The backbone of DNA is made up of alternating sugar (deoxyribose) and phosphate groups. The sugar and phosphate groups form a repeating pattern, with the sugar molecules connected to the phosphate groups by phosphodiester bonds. The sugar-phosphate backbone provides structural support to the DNA molecule and links the nucleotides together. The nitrogenous bases (adenine, thymine, cytosine, and guanine) are attached to the sugar molecules and form the "rungs" of the DNA ladder. The specific base pairing between the nitrogenous bases provides a way for DNA to make exact copies of itself.