Question

You have a piece of double stranded DNA that is 300 base pairs and 50% guanine

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

According to Chargaff's rule, The Ratio of Purines (A & G) and Pyrimidines (C &T) in DNA equals to 1.

So, according to base pairing rule, A = T and G = C

300 base pairs means 300 x 2 = 600 bases.

If DNA contains 50% Guanine (G), it will contain 50% Cytosine (C) also.

So, out of 600 bases, no. of Guanine will be (600 x 50) / 100 = 300.

Cytosine will also be 300 as no. of Guanine equals no. of Cytosine.

So, no. A and T will be 0 as G and C together will make up 300 + 300 = 600 bases.

Between G and C (one pair), there are three hydrogen bonds

So among 300 G, C pairs, there will be 300 x 3 = 900 hydrogen bonds.

G C bonds are stronger than A T bonds . Hence, higher temperature is needed to " melt" this strand.

The answers of the questions are given below

1. Number of hydrogen bonds is 900.

2. Number of Purines ( Guanine) is 300.

3. Number of Pyrimidines (Cytosine) is 300.

4. This strand will melt at a higher temperature than AT rich DNA because GC bonds are stronger than AT bonds.