What determines the primary structure of a DNA molecule?
answer
the sequence of deoxyribonucleotides
question
DNA attains a secondary structure when hydrogen bonds form between the nitrogenous bases called purines and pyrimidines. What are the complementary base pairs that form in DNA?
answer
A-T and G-C
question
Which of the following rules apply to the synthesis of nucleic acids?
answer
complementary pairing between bases is required for copying nucleic acids.
question
Nucleic acids are directional, meaning that there are two different ends. What functional groups define the two different ends of a DNA strand?
answer
one end has a free phosphate group on the 5' carbon; the other end has a free hydroxyl group bonded to the 3' carbon.
question
What is responsible for the increased stability of DNA compared to RNA?
answer
DNA is a more stable molecule than RNA because it lacks a hydroxyl group on the 2' carbon and is therefore more resistant to cleavage, and because the two sugar-phosphate backbones are held together by many hydrogen bonds between nitrogenous bases.
question
Explain how Chargaff's rules relate to the complementary base pairing seen in the secondary structure of DNA. Would you expect these rules to apply to RNA as well? Explain why or why not.
answer
In DNA, the secondary structure requires that every guanine paris with a cytosine and every thymine paris with an adenine, resulting in consistent ratios between the nucleotides. Chargaff's rules do not apply to RNA, since it is single-stranded and the pairing is not consistent throughout the molecules.
question
If nucleotides from the DNA of a human were quantified and 30% of them consisted of adenine, what percentage of guanine nucleotides would be present?
answer
if 30% is adenine, then 30% would be thymine, since they are base-paired together. This means that 40% consists of G-C base pairs, which would be equally divided between the two bases.
question
What would be the sequence of the strand of DNA that is made form the following template: 5'-GATATCGAT-3' (Your answer must be written 5'-3') How would this sequence be different if RNA were made from this DNA template?
answer
The DNA sequence of the new strand would be 5'-ATCGATATC-3'. The RNA sequence would be the same, except each T would be replaced by a U.
question
A major theme in this chapter is that the structure of molecules correlates with their function. Explain how DNA's secondary structure limits it catalytic abilities compared with that of RNA molecules can catalyze a modest but significant array of reactions?
answer
DNA has limited catalytic ability because it (1) lacks functional groups that can participate in catalysis and (2) has a regular structure that is not conducive to forming shapes required for catalysis. RNA molecules can catalyze some reactions because they (1) have exposed hydroxyl functional groups and (2) can fold into shapes that can function in catalysis.
question
To replicate a ribozyme, a complete complementary copy must be made. Would you expect the double-stranded intermediate to maintain its catalytic activity? Justify your answer with an explanation.
answer
No. Catalytic activity in ribozymes depends on the tertiary structure generated from single-stranded molecules. Double-stranded nucleic acids do not form tertiary structures.
question
Suppose that Bartel's research group succeeded in producing a molecule that could make a copy of itself. Which of the five fundamental characteristics of life would support the claim that this molecule is alive?
answer
An RNA replicase would undergo replication and be able to evolve. It would process information in the sense of copying itself, and it would use energy to drive polymerization reactions. It would not be bound by a membrane and considered a cell. however, and it would not be able to acquire energy. It would best be considered as an intermediate step between nonlife and true life.
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