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  4. In a double-stranded DNA, one of the nitrogen base is adenine, whose amount is found to be always equal to the amount of

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Q. In a double-stranded DNA, one of the nitrogen base is adenine, whose amount is found to be always equal to the amount of

NTA AbhyasNTA Abhyas 2022

Solution:

The nitrogen bases in both strands of DNA are attached via hydrogen bonding forming base pairs. Adenine makes two hydrogen bonds with Thymine (5-methyl uracil) from the opposite strand and vice-versa. Similarly, guanine is bound to cytosine with three H-bonds. Hence, as a rule, pyrimidine will always bind to a purine. Erwin Chargaff gave the observation that for a double-stranded DNA, the ratios between Adenine and thymine and guanine and cytosine are constant and equals. Thus, the amount of adenine and thymine is the same and that of guanine and cytosine is the same.