Why can RNA polymerase only add nucleotides in a 5' to 3' direction?

RNA polymerase synthesizes RNA by adding ribonucleotides to a growing chain, and the directionality of this process is crucial for proper RNA synthesis. The correct answer highlights that RNA polymerase can only attach to the 3' hydroxyl group of nucleotides. This is because the formation of a new phosphodiester bond linking adjacent nucleotides occurs when the 3' hydroxyl group of the last nucleotide in the chain reacts with the phosphate group of the incoming nucleotide.

This unique biochemical mechanism necessitates that RNA polymerase adds new nucleotides to the free 3' end of the RNA strand, effectively constructing the RNA molecule in the 5' to 3' direction. This directionality is fundamental to maintaining the integrity and functionality of the RNA molecule, allowing it to carry the genetic information properly and participate in subsequent processes, such as translation.

Understanding this process also clarifies why the other options do not accurately describe the behavior of RNA polymerase. The binding of RNA polymerase or any requirement for energy from a specific region does not dictate the direction of nucleotide addition. Similarly, while the structure of the nucleotides does play a role in the overall biochemical interactions, it is specifically the presence of the reactive