Question

A novel species with double stranded genetic material consists of 5 bases namely P, Q, R, S, T with percentages given below.

| P| Q| R| S| T
---|---|---|---|---|---
Percentage| 22| 28| 22| 12| 16

Based on the above information, which ONE of the following inferences is NOT supported by the observation?
A. S base pairs with T and Q base pairs with R
B. S base pairs with Q and T base pairs with Q
C. P base pairs with R and S base pairs with Q
D. P base pairs with R and T base pairs with Q

Answer 1

Base pairing in DNA occurs between the complementary base pairs- A & T, C & G, to form units called base pairs. The amount of adenine base is approximately similar to the amount of thymine base and the amount of cytosine is the same as the amount of guanine base. In other words, we can say that A=T and G=C. This relationship is known as Chargaff’s rule.

Complete answer: The model of DNA as proposed by Watson and Crick suggested that DNA is made up of two strands that are twisted around each other to form a right-handed helix. There are two forces that stabilize the DNA double helix. They are – base-stacking interactions and paired bases in opposite strands forming hydrogen bonds.
According to Chargaff's rules, any cell contains a 1:1 ratio between pyrimidine and purine bases in its DNA. According to it, the number of adenine bases is always equal to thymine and the amount of guanine should be equal to cytosine and.
In the novel organism, we find five bases named P, Q, R, S, and T. According to the Chargaff’s rules; we get:
The same percentage forming bases, i.e., P and R must bond with each other. The percentage of Q is equal to the total percentage of S and T and hence Q can bind to both S and T. So the correct answer is ' S base pairs with T and Q base pairs with R'.

Note:
According to Chargaff’s rules; purines consist of A and G and T and C constitute the pyrimidines. The base-pairing rule or the rule of base equivalence which states that only one purine can combine with one pyrimidine is given by Chargaff’s rule. Thus it signifies that G forms bonds with C and A combines with T.