James watson and francis crick are the scientists who discovered

three dimensional double helical model for DNA.

Explanation:

In 1953, James Watson and Francis Crick deduced the double-helical structure of DNA which in turn described the structural basis for DNA replication. They proposed the three dimensional model for DNA according to which a DNA molecule consists of two helical DNA chains that are wound around the same axis to form a double helix. The alternating deoxyribose and phosphate groups make the hydrophilic backbones present outside of the helix towards the surrounding watery medium.

The purine and pyrimidine bases are stacked inside the double helix to keep their hydrophobic structure away from the water. According to the Watson and Crick model, base pairing occurs between guanine and cytosine and thymine and adenine by hydrogen bonding. They proposed that the two DNA strands of a double helix are anti-parallel with respect to the polarity.