Ch. 9 - DNA Structure and Analysis

Klug - Essentials of Genetics 10th Edition

Klug10th EditionEssentials of GeneticsISBN: 9780135588789Not the one you use?Change textbook

Klug 10th Edition

Ch. 9 - DNA Structure and Analysis

Problem 27d

Chapter 9, Problem 27d

A primitive eukaryote was discovered that displayed a unique nucleic acid as its genetic material. Analysis provided the following information:
About 75 percent of the sugars are deoxyribose, while 25 percent are ribose.
Postulate a model for the structure of this molecule that is consistent with the foregoing observations.

Verified step by step guidance

Step 1: Understand the composition of the nucleic acid based on the sugar content. Since 75% of the sugars are deoxyribose and 25% are ribose, the molecule likely contains both DNA and RNA components or a hybrid structure.

Step 2: Recall the structural differences between DNA and RNA. DNA contains deoxyribose sugars and typically forms a double helix, while RNA contains ribose sugars and is usually single-stranded but can form complex secondary structures.

Step 3: Consider possible models that incorporate both sugar types. One possibility is a nucleic acid polymer where some nucleotides have deoxyribose and others have ribose, either randomly interspersed or organized in distinct regions.

Step 4: Think about how the presence of both sugars might affect the overall structure and stability. The molecule might have regions resembling DNA double helices interspersed with RNA-like single-stranded or looped regions, potentially allowing unique folding or function.

Step 5: Formulate a structural model hypothesis that explains the mixed sugar composition, such as a chimeric nucleic acid with alternating or block segments of DNA and RNA, or a hybrid strand with modified backbone chemistry that incorporates both sugar types.

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Key Concepts

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

Nucleic Acid Structure and Sugar Components

Nucleic acids are polymers made of nucleotides, each containing a sugar, a phosphate group, and a nitrogenous base. DNA contains deoxyribose sugars, while RNA contains ribose sugars. The sugar type influences the nucleic acid’s stability and structure, with deoxyribose lacking an oxygen atom present in ribose.

Hybrid Nucleic Acid Molecules

A nucleic acid containing both deoxyribose and ribose sugars suggests a hybrid or chimeric molecule. This could imply a novel structure combining features of DNA and RNA, possibly with segments or alternating nucleotides containing different sugars, affecting its chemical properties and biological function.

Molecular Modeling Based on Chemical Composition

Constructing a model for an unusual nucleic acid requires integrating chemical composition data with known molecular structures. The 75% deoxyribose and 25% ribose ratio suggests a predominant DNA-like backbone with RNA-like insertions, guiding hypotheses about strand arrangement, sugar-phosphate backbone continuity, and base pairing.

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Related Practice

Textbook Question

One of the most common spontaneous lesions that occurs in DNA under physiological conditions is the hydrolysis of the amino group of cytosine, converting the cytosine to uracil. What would be the effect on DNA structure of a uracil group replacing cytosine?

Textbook Question

A primitive eukaryote was discovered that displayed a unique nucleic acid as its genetic material. Analysis provided the following information:

A major hyperchromic shift is evident upon heating and monitoring UV absorption at 260 nm.

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Textbook Question

A primitive eukaryote was discovered that displayed a unique nucleic acid as its genetic material. Analysis provided the following information:

Base-composition analysis reveals four bases in the following proportions: Adenine = 8%; Guanine = 37%; Xanthine = 37%; Hypoxanthine = 18%

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.

Propose a general model of this molecule. Describe it briefly.

Textbook Question

A primitive eukaryote was discovered that displayed a unique nucleic acid as its genetic material. Analysis provided the following information:

The general X-ray diffraction pattern is similar to that of DNA, but with somewhat different dimensions and more irregularity.

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Textbook Question

In some organisms, cytosine is methylated at carbon 5 of the pyrimidine ring after it is incorporated into DNA. If a 5-methyl cytosine molecule is then hydrolyzed, what base will be generated?