Question

DNA was extracted from Bacterial Species. The Proportion of Cytosine was Found to be 30%, Then Calculate the Proportion of Purine.

Answer 1

Chargaff’s rule:

1. According to Chargaff’s rule, in any double-stranded DNA (deoxyribonucleic acid), the number of cytosine equals the number of guanines, and the number of adenine units equals the thymine units.
2. Adenine and guanine are purines and cytosine and thymine are pyrimidines.

Step 1: Given data

The proportion of cytosine = $30\%$

There is a need to find the proportion of purines, that is, adenine and guanine.

Step 2: Applying the formula

According to Chargaff’s rule,

$\mathrm{A}+\mathrm{G}=\mathrm{T}+\mathrm{C}$

Where A is adenine, G is guanine, T is thymine, and C is cytosine.

Step 3: Calculation

The proportion of C = $30\%$

As C is equal to G, therefore, G will also be $30\%$.

Therefore, out of $100\%$ bases, C and G make up $60\%$.

Now the remaining will be of A and T.

As $\mathrm{A}=\mathrm{T}$, so $\mathrm{A}=\mathrm{T}=20\%$

Therefore, purines, that is, adenine and guanine forms,

$30\%+20\%=50\%$ of the total bases.

Final answer: The proportion of purines is $50\%$.