Question

You have a piece of double stranded DNA that is 300 base pairs and 50% guanine remember there are 2 nucleotides in a base pair 1, How many H bonds hold the strands together? 2. How many purines are in the molecule? 3. How many pyrimidines in this molelcule? 4 Would you expect this strand to "melt" at a higher or lower temperature than AT rich DNA?

Answer 1

1. Since this 300 base pair DNA as per the Chargaffs rule Guanosine pairs with Cytosine by 3 H2 bonds and Adenine binds with Thymine by 2 H2 bonds , Also 50% Guanosine is present hence 150 GC pair is present 150 AT pair is present.

150 GC pair = 150 × 3 Hydrogen bond = 450

150 AT pair= 150 × 2 Hydrogen bond = 300

Hence total 750 hydrogen bond is present.

2. 300 purines are present : 150 G + 150 A

3. 300 pyrimidines are present. 150 C + 150 T

4. This GC pair melts at a higher temperature than AT rich pair as AT rich pair is very weak and melts at a very low temperature