Which option best describes the main purpose of RNA polymerase during transcription?

• A. It unwinds the DNA helix ahead of the replication fork to allow DNA copying.
• B. It translates mRNA codons into a polypeptide chain at the ribosome.
• C. It joins Okazaki fragments together to complete synthesis of the lagging DNA strand.
• D. It synthesizes an RNA strand by catalyzing the addition of ribonucleotides complementary to the DNA template strand.

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the process of transcription, which is the synthesis of RNA from a DNA template. This process is essential for gene expression.
2. Step 2: Identify the role of RNA polymerase, the enzyme responsible for reading the DNA template strand and synthesizing a complementary RNA strand.
3. Step 3: Recognize that RNA polymerase catalyzes the formation of phosphodiester bonds between ribonucleotides, adding them in the 5' to 3' direction complementary to the DNA template strand.
4. Step 4: Differentiate RNA polymerase's function from other enzymes involved in DNA replication and translation, such as helicase (unwinding DNA), DNA ligase (joining Okazaki fragments), and ribosomes (translating mRNA).
5. Step 5: Conclude that the main purpose of RNA polymerase during transcription is to synthesize an RNA strand by catalyzing the addition of ribonucleotides complementary to the DNA template strand.