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# Class 12th Biology 2015 Set1 Delhi Board Paper Solution

Question 14

## (a) 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?(b) Draw a diagrammatic sketch of a portion of DNA segment to support your answer.

(a) Cytosine and thymine are pyrimidines.

According to Chargaff's rule, ratio of purines to pyrimidines is equal.

Thus, the number of adenine (A) will be equal to the number of thymine (T). Number of adenine (A) containing nucleotides = 240

Thus, A = T = 240

Therefore, 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 nitrogenous bases

= 1000 - 480 = 520

Therefore, G =260, C = 260

Number of guanine will be equal to number of cytosine, which will be 260.

Therefore, the number of pyrimidines that the segment possess

= C + T

= 260 + 240 = 500

(b) A diagrammatic sketch of a portion of DNA segment is given below: 