The Percentage of Guanine in DNA Double Helix

To determine the percentage of guanine in a DNA double helix when the percentage of thymine is known, we need to understand the base pairing rules in DNA and the concept of complementary base pairs.

Base Pairing in DNA
DNA is made up of nucleotides, which consist of a sugar molecule (deoxyribose), a phosphate group, and a nitrogenous base. The four nitrogenous bases found in DNA are adenine (A), thymine (T), cytosine (C), and guanine (G).

The base pairing rules in DNA state that adenine always pairs with thymine (A-T), and cytosine always pairs with guanine (C-G). This means that the percentage of adenine is equal to the percentage of thymine, and the percentage of cytosine is equal to the percentage of guanine in a DNA double helix.

Calculating the Percentage of Guanine
Given that the percentage of thymine in the DNA double helix is 35%, we can deduce that the percentage of adenine is also 35% due to the base pairing rules.

Since the total percentage of the four bases must add up to 100%, we can now calculate the percentage of cytosine and guanine by subtracting the percentages of adenine and thymine from 100%.

Percentage of adenine (A) = 35%
Percentage of thymine (T) = 35%

Percentage of cytosine (C) = 100% - Percentage of adenine (A) - Percentage of thymine (T)
= 100% - 35% - 35%
= 30%

Since cytosine always pairs with guanine, the percentage of guanine (G) will also be 30%.

Summary
In conclusion, if the percentage of thymine in a DNA double helix is 35%, the percentage of guanine will also be 30%. This is because of the complementary base pairing rules in DNA, where adenine pairs with thymine and cytosine pairs with guanine. By subtracting the percentages of adenine and thymine from 100%, we can determine the percentages of guanine and cytosine.