Question

A double stranded DNA has
20 turns with 60
nucleotides of Adenine What will be the no. of
Cytosine bases?
a 140 b) 60
c) 120
d) 1200​

Answer 1

Answer:

140 cytosine bases ,this is correct one

Answer 2

Answer:

Option (a) 140

Explanation:

• Erwin Chargaff formulated important generalizations about DNA structure. These generalizations are called Chargaff's rules which can be summarized as:

• The nitrogenous bases (Purines and pyrimidines) are always present in equal amounts. i.e. A+G = T+C

• On simplifying, it is → A = T and G = C i.e- the amount of Adenine is always equal to that of thymine, and the amount of Guanine is always equal to that of cytosine. i.e.- A=T and G=C and the base ratio A+T / G+C are constant for a given species.

• As we know, in one turn of DNA, 10 base pairs are present, which is equal to 20 nucleotides.

Hence, in 20 turns ⇒20 × 20 = 400 nucleotides.

• As the given data says, it has 60 Adenine nucleotides. which means out of 400 nucleotides,60 are adenine.

• Now using the above mentioned Chargaff's rule,

the amount of Thymine can be calculated as A= 60,⇒T= 60.

• So total remaining = 400 - (A+T) ⇒400-(60+60)⇒400-120= 280 nucleotides

So G+C = 280 nucleotides

• Now the number of Cytosine bases will be '280÷2= 140'

Hence option (a) is the correct answer.