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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 29

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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1
Understand the structure of cytosine and the position of methylation: cytosine is a pyrimidine base, and methylation occurs at carbon 5 of the pyrimidine ring, producing 5-methylcytosine.
Recall that hydrolysis of cytosine (or 5-methylcytosine) typically involves deamination, where the amino group (-NH2) is replaced by a keto group (=O).
Recognize that deamination of cytosine converts it into uracil, while deamination of 5-methylcytosine converts it into thymine because the methyl group at carbon 5 remains intact.
Therefore, when 5-methylcytosine undergoes hydrolytic deamination, the base generated is thymine.
Summarize that the key concept is that 5-methylcytosine deaminates to thymine, which is important in understanding mutation processes such as C to T transitions in DNA.

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Key Concepts

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

DNA Methylation

DNA methylation involves the addition of a methyl group to the 5th carbon of cytosine bases, commonly forming 5-methylcytosine. This epigenetic modification affects gene expression without altering the DNA sequence and is important in processes like gene regulation and genomic imprinting.
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5-Methylcytosine Deamination

5-Methylcytosine can spontaneously deaminate, losing an amino group and converting into thymine. This chemical change is significant because it can lead to mutations if not repaired, as thymine is a normal DNA base but mismatches with guanine.
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Base Hydrolysis and Resulting Products

Hydrolysis of a methylated cytosine base breaks chemical bonds, leading to the release of the base from the DNA backbone. When 5-methylcytosine is hydrolyzed and deaminated, the base generated is thymine, not cytosine, due to the methyl group and deamination altering its structure.
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Related Practice
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%

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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

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.

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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.

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

531
views
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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