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