Question

A DNA sample is $13.6nm$ long. It has 10% cytosine molecules calculate the number of weak hydrogen bonds in the given sample.

• A. $88$
• B. $112$
• C. $80$
• D. $120$

Answer 1

Answer: (A)

Length of DNA sample $=13.6nm$ . Length of $1turn=3.4nm$ . Thus No. of turns in the sample $=\frac{13.6nm}{3.4nm}=4$
No. of nucleotides in 1 turn $=20$ . Thus, No. of nucleotides in the given sample $=20\times 4=80$
Since cytosine is 10%, By Chargaff's rule, $G=C=10\%$ while $A=T=40\%$
No. of Guanine molecules $=\frac{1}{1}x8=8$ . No. of Cytosine $=8$
No. of Adenine $=\frac{40}{100}\times 80=32$ . No. of Thymine $=32$
Thus, no. of horizontal steps with GC linkage $=8$ and No. of horizontal steps with AT linkage $=32$
GC are linked by 3 weak hydrogen bonds. Thus no. of weak hydrogen bonds due to GC linkage $=8\times 3=24$
AT are linked by 2 weak hydrogen bonds. Thus no. of weak hydrogen bonds due to AT linkage $=32\times 2=64$
Total no. of weak hydrogen bonds in the given sample $=24+64=88$