Question

If total length of DNA molecule is $85\stackrel{\circ }{A}$ and it is having 20% of adenine. Then the number of $H-$bonds and $C-N$ rings present in it respectively will be?

• A. $65,75$
• B. $65,50$
• C. $75,75$
• D. $65,65$

Answer 1

The correct option is B $65,50$

A number of nucleotides present in one strand of DNA molecule of 85 Å are 85/3.4 = 25, out of which 20 are Adenine. An equal number of thymine must be present too, which forms a double bond with Adenine. While rest 5 nucleotides in one strand are either guanine or cytosine forming a base pair by a triple bond with another guanine/ cytosine from another strand. So, the total number of hydrogen bonds present are 20×2 + 5×3 = 65.

The two-carbon nitrogen ring bases (adenine and guanine) are purines, while the one-carbon nitrogen ring bases (thymine and cytosine) are pyrimidines. Hence, 20+20+5+5 = 50 C - N rings are present.

So, the correct option in '65, 50'.