'G' bonds are produced when ________ chromosomes are subjected to mild heat and then stained with Giemsa reagent.

The correct answer is option 3

Key PointsG-banding is a technique used in cytogenetics to produce a visible karyotype by staining condensed chromosomes. It is the most common chromosome banding method. It is useful for identifying genetic diseases (mainly chromosomal abnormalities) through the photographic representation of the entire chromosome complement.

• The metaphase chromosomes are treated with trypsin (to partially digest the chromosome) and stained with Giemsa stain.
• Heterochromatic regions, which tend to be rich with adenine and thymine (AT-rich) DNA and relatively gene-poor, stain more darkly in G-banding.
• In contrast, less condensed chromatin (Euchromatin)—which tends to be rich with guanine and cytosine (GC-rich) and more transcriptionally active—incorporates less Giemsa stain, and these regions appear as light bands in G-banding.
• The 'G' bonds are produced by the interaction of the Giemsa stain with the AT-rich regions of the chromosomes. The AT-rich regions are more susceptible to denaturation by heat, which exposes the DNA to the Giemsa stain.
• The Giemsa stain then binds to the DNA, forming the 'G' bonds.
• Hence option 3 is correct.