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Difficulty: Very hardGene Mutations and Chromosomal Aberrations

Match each type of genetic alteration listed on the left with its precise molecular or cytogenetic mechanism on the right.

  • Transition mutationReplacement of a purine nucleotide by another purine, or a pyrimidine by another pyrimidine, preserving chemical ring classification.
  • Transversion mutationReplacement of a double-ring purine nucleotide by a single-ring pyrimidine (or vice versa), altering ring structure across base pairs.
  • Pericentric inversionTwo chromosomal breaks occurring on opposite sides of the centromere followed by a 180∘180^\circ rotation, incorporating the centromere into the inverted segment.
  • Robertsonian translocationCentromeric fusion of the long arms (qq arms) of two non-homologous acrocentric chromosomes accompanied by loss of tiny short arms, reducing total chromosome count.

Answer

Transition mutation matches replacement of a purine by another purine (or pyrimidine by pyrimidine). Transversion mutation matches substitution of a purine with a pyrimidine (or vice versa). Pericentric inversion matches chromosomal breaks flanking the centromere with 180∘180^\circ inversion containing the centromere. Robertsonian translocation matches centromeric fusion of two acrocentric long arms resulting in loss of short arms and reduced chromosome number.
Transition mutation corresponds to swapping purine-for-purine (A↔GA \leftrightarrow G) or pyrimidine-for-pyrimidine (C↔TC \leftrightarrow T). Transversion mutation corresponds to swapping purines for pyrimidines (A/G↔C/TA/G \leftrightarrow C/T). Pericentric inversion includes the centromere between two break points prior to rotation. Robertsonian translocation specifically joins the qq arms of acrocentric chromosomes near the centromere, shedding the non-essential heterochromatic pp arms.

Step-by-Step Solution

1
Classify point mutations by chemical base structure alteration
Transition mutations exchange like-for-like ring structures (purine to purine or pyrimidine to pyrimidine), whereas transversion mutations swap single-ring pyrimidines with double-ring purines or vice versa.
This establishes the precise molecular distinction between point substitution categories.
2
Differentiate structural chromosomal inversions
Pericentric inversions involve breaks on both sides of the centromere (including it in the inverted segment), unlike paracentric inversions which occur entirely within one chromosome arm (excluding the centromere).
Including the centromere can alter arm ratios and morphological appearance of the chromosome.
3
Identify special translocation mechanisms involving acrocentric chromosomes
Robertsonian translocation specifically involves breakage near centromeres of acrocentric chromosomes, causing long arms to fuse into a single metacentric or submetacentric chromosome.
This reduces the overall functional chromosome count (2n=452n = 45 in balanced carriers).

Key Concept

Distinction between point gene mutation mechanisms (transitions vs transversions) and structural/numerical chromosomal aberrations (pericentric inversions vs Robertsonian translocations).
Estimated Time:2m 0s
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Topics

Gene Mutations and Chromosomal Aberrations

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