Question

A DNA molecule contains 30% G. What percentage of its bases are A? a. 30% b. 20% c. 15% d. 10% e. none of the above

Short answer

b. 20%

Step-by-step solution

Understand the complementary base pairing

Remember the rule of base pairing in DNA - A always pairs with T and G always pairs with C. The proportion of each pair is always the same. This means if the proportion of G is 30%, so will be the proportion of C.

Calculate concentration of A and T

Since it's known that G (and therefore C) each make up to 30% of the total, therefore, they collectively account for 60% of the DNA molecule. This leaves 40% to be divided equally between A and T (as they are complementary). Since we want to find the percentage of 'A', A makes up 20% of the molecule.

Key concepts

Complementary Base Pairing

Complementary base pairing is a fundamental principle of DNA structure that dictates how bases pair with each other. In DNA, the four nucleotides are adenine (A), thymine (T), cytosine (C), and guanine (G). According to the rule, A always pairs with T, and G always pairs with C. This means that the amount of A will always equal the amount of T, and the amount of G will always equal the amount of C.

• If a DNA strand has 30% guanine, it will also have 30% cytosine since G and C are complementary.
• The remaining percentage must be divided equally between adenine and thymine, which also pair together.

The concept of complementary base pairing ensures that both strands of DNA are able to complement each other, leading to the double helix structure we are so familiar with.

Guanine Content

Guanine content is an important measure in understanding DNA composition. In the case of your exercise, guanine makes up 30% of the DNA molecule. This information is crucial since it directly determines the percentage of another base, cytosine, due to complementary base pairing.

• 30% guanine means 30% cytosine.
• Together, G and C make up 60% of the DNA strand.

This not only shows the rule of complementary base pairs but also allows us to calculate the remaining bases for genetic analysis and academic exercises.

Adenine Percentage

Adenine percentage helps complete the puzzle of DNA composition. Since guanine and cytosine make up 60% of the DNA, adenine and thymine together must make up the remaining 40%. This is because the sum of all four bases in any DNA strand is always 100%.

• A and T, being complementary, will have equal percentages.
• If 40% is left for A and T together, then each must contribute 20%.

Thus, in this particular DNA molecule, adenine accounts for 20% of the total bases. This is derived from the principle that the proportion of A and T must balance out the existing proportions of G and C.

DNA Structure

DNA structure is a marvel of molecular biology, explained in part by its base pairing rules and overall composition. Understanding how adenine, thymine, guanine, and cytosine interact allows us to appreciate the double helical structure.

• The double helix consists of two strands running in opposite directions.
• The nitrogenous bases pair between the strands, holding them together.

Both the structure and function of DNA are reliant on the balance and ratios of these bases. The exact distribution of adenine, thymine, guanine, and cytosine within a DNA strand not only influences structural stability but also influences genetic information encoding.