 So, as you have seen, a nucleotide is made up of three components, but a DNA molecule is a polymer of these deoxyribonucleotides. Then next question arises, how different deoxyribonucleotides are joined together to make a polymer of DNA, poly nucleotide. So, the successive nucleotides of DNA are joined together through phosphodiester linkage. How this phosphodiester linkage is formed and what type of linkage is this? Let us see in this picture. So, in this picture you can see these two deoxyribonucleotides, how they are joined. This is a deoxyribonucleotide and look at this. This phosphate group is attached to the 5-prime carbon of pentose sugar and this phosphate is next attached to the 3-prime carbon of the next deoxyribonucleotide. So, a hydroxyl group is present at this 3-prime carbon, similarly another hydroxyl group is present on this phosphoric acid. So, these two hydroxyl groups, they combine together and a water molecule is eliminated and an astrolinkage is established. Again, this is an astrolinkage because you know this is an organic acid and this is alcoholic group and when an organic acid and an alcoholic group react, they form an astrolinkage. So, this astrolinkage is established between pentose of one nucleotide and phosphoric acid of the other nucleotide. So, one nucleotide is attached with this astrolinkage with the other nucleotide. Then why this linkage is called phosphodiastrolinkage? Because this is one astrolinkage and this is another astrolinkage. So two astrodes and a phosphate group are included in this linkage, in this whole linkage. So, that's why this linkage is called phosphodiastrolinkage. So, all these successive nucleotides are linked through this phosphodiastrolinkage. So, this molecule, this whole molecule, it is a dinucleotide. So, in this dinucleotide, you can see this phosphate, which is attached at 5 prime, is again free to form astrolinkage with another nucleotide. Similarly, this hydroxyl, which is present at 3 prime, this is again available to make an astrolinkage with another nucleotide. So, in this way, this chain grows and it make a polynucleotide chain. So, look at this picture. This is a polynucleotide chain and these, in this picture, the nucleotides are linked together through phosphodiastrolinkages. So, if a chain containing 50 or lesser than 50 deoxyribonucleotides, it is called polygonucleotide chain and if this chain contains more than 50 deoxyribonucleotides, this is called a polynucleotide chain. So, as you know, DNA is a polynucleotide chain. So, in this way, a polynucleotide chain is formed.