Question

A DNA double helix is $340nm$ long. The number of nucleotides in this DNA is

• A. $2000$
• B. $1000$
• C. $200$
• D. $20$

Answer 1

The correct option is A $2000$

One complete turn of the double helix is 3.4 nm long and each of which has 10 base pairs. A nucleotide is made up of sugar, the nitrogenous base and phosphate moiety and each nucleotide base of one strand is paired in the same plane with a base of the other strand. Therefore, there are total 10 x 2= 20 nucleotides in one complete turn of the double helix which is 3.4 nm long. Hence a DNA strand of 340 nm would have 20/3.4 $\times$ 340 = 2000 nucleotides. This makes option A correct.