Question

The length of a fragment of B-DNA is 3400Å. On molecular analysis, it is found that adenine constitutes about 30% of total nitrogenous bases. What will be the total number of hydrogen bonds in this fragment of DNA?

• A. $1200$
• B. $2400$
• C. $1800$
• D. $600$

Answer 1

The correct option is B $2400$

Total length of B-DNA fragment $=3400\dot{A}$

So, number of pitches = $\frac{3400A}{34A}=100$

Each pitch has 10 base pairs

So, total number of base pairs

$=100\times 10=1000$

So, total number of nitrogenous bases

$=1000\times 2=2000$

Number of adenine given is = 30%

$=\frac{2000\times 30}{100}=600$

$A=T=600$

Two hydrogen bonds are present between adenine and thymine.

So, total number of hydrogen bonds

$600(A=T)\times 2=1200$ hydrogen bonds

Total number of G ≡ C content

$=2000–1200=800$

So, $G=C=400$

Three hydrogen bonds are present between G and C.
Hydrogen bonds =$400\times 3=1200$

So, total number of hydrogen bond

$=1200+1200=2400$.