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.
Consider the information in your sketch and speculate as to why proofreading would be problematic.

Verified step by step guidance

1. Understand the directionality of DNA synthesis: DNA polymerases synthesize DNA in the 5' to 3' direction by adding nucleotides to the 3' hydroxyl (-OH) group of the growing strand. This directionality is crucial for the mechanism of DNA replication.
2. Recognize the role of proofreading: DNA polymerases have a proofreading function, which involves the removal of incorrectly paired nucleotides using their 3' to 5' exonuclease activity. This ensures high fidelity during DNA replication.
3. Consider the energy requirements for proofreading: In the 5' to 3' synthesis direction, the incoming nucleotide provides the energy for bond formation via its triphosphate group. If synthesis occurred in the 3' to 5' direction, the energy source for proofreading would be unavailable because the triphosphate group would already be part of the growing strand.
4. Speculate on the consequences of 3' to 5' synthesis: If DNA synthesis occurred in the 3' to 5' direction, removing an incorrect nucleotide during proofreading would leave the strand without a triphosphate group at the 3' end. This would prevent the addition of new nucleotides, halting DNA synthesis entirely.
5. Conclude why 5' to 3' synthesis is advantageous: The 5' to 3' direction ensures that proofreading can occur without disrupting the energy supply for continued synthesis, maintaining the efficiency and accuracy of DNA replication.

Key Concepts

DNA Polymerase Function

DNA polymerases are enzymes responsible for synthesizing new DNA strands by adding nucleotides to the growing chain. They can only add nucleotides to the 3' end of the strand, which means that DNA synthesis occurs in a 5' to 3' direction. This unidirectional synthesis is crucial for maintaining the integrity and accuracy of DNA replication.

Proofreading Mechanism

Proofreading is a critical function of DNA polymerases that allows them to correct errors during DNA synthesis. When an incorrect nucleotide is added, the enzyme can detect the mismatch and remove the erroneous nucleotide before continuing synthesis. This mechanism relies on the enzyme's ability to reverse its direction temporarily, which would be energetically inefficient if DNA synthesis occurred in the 3' to 5' direction.

Energetics of DNA Synthesis

The energetics of DNA synthesis refers to the energy requirements for adding nucleotides to a growing DNA strand. The process is driven by the hydrolysis of nucleoside triphosphates (NTPs), which release energy when incorporated into the DNA. If DNA polymerases were to synthesize DNA in the 3' to 5' direction, it would complicate the energy dynamics and potentially hinder the proofreading process, making it less efficient.