Section 26
Chapter 25,725

The determination of the dosage relations of chromosomal alterations in B. romana induced by x-rays

Marquardt, H.

Zeitschr F Bot 37(6/7): 241-317, / 42


Accession: 025724084

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Flower buds of B. romana were irradiated with exact doses of x-rays (50-450 r). Two days later 6392 chromosomes in metaphase and early anaphase of the first pollen-grain mitosis were analyzed for chromatid and chromosomal fragmentations and restitutions. The basis of exact dosage-dependence detn. (morphology of the alterations, possibility of quantitative observation, influence of the time factor in radiating) is discussed. The data were subjected to an exact statistical and critical evaluation. Restitution number is related exponentially to dosage, while fragmentations show a linear relationship, the curve flattening with dosages 150-200 r. Consequently, fragmentation is to be regarded as resulting from a single hit, restitution from 2 or more hits. The new hypothesis of the author distinguishes between "restitution-hits" and "fragmentation hits," the former producing a surface instability only, with return to the normal state, the latter causing a more extended "fragmentation condition" which may return to the normal state or lead to definitive fragmentation. Restitution occurs when both these primary events or their consequences coincide, and at higher dosages when 2 "fragmentation conditions" are also present. The author shows how non-visible chromosomal alterations may be calculated if the laws of probability of the possible break combinations are known. Consequently, the true frequency of chromosomal and chromatid mutations in the stages of mitosis may also be evaluated. This opens up a new way to analyse the physiological origin of mutation.

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