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Ch. 9 - DNA Structure and Analysis
Klug - Essentials of Genetics 10th Edition
Klug10th EditionEssentials of GeneticsISBN: 9780135588789Not the one you use?Change textbook
Chapter 9, Problem 27c

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%

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Step 1: Understand the problem context. The organism has a unique genetic material with four bases: Adenine (A), Guanine (G), Xanthine (X), and Hypoxanthine (H), with given percentages. Unlike typical DNA, which has A, T, G, and C, this organism has unusual bases, so classical base pairing rules may not apply.
Step 2: Analyze the base composition percentages. Note that the total percentage should sum to 100%. Verify this by adding the given percentages: A (8%) + G (37%) + X (37%) + H (18%) = 100%. This confirms the data is consistent.
Step 3: Consider the implications for base pairing. In standard DNA, Chargaff's rules state that %A = %T and %G = %C due to complementary base pairing. Here, since the bases are different, check if any pairs have equal percentages, which might suggest pairing partners. For example, G and X both have 37%, which might indicate a pairing relationship.
Step 4: Formulate hypotheses about the pairing scheme. Since A is 8% and H is 18%, they are not equal, so they might not be complementary pairs. Alternatively, the organism might use non-standard pairing or single-stranded nucleic acid. Consider the possibility that G pairs with X and A pairs with H, or other combinations.
Step 5: Summarize the findings and suggest further analysis. To fully understand the genetic structure, one would need to investigate the chemical nature of X and H, their pairing properties, and whether the nucleic acid is single or double stranded. Experimental methods like sequencing or structural analysis would help confirm the pairing rules.

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Key Concepts

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

Nucleic Acid Base Composition

Nucleic acids are composed of nitrogenous bases whose proportions influence the structure and function of genetic material. Typical DNA bases include adenine, thymine, guanine, and cytosine, while RNA contains uracil instead of thymine. Unusual bases like xanthine and hypoxanthine suggest a non-standard nucleic acid, affecting base pairing and genetic coding.
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Base Distortions

Base Pairing Rules and Complementarity

In standard DNA, bases pair specifically (A with T, G with C) to maintain structure and enable replication. The presence of atypical bases like xanthine and hypoxanthine may alter or create new pairing rules, impacting the stability and replication fidelity of the genetic material in this primitive eukaryote.
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Complementation

Genetic Material Diversity in Early Eukaryotes

Primitive eukaryotes may possess unique genetic materials differing from canonical DNA or RNA. Studying such organisms helps understand the evolution of genetic systems and the possible existence of alternative nucleic acids with distinct bases, which could reveal new mechanisms of heredity and molecular biology.
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History and Experiments
Related Practice
Textbook Question

A genetics student was asked to draw the chemical structure of an adenine- and thymine-containing dinucleotide derived from DNA. The answer is shown here:

The student made more than six major errors. One of them is circled, numbered 1, and explained. Find five others. Circle them, number them 2 through 6, and briefly explain each in the manner of the example given.

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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:

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

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:

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.

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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?

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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?

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