 Dear students, in this module I am going to introduce to you two new types of molecules and these are nucleosides and nucleotides. So a molecule containing all these three components that is a pentose sugar, a nitrogenous base and a phosphoric acid is termed as a nucleotide while a molecule without a phosphate group is termed as a nucleoside or in other words you can say a nucleotide molecule with its phosphate group removed is called a nucleoside. So in this picture you can see this is a pentose sugar and this pentose is in its furanose configuration. So when a nitrogenous base is attached to this pentose it becomes a nucleoside and to this nucleoside if a phosphate group is attached it becomes a nucleotide. So this molecule is a nucleotide with all its three components. Now let us see how these three components are joined together. So as you can see here this is a one prime carbon and at this one prime carbon an OH group is present. So this nitrogen of the nitrogenous base this attaches with this one prime carbon of the pentose sugar. So this is nitrogen number 9 and this nitrogen contains a hydrogen atom. So the hydrogen of the nitrogenous base and the hydroxyl of the pentose sugar these two combine and with the elimination of a water molecule these two groups that is a pentose sugar and a nitrogenous base these are attached and what this attachment is called? This attachment is called N-beta glycosyl linkage. Why N-beta glycosyl linkage? Because this is a covalent interaction covalent bond and in this covalent bond the nitrogen of the nitrogenous base is included and this pentose this pentose this is in its beta configuration. So that's why the name to this covalent bond N-beta glycosyl is given. So all the purines if this nitrogenous base is a purine it will be attached from its nitrogen 9 to the one prime of pentose and if this nitrogenous base is a pyrimidine and as you know pyrimidine is just like this molecule and here it is nitrogen number one. So if this nitrogenous base is a pyrimidine it is attached from nitrogen one with the one prime of the pentose. Now come to the phosphoric acid how this phosphate this phosphate group is attached with this pentose sugar. So this you can see this linkage this linkage is called an ester linkage ester how this ester is formed ester is formed when a phosphoric acid or any other organic acid is attached with hydroxyl or alcohol. So this is this is 5 prime of pentose and at this 5 prime a hydroxyl group is present to this hydroxyl group the hydroxyl group of phosphoric acid these are attached. When these two hydroxyl groups are attached they eliminate a water molecule and with the elimination of this water molecule a bond is established and this bond is called ester bond. So in this way all three components are joined to make nucleotide. So simply you can say in other words a nucleotide is equal to nucleoside plus phosphoric acid while a nucleoside is equal to a nucleotide minus phosphoric acid.