The process that converts DNA into mRNA is called transcription. During transcription, the enzyme RNA polymerase binds to a specific region of the DNA and unwinds the double helix. It then synthesizes a single strand of mRNA using one of the DNA strands as a template.
RNA polymerase reads the DNA sequence and assembles nucleotides to form the complementary mRNA strand. In this sense, adenine (A) in DNA pairs with uracil (U) in RNA instead of thymine (T), which is found in DNA. This is a crucial step in the central dogma of molecular biology, where genetic information flows from DNA to RNA and then to proteins.