What is the first step of gene expression where a segment of DNA is copied into RNA?

The first step of gene expression involves the process known as transcription. During transcription, a specific segment of DNA is utilized as a template to synthesize RNA. This process is initiated when the enzyme RNA polymerase binds to the promoter region of the gene on the DNA. Once bound, RNA polymerase unwinds the DNA strands and begins to synthesize a complementary RNA strand by adding ribonucleotides that are complementary to the DNA template.

Transcription is fundamental because it generates the RNA molecules necessary for the subsequent steps of gene expression, including translation, where the information in the RNA is used to build proteins. This distinction between transcription and other processes is crucial: while replication refers to the duplication of DNA molecules, and processing involves modifications to RNA after transcription, transcription itself is specifically the step that converts genetic information from DNA to RNA.