Transcription

A transcription unit contains two differently functioning DNA strands: the template strand, which is actually read by RNA polymerase, and the coding strand, which is not transcribed. The coding strand has a base sequence identical to the mRNA produced, except that thymine in DNA is replaced by uracil in RNA. This identity arises because the mRNA is complementary to the template strand, and the coding strand is itself complementary to the template, so mRNA and coding strand end up matching. The template strand is incorrect because it is the complementary strand actually used as the reading guide, having the opposite sequence to the mRNA. The lagging strand is a DNA-replication term describing discontinuous synthesis and has no role in defining transcription sequence identity. The antisense strand is another name for the template strand, so it too carries the complementary, not identical, sequence. As stated in NCERT Class 12, Chapter 6 (Molecular Basis of Inheritance), by convention the coding strand sequence is written to represent a gene. As a verification step, replacing every T with \cup in the coding strand reproduces the mRNA sequence exactly, confirming that the coding strand matches the transcript.