If nucleotides from the DNA of a human were quantified and 30 percent of them contained the base adenine, what percentage of them would contain the base guanine?
a. 20 percent
b. 30 percent
c. 40 percent
d. 70 percent

Verified step by step guidance

1. Understand the base pairing rules in DNA: Adenine (A) pairs with Thymine (T), and Cytosine (C) pairs with Guanine (G). This means the percentage of adenine will equal the percentage of thymine, and the percentage of cytosine will equal the percentage of guanine.
2. Given that 30% of the nucleotides contain adenine, it follows that 30% must also contain thymine due to the base pairing rule.
3. Calculate the combined percentage of adenine and thymine: Since adenine and thymine each make up 30% of the nucleotides, together they account for 60% of the DNA.
4. Determine the remaining percentage for cytosine and guanine: Since adenine and thymine together make up 60%, the remaining 40% of the nucleotides must be cytosine and guanine.
5. Divide the remaining percentage equally between cytosine and guanine: Since cytosine and guanine pair together, they must each make up half of the remaining 40%, resulting in 20% for guanine.

Key Concepts

Base Pairing Rules

In DNA, base pairing rules dictate that adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). This means that the amount of adenine is equal to thymine, and the amount of guanine is equal to cytosine. Understanding these rules is crucial for determining the composition of DNA strands.

Chargaff's Rules

Chargaff's rules state that in any given DNA molecule, the amount of adenine equals thymine, and the amount of guanine equals cytosine. This implies that the total percentage of purines (adenine and guanine) equals the total percentage of pyrimidines (thymine and cytosine), which is essential for calculating nucleotide percentages.

Percentage Calculation in DNA

To calculate the percentage of a specific base in DNA, use the known percentage of its complementary base. If adenine is 30%, then thymine is also 30%. The remaining 40% of the DNA is split equally between guanine and cytosine, making each 20%. This calculation is based on the total percentage of all bases summing to 100%.