Class 12th Biology 2015 Set2 Delhi Board Paper Solution
(a) A DNA segment has a total of 1,500 nucleotides, out of which 410 are Guanine containing nucleotides. How many pyrimidine bases this 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, the ratio of purines to pyrimidines is equal.
Thus, the number of cytosine (C) will be equal to the number of guanine (G).
Number of guanine-containing nucleotides = 410
Thus, G = C = 410
Now, G + C = 410 + 410 = 820
The number of adenine (A) will be equal to the number of thymine (G).
Thus, A + T = Total number of nucleotides – Nucleotides containing G and C nitrogenous bases
= 1500 – 820 = 680
Therefore, A = 340 and T = 340
The number of adenine will be equal to number of thymine, which is 340.
Therefore, Number of pyrimidines that the segment possess = C + T
= 410 + 340 = 750
(b) A diagram of a portion of DNA segment is given below.
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