What is Chargaff's Rule of DNA?

Erwin Chargaff was an Australian biochemist who analyzed nucleic acids of many organisms. From his research, he concluded that the number of purine bases (A, G) equals the pyrimidine bases (C, T) in any double stranded DNA of an organism.

It means,

•  A+G / T+C = 1; 
• A+T / C+G may vary from organism to organism.
• % A = % T, and % G = % C

Based on Chargaff's rule, many questions are asked in various competitive exams including AIPMT, CSIR NET, GATE, etc.

Q1. One of the strands of a double stranded DNA has the following number of bases:
A= 3800; T= 2600. What would be the base composition of the double stranded DNA?

Solution:

From the question, it is a double stranded DNA (Chargaff's rule is valid for only double stranded DNA), and only one strand with its base composition is given. So, the total number of bases would be 20k (10k bases on each strand).

Now, A and T bases with their numbers are given on one strand, so A will have T and T will have A on their complementary DNA. It means, on one strand, the total number of A would be 3800+2600 because % A = % T. Similarly, the total number of T would be 2600 + 3800 = 6400

The total number of A and T on both strands would be 6400 + 6400 = 12800

The remaining bases would be G and C = 20,000 - 12800 = 7200

Since % G =  % C; G = 7200/2 = 3600; C = 3600