Multiple Choice
In a DNA strand, how can you identify the 5' and 3' ends based on the chemical structure of the sugar-phosphate backbone?
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7. DNA and Chromosome Structure
DNA Structure
Multiple Choice
The pairing of nitrogenous bases in DNA is specific because which feature ensures that adenine pairs with thymine and guanine pairs with cytosine?
A
Hydrogen-bonding patterns and complementary base shapes allow only specific purine–pyrimidine pairs to fit the helix geometry
B
All nitrogenous bases form the same number of hydrogen bonds, so any base can pair with any other base equally well
C
The phosphate groups on opposite strands attract each other and force specific base pairing
D
Covalent bonds between bases determine pairing specificity by permanently linking A to T and G to C
Verified step by step guidance1
Understand that DNA base pairing is governed by the chemical structure and bonding properties of the nitrogenous bases.
Recall that adenine (A) and guanine (G) are purines (larger, two-ring structures), while thymine (T) and cytosine (C) are pyrimidines (smaller, single-ring structures).
Recognize that the DNA double helix structure requires base pairs to fit together in a way that maintains a uniform width, which means a purine must pair with a pyrimidine.
Focus on hydrogen bonding: A pairs with T through two hydrogen bonds, and G pairs with C through three hydrogen bonds, which are specific and complementary.
Conclude that the specificity of base pairing arises from the hydrogen-bonding patterns and the complementary shapes of the bases, ensuring only A-T and G-C pairs fit properly within the helix.
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