Question

What will be the percentage of Guanine in a DNA molecule having $20\mathrm{\%}$ Adenine.
A. $20\mathrm{\%}$
B. $30\mathrm{\%}$
C. $40\mathrm{\%}$
D. $60\mathrm{\%}$

Answer 1

Hint: DNA carries the hereditary material in an organism. It is a type of nucleic acid and is polyfunctional. They are made of nucleotides and they include- a nitrogenous base (purine and pyrimidine), a pentose sugar, and a phosphate. DNA has a $1:1$ ratio of purine (Adenine and Guanine) and pyrimidine (Cytosine and Thymine).

Complete answer:
The nucleotides are composed of nitrogenous bases such as-
• Purines- Adenine and Guanine
• Pyrimidines- Cytosine and Thymine.
The nucleotides are present according to Chargaff’s rule on the ratio of $1:1$. It is also called the Base pair rule.

Adenine bonds or pairs with Thymine always and Guanine bonds or pairs with Cytosine always. We already know the percentage of Adenine which is $20\mathrm{\%}$ So, the percentage of Thymine also becomes $20\mathrm{\%}$ , they both make a total of $40\mathrm{\%}$ and the remaining quantity i.e. $60\mathrm{\%}$ gets equally divided between Cytosine and Guanine.i.e. $30\mathrm{\%}$ each.
$A+T=20\mathrm{\%}+20\mathrm{\%}=40\mathrm{\%}$
$C+G=100-(A+T)=100-40=60\mathrm{\%}$
Each get’s an equal quantity of $30\mathrm{\%}$.

Hence, the correct answer is option (B).

Additional information:
Uracil is a pyrimidine found in RNA instead of Thymine.
The other names for nucleotides are as follows:
- Adenine- 6-Amino Purine
- Guanine- 2-amino, 6-oxopurine
- Cytosine- 2-oxo 4 aminoPyrimidine
- Uracil- 2,4 dioxo Pyrimidine
- Thymine- 2,4 dioxo 5 methyl pyrimidine
- Nucleosides are devoid of Phosphoryl groups.

Note: If false pairing takes place then mutation may arise but before that a process called DNA repair mechanism undergoes, and mismatching is corrected. A nucleotide has three components namely ribose sugar, nitrogen base, and phosphate group. In a nucleotide if adenine percentage is $20$ then the percentage of guanine and cytosine will be $30$ each. However, in RNA Uracil is present instead of thymine.