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Question
If there are 120 adenine molecules in a DNA double helical structure showing 20 coils, what is the number of pyrimidine nucleotides forming three hydrogen bonds in it?
If there are 120 adenine molecules in a DNA double helical structure showing 20 coils, what is the number of pyrimidine nucleotides forming three hydrogen bonds in it?
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Solution
The correct option is A 80
DNA has 10 base pairs per turn (coil) of the helix.
If there are 20 coils, then the total number of base pairs is 200 and the number of nucleotides (bases) is 400.
Out of 400 bases, 120 are adenines and hence thymines will also be 120 as A=T (Chargaff's rule).
Hence, the total number of adenine and thymine bases will be 240.
The remaining 160 bases will be guanine and cytosine.
According to Chargaff's rule, G = C, so, the number of guanine bases will be 80 and cytosine bases will also be 80.
Adenine and guanine form a base pair consisting of two hydrogen bonds, whereas guanine and cytosine form a base pair with three hydrogen bonds.
Among guanine and cytosine, cytosine is the pyrimidine. Thus, there are 80 pyrimidine nucleotides which form 3 H-bonds.
DNA has 10 base pairs per turn (coil) of the helix.
If there are 20 coils, then the total number of base pairs is 200 and the number of nucleotides (bases) is 400.
Out of 400 bases, 120 are adenines and hence thymines will also be 120 as A=T (Chargaff's rule).
Hence, the total number of adenine and thymine bases will be 240.
The remaining 160 bases will be guanine and cytosine.
According to Chargaff's rule, G = C, so, the number of guanine bases will be 80 and cytosine bases will also be 80.
Adenine and guanine form a base pair consisting of two hydrogen bonds, whereas guanine and cytosine form a base pair with three hydrogen bonds.
Among guanine and cytosine, cytosine is the pyrimidine. Thus, there are 80 pyrimidine nucleotides which form 3 H-bonds.
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