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  4. Calculate the number of weak hydrogen bonds in a DNA strand 204 Å long with 15% thymine bases.

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Q. Calculate the number of weak hydrogen bonds in a DNA strand $204 \, Å$ long with 15% thymine bases.

NTA AbhyasNTA Abhyas 2020Molecular Basis of Inheritance

Solution:

DNA length of one turn is $34 \, Å$ . It means one turn contains 10 base pairs. So $34 \, Å=10$ base pairs.
Hence, $204 \, Å=204\times 10 / 34= \, 60$ base pairs. As given 15% is thymine $=15 / 100\times \, 60=9$ thymine.
Thymine pairs with adenine and hence adenine is also 9. So AT is total of 18. Total $GC=60-18=42$ .
Total hydrogen bonds $= \, 2x \, AT+3x \, GC$ (AT shows double bond and GC shows triple bond) $= \, 2 \, \times \, 18 \, + \, 3 \, \times \, 42=36+126=162.$