non‑template) strand, and the other strand (strand 2) must be the template used by RNA polymerase.​

Therefore, the strand of DNA serving as template is strand 2.

Correct option: b. Strand 2

Explanation of each option

Option a: Strand 1

• If strand 1 were the template, mRNA would have complementary bases: where strand 1 has A, mRNA should have U; where strand 1 has T, mRNA should have A; C ↔ G and G ↔ C.​​

• The observed mRNA composition instead mirrors strand 1 (A≈19, G≈26, C≈31) with T replaced by U, which is the hallmark of a coding strand, not a template strand.​

• Hence, strand 1 is not the template; it is the coding or non‑template strand.

Option b: Strand 2 (correct)

• Because mRNA matches strand 1, strand 1 and mRNA are complementary to strand 2.​

• The template strand is always complementary to the mRNA, and the other strand (with same sequence as mRNA except T/U) is called the coding strand.​

• Thus, RNA polymerase must have used DNA strand 2 as the template for mRNA synthesis, making option b correct.

Option c: Both strands

• In cells, for a given transcript, only one DNA strand is used as the template at a time; the two strands are antiparallel and a single RNA polymerase tracks along one strand per gene.​

• Although different genes on the same DNA molecule can use opposite strands as templates, a single mRNA molecule arises from one specific template strand, not both simultaneously.​

• The data here clearly point to one coding-like strand (strand 1) and one complementary template (strand 2), so both strands cannot be templates for this mRNA.

Option d: Neither strand

• This would be possible only if the mRNA base composition did not correspond to either DNA strand.

• Because mRNA composition closely matches strand 1, with U replacing T, and thus is complementary to strand 2, at least one of the given strands must be involved in transcription.​

• Therefore, “neither strand” is inconsistent with the base composition data and is incorrect.

Brief introduction for SEO

In molecular biology and exam preparation, questions often ask which DNA strand serves as template for mRNA synthesis when given base composition or sequence data. Understanding the relationship between the DNA template strand, the coding strand, and the resulting mRNA is crucial for solving such MCQs accurately and quickly.​