Question

How may H-bonds are formed between the two strands of dsDNA of $170A^o$ length if the cytosine is $20\%?$

Answer 1

Length of a base pair is 3.4 Å. Hence a DNA segment of length 170 Å has 170/3.4 = 50 base pairs.

It is given in question that 20% of DNA is cytosine. Cytosine forms triple hydrogen bond with guanine resulting in a base pair Hence, an equal percentage ( 20% ) of guanine is present too. Based on this it concluded that 40% of base pairs ( 50×40% = 20 base pairs ) are cytosine-guanine. Each of them has triple bond hence 20×3 = 60 hydrogen bonds.

While rest, 60% ( 50×60% = 30 base pairs ) are adenine-thymine. Each of them has a double bond hence 30×2 = 60 hydrogen bonds.

Hence this dsDNA segment has 60+60 = 120 hydrogen bonds.