Question

In Q-banding experiments, the light fluorescent band indicates that the chromosomal region is rich in base pairs-

• A. T only
• B. A only
• C. A and T
• D. G and C

Hint:

Mnemonic: "Bright Q-bands stay AT the top" - Bright Q-bands signify AT-rich regions.
Remember that Q-banding was the first chromosome banding method developed (by Caspersson et al. in 1970).

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks about the molecular basis of the staining pattern observed in Q-banding, a cytogenetic technique used for chromosome identification.
Q-banding involves staining chromosomes with a fluorescent dye called quinacrine mustard or quinacrine dihydrochloride.
The resulting patterns of bright and dull bands are characteristic for each chromosome pair.

Step 2: Detailed Explanation:
1. Chromosome banding techniques like Q-banding and G-banding (Giemsa) allow scientists to distinguish between different regions of the genome based on their DNA composition and chromatin structure.
2. Quinacrine is a fluorescent dye that intercalates between the base pairs of DNA.
3. The intensity of fluorescence depends on the local base composition of the DNA.
4. Regions of the chromosome that are rich in Adenine (A) and Thymine (T) base pairs tend to show bright (light) fluorescence.
5. This is because AT-rich regions enhance the fluorescence of the quinacrine molecule.
6. Conversely, regions rich in Guanine (G) and Cytosine (C) base pairs appear dull or dark because the GC pairs quench the fluorescence of quinacrine.
7. Q-bands often correspond closely to the dark bands seen in G-banding (G-bands), both typically representing late-replicating, gene-poor, AT-rich heterochromatin.
8. Therefore, the "light" or bright fluorescent bands seen in a Q-banding experiment indicate regions highly concentrated with AT base pairs.
Final Answer:
The bright or "light" fluorescent bands in Q-banding specifically indicate regions rich in A and T base pairs.
Thus, option (C) is the correct choice.