In transcription, the DNA template strand serves as a guide for the synthesis of messenger RNA (mRNA) by complementary base pairing with ribonucleotides. During this process, specific nitrogenous bases on the DNA strand pair with their complementary bases on the incoming ribonucleotides. Here are the base-pairing rules during transcription:

- Adenine (A) on the DNA template strand pairs with Uracil (U) on the RNA strand.

- Thymine (T) on the DNA template strand remains unpaired, as there is no corresponding base in RNA.

- Cytosine (C) on the DNA template strand pairs with Guanine (G) on the RNA strand.

- Guanine (G) on the DNA template strand pairs with Cytosine (C) on the RNA strand.

During transcription, an RNA polymerase enzyme reads the DNA template strand in a 5' to 3' direction and synthesizes a complementary RNA molecule by sequentially adding ribonucleotides in the 5' to 3' direction. This base-pairing mechanism ensures that the RNA transcript is an accurate copy of the DNA template, except for the presence of Uracil (U) in place of Thymine (T), making it suitable for translation into proteins.