Structural paradox of polytene chromosomes.

Area of Science:

• Molecular Biology
• Genetics
• Cell Biology

Background:

• Polytene chromosomes in Diptera exhibit distinct band and interband regions.
• Previous estimates suggested interbands contain minimal DNA (3-5%) and mass.

Purpose of the Study:

• To accurately estimate the chromatin and DNA content of interbands in Dipteran polytene chromosomes.
• To resolve the discrepancy between observed thick interband fibers and low DNA estimates.

Main Methods:

• Whole-mounted polytene chromosomes from Drosophila melanogaster were analyzed.
• High-voltage electron microscopy and densitometry were used to measure cross-sectional dry mass.

Main Results:

• The average ratio of band to interband mass was estimated to be less than 4:1.
• Interband chromatin contributes at least 26% of the total chromosome mass.
• DNA proportion in interbands is similar to bands, implying 26% of the euchromatic genome is in interbands.

Conclusions:

• Interbands contain substantially more DNA than previously thought, challenging existing models.
• A revised model of polytene chromosomes incorporates local variations in polyteny levels.
• This reevaluation necessitates reinterpretation of Dipteran genetic and molecular data.