During cytogenetic preparation, chromosomes are commonly isolated from metaphase cells by applying which osmotic treatment to swell cells and spread chromosomes?

Introduction / Context:
High-quality metaphase spreads are essential for karyotyping and chromosome banding. A standard step is to expose cells to a hypotonic solution so they swell, facilitating chromosome dispersion after fixation and dropping onto slides.

Given Data / Assumptions:

• Cells are arrested in metaphase (e.g., by colchicine).
• Goal is to separate and spread condensed chromosomes for microscopy.
• Hypotonic treatment swells cells by water influx.

Concept / Approach:
Hypotonic solutions (classically 0.075 M KCl) create an osmotic gradient driving water into cells. This swelling weakens the plasma membrane and disperses chromosomes upon fixation and drop technique. Hypertonic conditions would shrink cells and compact contents, defeating the purpose.

Step-by-Step Solution:

Verification / Alternative check:
Protocols from clinical cytogenetics consistently specify hypotonic treatment before fixation, confirming the correct choice.

Why Other Options Are Wrong:

• Hypertonic lysis compacts cells and is counterproductive.
• “Either” is incorrect because hypertonic does not give identical results.
• Isotonic conditions maintain cell volume; spreads are poor.
• Detergent alone without hypotonic swelling yields clumped metaphases.

Common Pitfalls:
Insufficient hypotonic time leading to crowded plates; overexposure causing chromosome loss.

Final Answer:
Hypotonic lysis