DNA polymerases in all organisms add only 5' nucleotides to the 3' end of a growing DNA strand, never to the 5' end. One possible reason for this is the fact that most DNA polymerases have a proofreading function that would not be energetically possible if DNA synthesis occurred in the 3' to 5' direction. Sketch the reaction that DNA polymerase would have to catalyze if DNA synthesis occurred in the 3' to 5' direction.

Key Concepts

DNA Polymerase Function

DNA polymerases are enzymes responsible for synthesizing new DNA strands by adding nucleotides to the 3' end of a growing chain. This unidirectional synthesis is crucial for maintaining the integrity of the genetic code and ensuring accurate replication. The enzyme catalyzes the formation of phosphodiester bonds between nucleotides, which is essential for DNA strand elongation.

Directionality of DNA Synthesis

DNA synthesis occurs in a 5' to 3' direction, meaning that nucleotides are added to the 3' hydroxyl group of the last nucleotide in the chain. This directionality is fundamental to the structure of DNA and the mechanisms of replication and transcription. If synthesis were to occur in the 3' to 5' direction, it would disrupt the established biochemical pathways and the energetics of nucleotide addition.

Proofreading Mechanism

Many DNA polymerases possess a proofreading function that allows them to correct errors during DNA synthesis. This function relies on the enzyme's ability to detect mismatched nucleotides and excise them before continuing synthesis. If DNA synthesis occurred in the 3' to 5' direction, the proofreading mechanism would be less efficient and energetically unfavorable, potentially leading to increased mutation rates.