Why DNA is Negatively Charged?
The deoxyribo nucleic acid or DNA is negatively charged and it moves in the solution towards the cathode which is a positively charged electrode. Cations move towards cathode and anion towards anode in electrophoresis. Cathode is negatively charged electrode and it has the tendency to attract positively charged ions called cations. Anode is positively charged electrode and it has the tendency to attract negatively charged ions.
DNA is an anion as it will get attracted towards the anode electrode. As it moves towards positive charge, DNA must be possessing negative charge. DNA consists of phosphate ions all over the strands. Each of the phosphate ions has negative charges and as the entire strand consists of many phosphate ions, negative charge is centered in DNA.
DNA is made up of nucleotides. Every nucleotide comprises of a nitrogen base, a pentose sugar molecule and a phosphate group. The adjacent nucleotides are attached between the oxygen atom of the phosphate group and the ribose sugar molecule. The negative charge on the phosphate group is stabilized as the charge is distributed all over the molecule.
As DNA is negatively charged, it gets attracted towards positive electrode or anode in electrophoresis. The oxygen atoms in the phosphate group are negatively charged. In solution, DNA forms a phosphoric ester where a H atom of phosphoric acid is replaced by a nitrogen base and sugar molecule. This phospho ester that is formed takes another water molecule and loses two hydrogen atoms. The resultant complex consists of 2 negative charges on the oxygen atom of phosphate. These negative charges accumulate on the DNA molecule to make it entirely negatively charged.
The phospho-di-ester bonds in the DNA stand as the reason for the presence of slightly negative charge on it. The phosphate groups in between the deoxyribose sugars in the chain of nucleotides impart acidity to the DNA and hence the molecule is called deoxyribo ‘nucleic acid’.