If nucleotides from the dna of a human were quantified and 30 percent of them consisted of adenine, what percentage of guanine nucleotides would be present?

Given: 30% of the DNA contains adenine, denoted by A.

To find: The percentage amount of guanine (denoted by G) in the DNA.

Method: We'll use the Chargaff's rule to solve the problem. Chargaff's rules state that DNA from any cell of a particular organism should have equal amounts of pyrimidines and purines. In short, guanine-cytosine will be present in equal amounts, and thymine-adenine in equal amounts.  

Solution:

Let the DNA be 100 base pair long.

Therefore, 30% of DNA is adenine which amounts to 30 A's.

Now adenine pairs up with thymine. So, 30 A's will pair with 30 T's.

This makes 60 A-T pairs in the DNA.

So we are left with 100 - 60 = 40 nucleotides. These nucleotides will contain G and C in equal numbers.

So the amount of G will be, the half of 40, which is 20.

Hence 20% of the DNA will contain guanine.

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