Question

There are $20\%$ adenine among the bases in a DNA fragment measuring $6.8nm$ in length. The number of pentose sugars, nitrogen base pairs, phosphate groups and hydrogen bonds in this DNA fragment are respectively

• A. $40,20,40,52$
• B. $40,52,40,20$
• C. $20,40,52,40$
• D. $52,20,20,40$

Answer 1

The correct option is A $40,20,40,52$

Number of base pairs
Length of one complete turn of the DNA double helix is $34Å=3.4nm$
One turn of DNA helix has 10 nucleotides or 10 base pairs
Thus, $3.4nm$ of DNA has $10bp$
So, $6.8nm$ of DNA will have $=\left(10/3.4\right)x6.8bp=20bp$
Number of bases
If the DNA double helix has 20 base pairs formed by hydrogen bonding between complementary bases from each strand, then the total number of bases present in 6.8 nm of DNA$=20x2=40$nitrogenous bases
Number of pentose sugars and phosphate group
Each nucleotide of a DNA strand is composed of 1 pentose sugar, 1 nitrogenous base and 1 phosphate group.
If the DNA double helix has a total of 40 nitrogenous bases, then,
Total number of pentose sugars = 40
Total number of phosphate groups = 40
Part 4: Number of hydrogen bonds
Total number of bases = 40
Number of Adenine bases $=20\%of40=\left(20x40\right)/100=8$
According to Chargaff’s rule, A = T
Therefore, number of Thymine bases = 8
Each Adenine and Thymine base pair is held by 2 hydrogen bonds.
Thus, number of hydrogen bonds needed for A-T base pairing $=8x2=16$

Out of 40 base pairs, 16 are Adenine and Thymine so the total number of guanine and cytosine bases $=40-16=24$
According to Chargaff’s rule, $G=C$
Thus the number of guanine molecules $=24/2=12$
The number of Cytosine molecules $=24/2=12$
Each Guanine and Cytosine base pair is held by 3 hydrogen bonds.
Thus, number of hydrogen bonds needed for G-C base pairing $=12x3=36$
Thus total number of hydrogen bonds $=16+36=52$
Final answer
(D)$40,20,40,52$