SAT II Biology E : DNA Structure and Function

Study concepts, example questions & explanations for SAT II Biology E

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Example Questions

Example Question #1 : Dna Structure And Function

Which type of bond makes up the backbone of DNA strands by linking together adjacent nucleotides?

Possible Answers:

Hydrogen bond

Phosphodiester bond

Glycosidic bond

Peptide bond

Ester bond

Correct answer:

Phosphodiester bond

Explanation:

DNA and RNA nucleotides are linked together through phosphodiester bonds. A strong covalent bond (ester bond) forms between the 3' carbon atom of the sugar pentose of one nucleotide and a phosphate group, and a second ester bond forms between the phosphate group and the 5' carbon atom of the sugar pentose of another nucleotide. This alternation of sugar and phosphate groups forms a strong backbone and is also the reason why DNA is antiparallel and forms in the 5' to 3' direction.

Example Question #2 : Dna Structure And Function

When synthesizing a strand of double-stranded DNA, which of the following could be a plausible combination of nitrogen bases?

Possible Answers:

None of these

23% Adenine, 23% cytosine, 27% guanine, 27% uracil

24% Adenine, 24% cytosine, 26% guanine, 26% thymine

48% Adenine, 52% cytosine, 52% guanine, 48% thymine

28% Adenine, 22% cytosine, 22% guanine, 28% thymine

Correct answer:

28% Adenine, 22% cytosine, 22% guanine, 28% thymine

Explanation:

DNA nucleotides all contain one of four possible nitrogen bases: adenine (A), thymine (T), cytosine (C), or guanine (G). In forming base pairs, an A must always pair with a T and a C must always pair with a G: [A-T], [C-G]. This means that for any DNA composition, the percent of adenine (A) must be equal to the percent of thymine (T) and, likewise, the percent of cytosine (C) must be equal to the percent of guanine (G). Looking across the answer choices, there is only one choice that satisfies this condition while also correctly summing to 100%. The choice with uracil can be eliminated immediately, since uracil only replaces thymine in RNA and is not present in DNA. 

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