 DNA is a polymer of deoxyribonucleotides or you can say deoxyribonucleotides are the building blocks of a DNA molecule. What is a deoxyribonucleotide? Deoxyribonucleotide is composed of three components and these are deoxyribose, nitrogenous base and a phosphoric acid. So first component deoxyribose. Deoxyribose sugar is a pentose sugar derivative. It is derived from ribose sugar. Here you can say this is ribose sugar and look at the numbering. Although this sugar is numbered conventionally but only a prime designation is added. So this is one prime, two prime, three prime, four prime and five prime carbons. So if this is ribose so if at two prime one oxygen is removed this ribose sugar becomes deoxyribose. So at two prime position one oxygen is missing. So this is deoxyribose sugar. So this is one component of deoxyribonucleotide. The second component is the nitrogenous base. Four different types of nitrogenous bases are present in a DNA molecule and these four nitrogenous bases they are derived from two parent compounds and these two parent compounds are purines and pyrimidine. These two are parent compounds and the rest the all the nitrogenous bases four nitrogen bases are derived from these two compounds. So these two adenine and goanine these two are purines. So look at the pictures there are only minor differences for example different slides here and here. So these adenine and goanine are derivatives of a parent compound purine. Look at the numbering again they are conventionally numbered but here no prime designation is given to distinguish this from the numbered atoms in the pentose sugar. So next two are pyrimidines and these are cytosine and thymine. Again these two are slightly different from each other for example differences here and here. So these two are derivatives of pyrimidines. The third component present in a deoxyribonucleotide is phosphoric acid. Phosphoric acid has three hydroxyl groups so it can form three bonds with other molecules. So all these three components that is a deoxyribose sugar, a nitrogenous base and a phosphoric acid they combine together and they make a deoxyribonucleotide which is the building block of the DNA molecule.