Molecular biology basics (gene expression): Which enzyme synthesizes RNA by using a DNA template and direction provided by the DNA sequence (promoter to terminator)?

Introduction / Context:
Transcription is the first stage of gene expression, where the information encoded in DNA is copied into RNA. The central enzyme that performs this task is RNA polymerase, which reads the DNA template strand and builds a complementary RNA strand. Understanding which enzyme is responsible is foundational for microbiology, genetics, and biotechnology.

Given Data / Assumptions:

• The process in question is synthesis of RNA directed by a DNA template.
• Promoters and terminators on DNA guide where transcription starts and stops.
• We are considering the core catalytic activity, not accessory factors.

Concept / Approach:
RNA polymerase binds to promoter sequences (often with help from sigma factors in bacteria or general transcription factors in eukaryotes), locally unwinds the DNA, and catalyzes the formation of phosphodiester bonds as ribonucleoside triphosphates are added to the growing RNA chain. The enzyme moves 3' to 5' along the DNA template while the RNA product grows 5' to 3'. Termination mechanisms then release the finished RNA.

Step-by-Step Solution:
Identify the biochemical transformation: DNA template → RNA product. Recall the enzyme class that catalyzes RNA synthesis: RNA polymerase. Exclude DNA polymerase (uses DNA template to make DNA, not RNA) and ligases (join breaks in nucleic acids). Select RNA polymerase as the only enzyme that fits the definition.

Verification / Alternative check:
In vitro transcription reactions require DNA template, ribonucleoside triphosphates, appropriate ions, and RNA polymerase to produce RNA. Removing RNA polymerase abolishes RNA synthesis, confirming the enzyme's essential role.

Why Other Options Are Wrong:

• DNA ligase: Seals nicks in DNA; does not synthesize RNA.
• DNA polymerase: Extends DNA, not RNA.
• RNA ligase: Joins RNA fragments; not template-directed synthesis from DNA.
• Reverse transcriptase: Uses RNA to synthesize DNA, the opposite direction.

Common Pitfalls:
Confusing transcription (DNA → RNA) with replication (DNA → DNA) or reverse transcription (RNA → DNA). Also, accessory factors aid binding and regulation but do not replace RNA polymerase's catalytic function.

Final Answer:
RNA polymerase.