Explanation

DNA is helically coiled macromolecule made up to two antiparallel polydeoxyribonucleotide chains held together by hydrogen bonds. One turn of spiral has a distance of 34Å. It contains 10 nucleotides in each chain so that the distance between adjacent nucleotides is 3.4Å. Hence, the length of DNA having 23 base pairs is 3.4×23=78.2Å.