Learning Objectives

• Explain the mechanism of Colchicine in arresting cells during mitosis.
• Describe the criteria used to order and number chromosomes.
• Identify clinical indications for Karyotyping, including prenatal and oncologic testing.

1. The Procedure: Arresting the Cell Cycle

Karyotyping requires cells that are actively dividing. To visualize chromosomes clearly, the process must be halted when they are most condensed.

• Colchicine: This alkaloid is added to cultured cells. It works by disrupting the assembly of mitotic spindles (microtubules).
• Mitotic Arrest: By preventing spindle formation, Colchicine arrests cells in metaphase of mitosis.

2. Chromosome Organization

Once arrested, chromosomes are stained (typically G-banding), ordered, and numbered. The classification is based on several morphological features:

• Size: Arranged from largest to smallest.
• Morphology: Position of the centromere (metacentric, submetacentric, or acrocentric).
• Arm-length ratio: The ratio between the short arm (p) and long arm (q).
• Banding Pattern: Unique light and dark bands created by staining.

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3. Clinical Utility

Karyotyping can be performed on various tissues, including blood, bone marrow, amniotic fluid, or placental tissue.

[Image of trisomy 21 Down syndrome karyotype]

• Chromosomal Imbalances: Used to diagnose autosomal trisomies (e.g., Down syndrome, Edwards syndrome) and sex chromosome disorders (e.g., Turner syndrome, Klinefelter syndrome).
• Oncology: Identifying translocations or extensive abnormalities in cancer cells (e.g., the Philadelphia chromosome in CML).

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