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Ch. 10 - DNA Structure and Analysis
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook
All textbooksKlug 12th EditionCh. 10 - DNA Structure and AnalysisProblem 34
Chapter 10, Problem 34

DNA and RNA are chemically very similar but are distinguished, in large part, by the presence of a 2'-OH group in RNA and a 2'-H group in DNA. Why do you suppose that both DNA and RNA have 3'-OH groups and we do not typically find nucleic acids within cells that have 3'-H groups?

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1
Consider the structure of nucleic acids: DNA and RNA are polymers made up of nucleotides, which consist of a sugar, a phosphate group, and a nitrogenous base.
Focus on the sugar component: In DNA, the sugar is deoxyribose, which lacks an oxygen atom at the 2' position, while in RNA, the sugar is ribose, which has an OH group at the 2' position.
Examine the role of the 3'-OH group: The 3'-OH group is crucial for the formation of phosphodiester bonds, which link nucleotides together to form the backbone of the nucleic acid strand.
Understand the importance of the 3'-OH group: The presence of the 3'-OH group allows for the addition of new nucleotides during DNA and RNA synthesis, as it provides a site for the formation of a new phosphodiester bond.
Consider the absence of 3'-H groups: If nucleic acids had 3'-H groups instead of 3'-OH groups, they would be unable to form the necessary phosphodiester bonds, preventing the elongation of the nucleic acid chain and thus inhibiting replication and transcription processes.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Nucleotide Structure

Nucleotides, the building blocks of DNA and RNA, consist of a sugar, a phosphate group, and a nitrogenous base. The sugar in RNA is ribose, which contains a hydroxyl (–OH) group at the 2' position, while DNA has deoxyribose, which lacks this group. This structural difference is crucial for the stability and function of each nucleic acid.
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3'-OH Group Functionality

The 3'-OH group in nucleic acids is essential for the formation of phosphodiester bonds, which link nucleotides together to form the backbone of DNA and RNA. This hydroxyl group allows for the addition of new nucleotides during DNA replication and RNA transcription, making it vital for the synthesis and integrity of genetic material.
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Chemical Stability and Reactivity

The presence of a 2'-OH group in RNA makes it more reactive and less stable than DNA, which can lead to RNA's shorter lifespan in cells. Conversely, the 2'-H group in DNA contributes to its stability, allowing it to serve as a long-term storage form of genetic information. This stability is crucial for the preservation of genetic material across generations.
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Related Practice
Textbook Question

Newsdate: March 1, 2030. A unique creature has been discovered during exploration of outer space. Recently, its genetic material has been isolated and analyzed. This material is similar in some ways to DNA in its chemical makeup. It contains in abundance the 4-carbon sugar erythrose and a molar equivalent of phosphate groups. In addition, it contains six nitrogenous bases: adenine (A), guanine (G), thymine (T), cytosine (C), hypoxanthine (H), and xanthine (X). These bases exist in the following relative proportions:

A =T = H and C = G = X

X-ray diffraction studies have established a regularity in the molecule and a constant diameter of about 30 Å. Together, these data have suggested a model for the structure of this molecule.

What base-pairing properties must exist for H and for X in the model?

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Textbook Question

Newsdate: March 1, 2030. A unique creature has been discovered during exploration of outer space. Recently, its genetic material has been isolated and analyzed. This material is similar in some ways to DNA in its chemical makeup. It contains in abundance the 4-carbon sugar erythrose and a molar equivalent of phosphate groups. In addition, it contains six nitrogenous bases: adenine (A), guanine (G), thymine (T), cytosine (C), hypoxanthine (H), and xanthine (X). These bases exist in the following relative proportions:

A =T = H and C = G = X

X-ray diffraction studies have established a regularity in the molecule and a constant diameter of about 30 Å. Together, these data have suggested a model for the structure of this molecule.

Given the constant diameter of 30 Å, do you think that either (i) both H and X are purines or both pyrimidines, or (ii) one is a purine and one is a pyrimidine?

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Textbook Question

During gel electrophoresis, DNA molecules can easily be separated according to size because all DNA molecules have the same charge-to-mass ratio and the same shape (long rod). Would you expect RNA molecules to behave in the same manner as DNA during gel electrophoresis? Why or why not?

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Textbook Question

Electrophoresis is an extremely useful procedure when applied to analysis of nucleic acids as it can resolve molecules of different sizes with relative ease and accuracy. Large molecules migrate more slowly than small molecules in agarose gels. However, the fact that nucleic acids of the same length may exist in a variety of conformations can often complicate the interpretation of electrophoretic separations. For instance, when a single species of a bacterial plasmid is isolated from cells, the individual plasmids may exist in three forms (depending on the genotype of their host and conditions of isolation): superhelical/supercoiled (form I), nicked/open circle (form II), and linear (form III). Form I is compact and very tightly coiled, with both DNA strands continuous. Form II exists as a loose circle because one of the two DNA strands has been broken, thus releasing the supercoil. All three have the same mass, but each will migrate at a different rate through a gel. Based on your understanding of gel composition and DNA migration, predict the relative rates of migration of the three DNA structures mentioned above.

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