Multiple Choice
Which of the following best describes a structural difference between DNA and RNA?
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3. Extensions to Mendelian Inheritance
Understanding Independent Assortment
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Which of the following base compositions is possible for a double-stranded DNA molecule? (%A = percent adenine, %G = percent guanine, %C = percent cytosine, %T = percent thymine, %U = percent uracil)
A
%A = 28, %G = 22, %C = 28, %T = 12, %U = 10
B
%A = 30, %G = 20, %C = 20, %T = 30, %U = 0
C
%A = 20, %G = 30, %C = 30, %T = 20, %U = 0
D
%A = 25, %G = 25, %C = 25, %T = 15, %U = 10
Verified step by step guidance1
Recall Chargaff's rules for double-stranded DNA, which state that the amount of adenine (%A) equals the amount of thymine (%T), and the amount of guanine (%G) equals the amount of cytosine (%C). This is because A pairs with T, and G pairs with C in the DNA double helix.
Check that the sum of all base percentages equals 100%, since the total composition must account for all nucleotides in the DNA molecule.
Verify that the percentage of uracil (%U) is zero, because uracil is found in RNA, not in double-stranded DNA. Any presence of uracil would indicate the molecule is not DNA.
For each given base composition, confirm that %A = %T and %G = %C. If these equalities do not hold, the composition cannot represent double-stranded DNA.
Eliminate any options where the sum of bases is not 100% or where uracil is present, and select the composition that satisfies all these conditions as the possible base composition for double-stranded DNA.
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