Now, this entire chain, as we can see, basically consists of these repeating units that are composed of a phosphate group and a sugar. So we have phosphate sugar, phosphate sugar, phosphate sugar. And this continues until that nucleic acid ends. And this is what we call the backbone of that sugar, of that nucleic acid. So the backbone of that nucleic acid consists of a chain of repeating sugar phosphate units as shown in the following diagram. And by the way, these are the bases, the third component of a nucleotide attached to this carbon number one of each one of these sugars.