During the process of transcription, complementary base pairing takes place as RNA polymerase aligns RNA nucleotides with the DNA template strand.

Transcription is a vital biological process in which the genetic information encoded in DNA is transcribed into RNA. This process is facilitated by an enzyme known as RNA polymerase. The RNA polymerase binds to a specific region of the DNA called the promoter and initiates the separation of the two strands of the DNA helix. This separation reveals the nucleotide bases on each strand, with one strand serving as the template for synthesizing an RNA molecule.

RNA polymerase moves along the DNA template strand in a 3’ to 5’ direction, matching RNA nucleotides to the exposed DNA bases through the principle of complementary base pairing. In DNA, adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). However, during transcription, thymine is replaced by uracil (U) in RNA. Consequently, if RNA polymerase encounters an adenine on the DNA template strand, it pairs it with uracil in the growing RNA molecule. Similarly, cytosine pairs with guanine, guanine with cytosine, and thymine with adenine.

This transcription process continues until RNA polymerase reaches a specific sequence of DNA known as the terminator, which signals the end of the gene. At this stage, RNA polymerase releases the newly synthesized RNA molecule and detaches from the DNA. The DNA strands then re-anneal, returning to their double-helix structure and preparing for potential future rounds of transcription.

The resulting RNA molecule is a complementary copy of the DNA template strand, with uracil substituting for thymine. This RNA can undergo further processing before being utilized in protein synthesis, a process referred to as translation. Thus, the complementary base pairing that occurs during transcription is a crucial step in the flow of genetic information from DNA to RNA and ultimately to protein.