Assertion: The sugar phosphate backbone of two chains in DNA double helix show anti-parallel polarity.
Reason: The phosphodiester bonds in one strand go from a 3′ carbon of one nucleotide to a 5′ carbon of adjacent nucleotide, whereas those in complementary strand go vice versa.

Both Assertion (A) and reason (R) are correct and R is the correct explanation of A.

DNA is a genetic material which is made up of nucleotides having nitrogenous base with a phosphate group and a ribose sugar. ∙ The backbone of each DNA strand is made up of ribose sugar that are connected with the help of phosphodiester Bond. ∙ This phosphodiester bond is between the three carbon atoms of a sugar and 5 carbon atoms of another sugar molecule which forms an ester bond. ∙ The sugar phosphate backbone of the two chains in DNA double helix have antiparallel polarity because the direction of one strand is 5 to 3 while the other is arranged in 3 to 5 direction which is the and parallel strand. ∙ This anti parallel direction is because of the phosphodiester bond and the two strands are made in such a way that they are held together in antiparallel direction with the help of hydrogen bonds.