Here's how it works:

* Nucleotides: DNA is made up of repeating units called nucleotides. Each nucleotide consists of a sugar (deoxyribose), a phosphate group, and a nitrogenous base (adenine, guanine, cytosine, or thymine).

* Sugar-phosphate backbone: The phosphate group of one nucleotide forms a covalent bond with the sugar of the next nucleotide. This bond is called a phosphodiester bond.

* Chain formation: This repeated bonding of phosphate to sugar creates a long chain, which is the backbone of the DNA molecule.

* Directionality: The DNA strand has a 5' end (where the phosphate group is exposed) and a 3' end (where the sugar group is exposed).

Therefore, phosphodiester bonds are responsible for the structural integrity and directionality of a DNA strand.