The ratio of thymine to cytosine in DNA molecule of 204 Armstrong in lenth which has 15 adenine molecules . explain in detail.
A)1:3 B)1:4 C) 4:1 D)3:1
Answers
One turn of DNA double helix is 3.4 nm long and has 10 bp; therefore, a 204 nm long DNA strand will have 3.4/10 * 204= 600 bp. According to the question, this DNA strand has 40% adenine. According to Chargaff’s rule in all cellular DNAs, regardless of the species, number of adenosine residues is equal to number of thymidine residues which means that A=T; and the number of guanosine residues is equal to the number of cytidine residues; G= C. Therefore, the DNA strand has 40 % adenine and 40% thymine. Out of total 600 bp, number of adenine residues = 40/100 *600= 240. And the number of thymine will also be 240. Out of total 600 bp, rest base pairs (600-240+240= 120) are C+G. Hence the number of cytosines = 120/2= 60 and the number of guanine base is also 60. As defined forms double hydrogen bonds with thymine, hence, a total number of double hydrogen bonds= 240. And guanine forms triple hydrogen bonds with cytosine; a number of triple hydrogen bonds= 60. Hence the ratio of double and triple hydrogen bonds is 240: 60= 4:1. Thus, the correct answer is option C.
