How does complementary base pairing occur during transcription?

During transcription, complementary base pairing occurs when RNA polymerase matches RNA nucleotides with the DNA template strand.

Transcription is a fundamental process in biology where the genetic information stored in the DNA is copied into RNA. This process is carried out by an enzyme called RNA polymerase. The RNA polymerase binds to a specific region of the DNA, known as the promoter, and begins to separate the two strands of the DNA molecule. This separation exposes the nucleotide bases on each strand, one of which serves as a template for the synthesis of an RNA molecule.

The RNA polymerase moves along the DNA template strand in a 3' to 5' direction, matching RNA nucleotides with the exposed DNA bases. This is where the principle of complementary base pairing comes into play. In DNA, adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). However, in RNA, the thymine is replaced by uracil (U). Therefore, during transcription, if the RNA polymerase encounters an adenine on the DNA template strand, it will pair it with a uracil in the RNA molecule. Similarly, a cytosine on the DNA will be paired with a guanine, a guanine with a cytosine, and a thymine with an adenine.

This process continues until the RNA polymerase reaches a sequence of DNA called the terminator, which signals the end of the gene. At this point, the RNA polymerase releases the newly formed RNA molecule and detaches from the DNA. The DNA molecule then zips back together, ready for the next round of transcription.

The resulting RNA molecule is a complementary copy of the DNA template strand, with uracil replacing thymine. This RNA molecule can then undergo further processing before it is used to synthesise proteins in a process called translation. Therefore, the complementary base pairing during transcription is a crucial step in the flow of genetic information from DNA to RNA to protein.