Mutation frequency as conditioned by the manner of application of radiation
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Date
1939Author
Makhijani, J. K.
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Abstract
(I) THE INEFFECTIVENESS OF TEMPERATURE IN
INFLUENCING THE PRODUCTION OF
MUTATIONS BY X -RAYS.
(with 2 Tables and 2 Diagrams) A study has been made of the effect of
temperature during irradiation on the frequency
of X -ray induced lethal mutations, and of trans -
locations between chromosomes X, II and III. The
flies were X -rayed simultaneously and under otherwise
identical conditions, at the temperatures 4° to 6 °C
and 36° to 38° C, resp ectively. No significant
difference was obtained in the two groups. This is
regarded as proving that the temperature coefficient
for lethal mutations and translocations during
X- raying is very nearly 1. Hence the completion
of the secondary chemical processes, leading from
the initial ionisations to the observed mutations,
must be very little affected by temperature. (II) TRANSLOCATION FREQUENCY IN RELATION
TO TIMING OF IRRADIATION (With 5 TABLES): Experiments on Drosophila melanogaster
planned with a view to determining the possible
effect of differences in the time factor on the
production of translocations by X -rays in spermatozoa
clearly show that even long intervals between and
after the exposures do not alter the frequency of
chromosome re-arrangements, provided the total dose
of X -rays, as measured either by ionisation or by the
production of sex-linked lethal mutations, remains the
same. Three groups were compared in the tests of
the frequency of translocations. In one, the
exposure of the inseminated females was continuous an
the flies, after treatment, were allowed to deposit
their eggs at once. The second group was irradiated
similarly and simultaneously, but the deposition of
eggs was postponed for one month, by keeping the
flies at 8°C on syrup food. In the third series
the dosage was divided into four equal fractions,
administered at weekly intervals, and the flies were
bred a week after the last irradiation, i.e., one
month after the first irradiation. The amount of
dosage applied in the three cases was the same, viz.
ca. 1500 r. The results showed no significant differences in the frequencies of translocations. Series to test translocations at a very low
dose (ca. 375 r), which was one-fourth of the above,
were run simultaneously with those mentioned, above.
The results of these tests, combined with those of a
parallel experiment by Raychaudhuri, showed that, for
the range of dosage between 375 r and 1500 r, the
number of translocations is proportional to a power o
the dosage which is certainly well above 1, and is
probably nearer to 2 than. to 1. Sex-linked lethals also were tested
simultaneously in all. these cases, and the results
obtained showed that the differences in timing of the
treatment and breeding caused no significant differences
in their frequencies. As this was expected
on the basis of previous work, these results also
served as a check on the dosage given for translocations. On the basis of the frequency-dosage
relations here found, especially when these are taken
in connection with the results of other experiments,
it is concluded that radiation produces gross rearrangements
by first breaking the chromosomes in
two or more places by means of separate ionisations,
and that the broken ends later undergo recombination. On the basis of the finding that the
results are independent of timing it is further
concluded that the broken ends cannot rejoin so long
as they are included within the free spermatozoon,
but that after fertilisation some change in
conditions make union of the pieces possible.