Transcription is best defined as a process by which?

Transcription is defined as the process by which RNA is synthesized from a DNA template. This process begins when the enzyme RNA polymerase binds to a specific region of the DNA called the promoter. Once bound, RNA polymerase unwinds the DNA and synthesizes a single-stranded RNA molecule by matching complementary RNA nucleotides to the DNA sequence. This process is crucial because it leads to the production of messenger RNA (mRNA), which carries genetic information from the DNA in the nucleus to the ribosomes in the cytoplasm, where protein synthesis occurs.

The other choices do not accurately describe transcription. For instance, a base pair substitution leading to a mutation pertains to genetic variation and alterations in the DNA sequence rather than the transcription process itself. The role of RNA in directing ribosome synthesis is more related to translation and the function of ribosomal RNA, not transcription. Additionally, the binding process described with DNA polymerase does not accurately represent the transcription phase since DNA polymerase is primarily involved in DNA replication, not transcription.