When a new nucleotide is added to a growing DNA strand, a new nucleotide is covalently linked to the 3' end of the existing strand.

• The incoming nucleotide forms a phosphodiester bond with the 3' hydroxyl group of the last nucleotide in the chain.
• This creates a continuous sugar-phosphate backbone, extending the DNA strand in the 5' to 3' direction.

In short: a nucleotide is attached to the 3' end of the existing strand.