Since this is a molecular genetics course, I want to take a little time to review DNA structure.
You can find most of this material in chapter 6 of your text.

If we take a nucleic acid sample and hydrolyse it to its components in acid,
we obtain three classes of compound.

The phosphoric acid links adjacent sugars via a phospho-diester linkage.

The sugar component of nucleic acids consists of two related sugar moities:

beta-D-ribofuranose, found in Ribonucleic acids (RNA),
and
beta-D-2-deoxy-ribofuranose, found in Deoxyribonucleic acids (DNA).

Notice that the two sugars are identical save for the lack of the 2' OH group on the deoxy-ribofuranose.

This simple change is responsible for the differential stability of DNA and RNA in the presence of base.

The presence of two OH groups on ribofuranose
allows the cyclization of the phosphate which links adjacent sugars,
resulting in breaking of the linear polymer
(see figure below).