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?

(a) Cytosine and thymine are pyrimidines.

As indicated by Chargaff's standard, proportion of purines to pyrimidines is equivalent.

Hence, the quantity of adenine (A) will be equivalent to the quantity of thymine (T). Number of adenine (A) containing nucleotides = 240

In this manner, A = T = 240

In this manner, A + T = 240 + 240 = 480

The quantity of cytosine (C) will be equivalent to the quantity of guanine (G).

In this way, G + C = Total number of nucleotides - Nucleotides containing An and T nitrogenous bases

= 1000 - 480 = 520

In this way, G =260, C = 260

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

In this way, the number of pyrimidines that the portion has

= C + T

= 260 + 240 = 500