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Ch. 9 - DNA Structure and Analysis
Klug - Essentials of Genetics 10th Edition
Klug10th EditionEssentials of GeneticsISBN: 9780135588789Non è quello che usi tu?Cambia libro di testo
Capitolo 9, Problema 28

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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Understand that cytosine normally pairs with guanine in DNA through three hydrogen bonds, maintaining the DNA double helix structure.
Recognize that when cytosine undergoes deamination, it is converted into uracil, which is not a normal base in DNA but is normally found in RNA.
Consider that uracil pairs preferentially with adenine through two hydrogen bonds, unlike cytosine which pairs with guanine.
Analyze how the presence of uracil in DNA would lead to a mismatch during DNA replication, potentially causing a C-G to T-A transition mutation if not repaired.
Conclude that the incorporation of uracil disrupts the normal base pairing and can lead to mutations, which is why cells have repair mechanisms like uracil-DNA glycosylase to remove uracil from DNA.

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Spontaneous DNA Lesions and Deamination

Spontaneous DNA lesions are natural chemical changes that occur without external influence. Deamination is the removal of an amino group from a base, such as cytosine converting to uracil, which alters the base's identity and can lead to mutations if unrepaired.
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Spontaneous Mutations

Base Pairing and DNA Structure

DNA structure relies on specific base pairing: cytosine pairs with guanine via three hydrogen bonds. Replacing cytosine with uracil disrupts this pairing because uracil pairs with adenine, potentially causing mismatches and destabilizing the double helix.
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DNA Repair Mechanisms for Uracil

Cells have repair systems like base excision repair to recognize and remove uracil in DNA, preventing mutations. Uracil-DNA glycosylase excises uracil bases, maintaining DNA integrity by restoring correct cytosine-guanine pairs.
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Repair Pathways
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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:

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