CYTOPHOTOMETRIC DETERMINATION OF DNA CONTENT INNICOTIANAMEGACHROMOSOMES

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Abstract
DNA content in normal Nicotiana chromosomes and in unusually long chromosomes called megachromosomes was determined by Feulgen cytophotometric techniques. DNA content estimates expressed as per cent transmittance were made at specific points on a megachromosome and the mean value for these measurements was compared to the mean transmittance values for normal chromosomes in the same pretreated metaphase cell. DNA measurements for interphase cells, unpretreated metaphase cells and the metaphase chromosome which forms the megachromosome were also obtained. Our observations support the conclusion that comparable unit areas of normal chromosomes and megachromosomes contain equivalent amounts of DNA. The finding of DNA amounts in megachromosomes comparable to DNA levels in normal chromosomes is interpreted as evidence for a replicative mechanism of megachromosome origin instead of a simple extension of the DNA in a chromosome of normal length throughout the length of a megachromosome.