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Difficulty: HardGene Mutations and Chromosomal Aberrations

Nitrous acid (HNO2\text{HNO}_2) acts as a chemical mutagen by inducing the oxidative deamination of cytosine, converting it into uracil (UU). If a double-stranded DNA molecule containing a newly formed uracil base undergoes two consecutive rounds of DNA replication without prior enzymatic repair, which specific base-pair substitution will ultimately be fixed in the mutant daughter DNA molecule?

  1. A C⋅G→T⋅AC\cdot G \rightarrow T\cdot A transition mutationAnswer
  2. B
    A C⋅G→A⋅TC\cdot G \rightarrow A\cdot T transversion mutation
  3. C
    A C⋅G→G⋅CC\cdot G \rightarrow G\cdot C inversion mutation
  4. D
    An inherited somatic adaptation acquired directly to resist chemical toxicity

Answer

A C⋅G→T⋅AC\cdot G \rightarrow T\cdot A transition mutation will be fixed in the daughter DNA molecule.
Deamination of cytosine creates uracil. Uracil pairs with adenine during the first replication round. In the subsequent round, adenine pairs with thymine, substituting the original cytosine-guanine pair with a thymine-adenine pair (C⋅G→T⋅AC\cdot G \rightarrow T\cdot A).

Step-by-Step Solution

1
Identify the primary chemical effect of nitrous acid on cytosine.
Nitrous acid removes an amino group (oxidative deamination) from cytosine, turning it into uracil (UU).
Uracil is a nitrogenous base with hydrogen-bonding properties similar to thymine.
2
Trace the base pairing during the first cycle of DNA replication.
The template strand containing uracil (UU) pairs with adenine (AA) during DNA synthesis, producing a U−AU-A base pair intermediate.
DNA polymerase recognizes uracil as equivalent to thymine during template reading.
3
Trace the base pairing during the second cycle of DNA replication.
The strand with adenine (AA) serves as a template and pairs with thymine (TT), producing a stable A−TA-T (or T−AT-A) double helix.
Standard complementary base pairing pairs adenine with thymine in normal DNA.
4
Compare the original wild-type base pair with the final mutant base pair.
The original C⋅GC\cdot G base pair has been replaced by a T⋅AT\cdot A base pair. Because a pyrimidine-purine pair is replaced by another pyrimidine-purine pair, it is classified as a transition mutation.
Point mutations that exchange a pyrimidine for a pyrimidine (C→TC \rightarrow T) are transitions.

Key Concept

Chemical Mutagenesis and Base-Pair Transition Mutations
Estimated Time:2m 0s
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Topics

Gene Mutations and Chromosomal Aberrations

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