Question

A double stranded DNA molecule was found to have a total of 10,000 A+T base residues and 14,000 G+C residues. Analysis of one of the strands of DNA showed that it had 3,000 A and 4,000 C residues. What is the number of T and G base residues on this strand?

Answer 1

Ans:

The number of T and G base residues on this strand are 2000 and 3000 respectively.

Explanation:

According to the chargaff's rules, the amount of adenine (A) is equivalent to thymine (T) and the amount of guanine (G) is equivalent to cytosine (C) and adenine always pairs with thymine and guanine always pairs with cytosine.

So, A=T and G=C

If one strand of DNA strand contains 3000 adenine residues and 4000 cytosine residues, the other strand contains same number of thymine and guanine.

Total number of A+T = 10000

Total number of given A+T in both strand = 3000+3000=6000

Remaining A+T residues = 10000-6000 = 4000

So, total of T residues on this strand = 4000/2=2000 (because equal number of A+T is remaining in both strand; so, each strand half number of A+T residues.

Total number of G+C = 14000

Total number of given G+C in both strand = 4000+4000=8000

Remaining A+T residues = 14000-8000 = 6000

So, total of G residues on this strand = 6000/2=3000 (because equal number of G+C is remaining in both strand; so, each strand half number of G+C residues).