Chargaff’s Rule Questions and Answers

Chargaff Rule Questions: Chargaff’s rule, proposed by Erwin Chargaff in the 1950s, is a fundamental principle in molecular biology that describes the base composition of DNA. The rule has two main components:

Base Pairing Equality: In a double-stranded DNA molecule, the amount of adenine (A) is equal to the amount of thymine (T), and the amount of cytosine (C) is equal to the amount of guanine (G). This is represented as:

A = T
C = G

Base Composition Variability: The total amounts of A+T and G+C vary between species, providing a unique “base composition signature” for each organism. However, within a given species, this ratio remains consistent.

Chargaff’s rule laid the groundwork for the discovery of the DNA double helix by Watson and Crick, as it highlighted the complementary nature of base pairing, which is crucial for DNA structure and replication. The following are some molecular biology problems based on Chargaff’s rule with answers.

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Problem 1: A DNA sample contains 30% adenine. What is the percentage of guanine in the DNA?

Answer: 20%

Explanation: According to Chargaff’s rule, the amount of adenine (A) equals thymine (T), and the amount of cytosine (C) equals guanine (G). If A = 30%, then T = 30%. This leaves 40% for G and C combined. Since G = C, G = 20%.

Problem 2: If a DNA molecule has 26% cytosine, what is the percentage of thymine in the molecule?

Answer: 24%

Explanation: Cytosine equals guanine, so G = 26%. The remaining 48% (100% – 52%) is split equally between adenine and thymine. Thus, T = 24%.

Problem 3: A double-stranded DNA sequence has 18% guanine. What percentage of the sequence is adenine?

Answer: 32%

Explanation: Since G = 18%, C also equals 18%. Together, they make up 36%, leaving 64% for A and T. Since A = T, A = 32%.

Problem 4: If the DNA content of adenine in a strand is 22%, calculate the percentage of cytosine.

Answer: 28%

Explanation: If A = 22%, then T = 22%, leaving 56% (100% – 44%) for C and G. Since G = C, C = 28%.

Problem 5: In a DNA molecule, 40% of the bases are cytosine and guanine combined. What percentage of the molecule is thymine?

Answer: 30%

Explanation: C and G together make up 40%, so A and T together make up 60%. Since A = T, T = 30%.

Problem 6: If the proportion of thymine in DNA is 27%, what is the percentage of guanine?

Answer: 23%

Explanation: Since T = 27%, A = 27%. This leaves 46% (100% – 54%) for G and C. Hence, G = 23%.

Problem 7: In a DNA sample, the proportion of guanine is 19%. What is the percentage of cytosine?

Answer: 19%

Explanation: Chargaff’s rule states that G = C. Thus, if G = 19%, C = 19%.

Problem 8: A DNA sequence has 35% adenine. What percentage of the sequence is cytosine?

Answer: 15%

Explanation: A = 35%, so T = 35%. This leaves 30% (100% – 70%) for C and G. Since G = C, C = 15%.

Problem 9: The DNA of an organism contains 28% cytosine. Calculate the percentage of thymine.

Answer: 22%

Explanation: If C = 28%, then G = 28%. Together, they make 56%, leaving 44% (100% – 56%) for A and T. Since A = T, T = 22%.

Problem 10: If the total adenine and guanine content in DNA is 60%, what is the percentage of cytosine?

Answer: 20%

Explanation: A + G = 60%, leaving 40% for T and C. Since A = T and G = C, C = 20%.

Exercise problems

Problem 1: In a DNA molecule, the combined percentage of adenine and cytosine is 55%. If the percentage of thymine is 25%, what is the percentage of guanine in the molecule?

Problem 2: A DNA strand contains 42% guanine and cytosine combined. If the difference between the percentages of adenine and cytosine is 8%, calculate the percentage of adenine in the DNA strand.

Problem 3: If a DNA sample contains 36% thymine, and the percentage of guanine is double that of cytosine, what is the percentage composition of cytosine in the DNA?

Problem 4: In a double-stranded DNA sequence, the percentage of cytosine is 18%, and the sum of adenine and guanine is 72%. What is the percentage of thymine in the sequence?

Problem 5: A DNA molecule has 40% of its bases consisting of adenine and thymine combined. If the percentage of guanine is three times that of adenine, calculate the percentage of each base in the DNA strand.

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