Question

(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.

Answer 1

a) Adenine and guanine are purines.

Thymine and cytosine are pyrimidines.
According to Chargaff's rule, the total number of purine bases is always equal to the total number of pyrimidine bases.
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 + T nitrogenous bases
= 1000 − 480
= 520
Therefore, G =260, C = 260
Therefore, the number of pyrimidines that the segment possess = C + T
= 260 + 240
= 500

b) Diagrammatic sketch of a portion of DNA has been attached.