Question

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

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

Answer 1

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

Here, the given DNA measure = 6.8nm = 68 Angstroms.
Length of DNA = Distance between base pairs $\times$ Total number of base pairs
Therefore, 68 $\mathring{A}$ = 3.4$\mathring{A}$ $\times$ Total number of nitrogen base pairs.
Hence, nucleotides = 40
1 nucleotide = 1 nitrogen base + 1 pentose sugar + 1 phosphate groups
So, pentose sugar = 40
Phosphate groups = 40
From the percent mentioned these are 8 combinations of A-T and 12 combinations of C=G in given fragment of DNA. Adenine is paired to thymine by two hydrogen bonds and cytosine is paired to guanine by three hydrogen bonds.
So 8$\times$ 2 = 16 and 12 $\times$ 3 = 36
16+ 36 = 53 hydrogen bonds.