 Let us come to the types of the deoxyribonucleotides. So as you have seen in a nucleotide molecule, there are three components and these are phosphoric acid, pentose sugar and a nitrogenous base. So if you see in these three components, two components are same in all the deoxyribonucleotides and these are phosphoric acid and pentose sugar. The third component, a nitrogenous base, it varies. So on the basis of type of nitrogenous base, there are four types of deoxyribonucleotides which are present in a DNA molecule. As you can see in this picture, so this molecule, a colored portion, this is a nucleoside because it contains a pentose sugar and a nitrogenous base. And this nitrogenous base is adenine. If this nitrogenous base is adenine, the adenosine will be called as deoxyadenosine. And if this phosphate group is also included in this molecule, it becomes a nucleotide and this nucleotide which contains adenine as its nitrogenous base, it is called deoxyadenylate. If you see this picture next, in this picture, this colored portion again, this is a nucleoside and in this nucleoside, this nitrogenous base is thymine. So this nucleoside is called deoxythymidine. So if this phosphoric acid is also included in this molecule, it will become nucleotide and this nucleotide is called deoxythymidylate. So these are two types of deoxyribonucleotides. These are the other two types of deoxyribonucleotides because only four types of nitrogenous bases are present in the DNA molecule and these are adenine, guanine, cytosine and thymine. So there are only four types of deoxyribonucleotides. So in this third type of deoxyribonucleotide, this colored portion, this colored portion contains pentose sugar and this nitrogen base is guanine. So this nucleoside is called deoxyguanosine and if this nucleoside also contains a phosphate group, it becomes a nucleotide and this nucleotide is deoxyguanilate. In the fourth type of deoxyribonucleotide, if the nitrogenous base is cytosine, the compound, this nucleotide, whole nucleotide is called deoxycytidylate and the only nucleoside, this nucleoside is called deoxycytidine. So these are four types of deoxyribonucleotides which are present in a DNA molecule. So these four deoxyribonucleotides, they make the structural units of a DNA molecule.