Where are nucleotides attached during DNA replication?

During DNA replication, nucleotides are added to the growing DNA strand at the 3' hydroxyl group of the previously incorporated nucleotide. This process occurs because DNA polymerases, the enzymes responsible for synthesizing new DNA strands, can only add nucleotides to an existing 3' end.

When a nucleotide is added, its 5' phosphate group forms a covalent bond with the 3' hydroxyl group of the last nucleotide on the DNA strand, creating a phosphodiester bond and extending the chain. This directional synthesis means that the new strand is built in a 5' to 3' direction, with each additional nucleotide contributing to the existing strand's 3' end. This process is crucial for maintaining the integrity and sequence of the DNA during cell replication.