Understanding DNA Base Composition Calculation

This question asks us to determine the percentage of Guanine in a double-stranded DNA molecule, given that the percentage of Thymine is 30%. To solve this, we need to apply the fundamental rules of DNA base pairing, often referred to as Chargaff's rules.

Chargaff's Rules and DNA Base Pairing

In a double-stranded DNA molecule:

• The amount of Adenine (A) is always equal to the amount of Thymine (T). They pair with each other (A=T).
• The amount of Guanine (G) is always equal to the amount of Cytosine (C). They pair with each other (G≡C).
• The total percentage of all four bases (A, T, G, and C) in the DNA molecule must add up to 100%.

Mathematically, Chargaff's rules can be expressed as:

$\text{%A} = \text{%T}$

$\text{%G} = \text{%C}$

$\text{%A} + \text{%T} + \text{%G} + \text{%C} = 100\%$

Step-by-Step Calculation

We are given that the percentage of Thymine (%T) is 30%.

Given: $\text{%T} = 30\%$

Step 1: Find the percentage of Adenine (%A).

According to Chargaff's rules, %A = %T.

$\text{%A} = \text{%T} = 30\%$

Step 2: Find the combined percentage of Adenine and Thymine.

$\text{%A} + \text{%T} = 30\% + 30\% = 60\%$

Step 3: Find the combined percentage of Guanine and Cytosine (%G + %C).

The remaining percentage of bases must be Guanine and Cytosine. The total percentage of all bases is 100%.

$\text{%G} + \text{%C} = 100\% - (\text{%A} + \text{%T})$

$\text{%G} + \text{%C} = 100\% - 60\% = 40\%$

Step 4: Find the percentage of Guanine (%G).

According to Chargaff's rules, %G = %C. So, the 40% is equally split between Guanine and Cytosine.

$\text{%G} = \text{%C} = \frac{\text{%G} + \text{%C}}{2}$

$\text{%G} = \frac{40\%}{2} = 20\%$

Therefore, the percentage of Guanine in the double-stranded DNA is 20%.

Summary of Base Percentages

The calculated percentage of Guanine is 20%, which corresponds to one of the given options.

Revision Table: Key Concepts in DNA Base Composition

Additional Information: Significance of Base Composition

The base composition (the relative amounts of A, T, G, and C) varies between different species, but it is constant within a species (with minor variations). The ratio of (A+T) to (G+C) is a characteristic feature of an organism's genome. Organisms with a higher percentage of G-C pairs tend to have DNA that is slightly more stable at higher temperatures due to the triple hydrogen bond between G and C, compared to the double hydrogen bond between A and T.

Understanding DNA base composition and Chargaff's rules is fundamental to molecular biology and genetics, providing insights into DNA structure, replication, and evolution.