**Question:** An enzyme that makes a double stranded DNA copy from a single stranded RNA template molecule is known as

A. RNA polymerase
B. DNA polymerase
C. DNA ligase
D. DNA polymerase

**Correct Answer:** A. RNA polymerase

**Core Concept:** The process of transcribing a single-stranded RNA template into a complementary double-stranded DNA molecule is called transcription. In this process, RNA polymerase is the enzyme that plays a crucial role. There are three types of RNA polymerases:

**Why the Correct Answer is Right:** RNA polymerase is responsible for synthesizing RNA from DNA templates. In prokaryotes, there is one type of RNA polymerase (RNA polymerase II in eukaryotes) that catalyzes the synthesis of mRNA (messenger RNA), converting the single-stranded RNA template into a double-stranded RNA molecule.

**Why Each Wrong Option is Incorrect:**

B. DNA polymerase: This is an enzyme responsible for DNA replication, not transcription. Transcription involves converting RNA from DNA, not the reverse process.
C. DNA ligase: DNA ligase is involved in DNA repair and ligation processes, not transcription.
D. DNA polymerase: Similar to option B, this enzyme is responsible for DNA replication, not transcription.

**Clinical Pearl:** Understanding the difference between DNA replication and transcription is essential in the context of gene expression regulation, as transcription is the first step in protein synthesis. Knowledge of these enzymes aids in comprehending cellular processes such as mRNA synthesis, gene expression, and the role of transcription factors in controlling gene expression. This knowledge is crucial for medical students, residents, and practicing physicians alike to comprehend the intricacies of gene regulation and cellular processes.