A DNA segment has a total of 1000 nucleotides, out of which 240 of them are adenine containing nucleotides. How many pyrimidine bases this DNA segment possesses?

Solution

According to Chargaff’s base pairing rule, The ratio of purines (A and G) to pyrimidines (C and T) is equal.

Thus, the number of adenine (A) will be equal to the number of thymine (T).

Therefore, if the number of A is 240, T will also be 240 as A = T

Thus A + T = 240 + 240 = 480

The number of cytosine (C) will be equal to the number of guanine (G).

Thus, G + C = Total number of nucleotides - Nucleotides containing A and T = 1000 - 480 = 520

Since G = C = 260 (520 divided by 2)

Thus, the number of pyrimidines that the segment possess = C + T
= 260 + 240 = 500

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