IK

Principles of Medical Biology - Cytogenetics

Cytogenetics

  • Study of the structure and properties of chromosomes.

  • Examines chromosome behavior during:

    • Somatic cell division (growth and development).

    • Reproduction (mitosis and meiosis).

    • Germ cell division.

  • Studies the influence of chromosomes on phenotype.

  • Investigates factors causing chromosomal changes.

Chromosome Structure and Nomenclature

  • DNA forms a double helix.

  • The double helix is wound around histones.

  • The DNA/histone complex forms the chromosome structure.

Standard Nomenclature for Karyotype Designation

  • Components:

    • Chromosome number.

    • Arm symbol (p or q).

    • Region number.

    • Band number.

  • Description of chromosome abnormalities.

  • Total number of chromosomes, including sex chromosomes.

  • Sex chromosome constitution.

  • Numerical abnormalities.

  • Example: Female Down syndrome (trisomy 21) is written as 47,XX,+21.

  • Structural changes are designated by letters (e.g., 'dup' for duplication).

  • Example of duplication: 46,XY,dup(1)(q22q25) indicates a duplication of a segment on the long arm (q) of chromosome 1, in region 2 between bands 22 and 25.

Chromosome Preparation

  • Source: Nucleated body cells from an individual.

    • Peripheral blood lymphocytes (3-day culture).

    • Skin biopsy fibroblasts (15–21 day culture).

  • Steps:

    1. Addition of colchicine: inhibits mitotic spindle formation.

    2. Hypotonic solution: disperses chromosomes.

    3. Fixation: chromosomes are fixed on a slide.

    4. Staining: chromosomes are stained.

Karyotype

  • Characterization of an individual's chromosomal complement.

  • Includes the number, form, and size of the chromosomes.

  • A photomicrograph of chromosomes arranged according to a standard classification.

  • Chromosome banding pattern: alternating light and dark stripes along the chromosome's length after staining.

  • Unique banding pattern identifies each chromosome.

Chromosome Banding Techniques and Staining

  • G-banding:

    • stains DNA regions rich in adenine and thymine.

    • Giemsa is the most commonly used stain.

    • Requires pretreating chromosomes with a proteolytic enzyme like trypsin.

  • R-banding:

    • Pretreating cells with a hot salt solution denatures DNA rich in adenine and thymine.

    • Chromosomes are then stained with Giemsa.

  • C-banding:

    • Stains areas of heterochromatin.

    • Heterochromatin is tightly packed and contains repetitive DNA.

  • NOR-staining:

    • NOR stands for