Question

Two students are given two different double stranded DNA molecules of equal length. They are asked to denature the DNA molecules by heating. The DNA given to student A has a following composition of base (A : G : T : C :: 35 : 15 : 35 : 15) while that given to student B is (A : G : T: C :: 12 : 38 : 12 : 38). Which of the following statement is true?

• A. Both the DNA molecules would denature at the same rate.
• B. The information given is insufficient to draw any conclusion.
• C. DNA molecule given to student B would denature faster than that of student A.
• D. DNA molecule given to student A would denature faster than that given to student B.

Answer 1

Answer: (D)

DNA molecule given to student A would denature faster than that given to student B.

DNA is a double helical polymer of nucleotides in which two antiparallel strands are joined to each other by complementary base pairing between nitrogenous bases. DNA has four bases; two purines and two pyrimidines; purines are namely adenine and guanine and the pyrimidines are cytosine, thymine. Adenine in one DNA strand makes two hydrogen bonds with thymine in other DNA strand and guanine makes three hydrogen bonds with cytosine. The rate of DNA strand separation depends on of A-T/G-C ratio; this makes option A wrong. Thus, a DNA strand with the higher A-T ratio is separated faster as compared to those having higher G-C content. In the question, student A got DNA with higher A-T residues and hence, will be separated faster (this makes option D correct) while student B has DNA with higher G-C content which needs comparatively more energy input to break three hydrogen bonds and thus will be separated slowly (this makes option C wrong). The base ratio given in question clearly shows the separation speed; this makes option B wrong.

Thus, the correct answer is option D.