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INSIGHTS OBTAINED FROM INTENSITY AND
MECHANISMS OF DOSE RATE EFFECTS:
FRACTIONATION STUDIES ON CHROMOSOME ABERRATION INDUCTION*
Sheldon Wolff
Biology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee

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*Research sponsored by the U. S. Atomic Energy Commission under contract
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Kunning head: Chromosome aberrations
Send proofs to: Dr. Sheldon Wölfr
Biology Division
Oak Ridge National laboratory
P. 0. Box Y
Oak Ridge, Tennessee
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Studies in radiation genetics have long been the foundation on which
much of the theory of modern radiation biology was built. In the main,
this wae true, because in radiation genetics, one studied the events induced
by the radiation, rather than the lack of the event. For instance, instead
of looking at individuals not affected by radiation (1.e., survivors),
geneticists looked for those arfected (mutants),
The basic studies performed over the years have not only fit the concepts
of target theory, but have actually provided some of its strongest support.
Target theory holds that radiation effects are caused by a simple interaction
of radiation with specific susceptible loci within the cell. Some of the
effects require but a single interaction and are considered to be one-hit
whereas other effects require multiple interactions of radiation and cellular
targets and are therefore multi-hit.
Simple mutation induced in sperm have long been known to be a one-hit
event. By this we mean that the numbers of mutations produced by X-rays
increase linearly with the dose. An 11lustration of this may be seen in Fig.
1 in which are presented some of the early dose action curves for visible
mutations induced in Drosophila sperm. The same phenomenon is quite general
and has been observed in many other organisms.
The meaning of a linear curve of this nature 18 two-fold: (1) That
the mutations are induced with a certain probability that 18 directly
proportional to the dose, and (2) that there is no interaction of the various
lonizations or clusters of ionizations in inducing mutations but that each
mutation is caused by a single ionizing particle or hit. If the latter were

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not true, und an interaction of hits were necessary for the formation of
mutations, then the yield would increase faster than linearly with dose. If
two hits were necessary, for example, then the probability of getting both
hits would be the product of the individual probabilities for each hit. This
Increase as the square of the dose.
In addition to linear dose curves, there 18 further evidence that
mutations are the result of a single ionization or cluster of ionizations
This evidence 18 the generally observed (until recently) lack of a dose
intensity or dose fractionation effect. The argument from this type of evidence
is that if an interaction between independent ionizations were necessary to
produce a mutation, 1.e., mutations were two-nit, then if radiation were
given at low intensities, 4 decay or repair of the effects or some of the
earlier ionizations could occur before the whole dose was given. Thus, the
total numbers of ionizations wouild not be in the cell concurrently. The same
into various portions with rest periods between. A decay or repair of the
effects of one part of the dose could now occur in the interdose intervals,
again leading to a decrease in mutation rate. It mutations, however, are
truly one-hit, i.e., required no interaction of independently induced lonizations,
and are induced immediately, the classical target theory would predict t.lat
there would be no intensity or fractionation effect.
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The early experiments on Induction of sex-linked recessive ellería
in Drosophila sperma show no such decrease either at low intensities or
after fractionation. As a matter of ract, if the results of several workers
are pooled, it is found that an approximately three million-fold variation in
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intensity (from 0.0001r/min to 2'700r/min) does not effect the yield of
mutations. In the mouse, too, it has been found that there 18 no intensity
effect for viøible mutations induced in sperm.
The lack of repair noted for point mutations induced in sperm, however,
has not been the caso in studies of two-hit intergendo mutations that are
the result of chromosome breakage and the subsequent rejoining of the breaks
in new configurations. Some of these new configurations (such as those that
result in inversions, deletions, and translocations) can alter linkage groups
and affect recombination. These would be considered true intergenic
deletions) are generally inviable after one or a few cell divisions and can
be considered intergenic mutations that are dominant cell lothals. Because
this latter class of chromosome aberrations is easily studied by cytologists,
more work of a quantitative nature has been done with these than with
Intárgenic changes that have to be detected by genetic means.
Those chromosome aberrations that require the interaction of two breaks
have long been known to be two-hit (increase as the square of the dose) with
sparsely ionizing radiation. They have similarly been known to be subject
to a repair as might be expected for a two-hit phenomenon. That 18, 1f the
dose 18 fractionated or given at low intensity, fewer aberrations are induced
than 1f the radiation were given in one continuous dose at high intensity.
The rational for this is that many more broken ends are produced than finally
end up in aberrations and that most of the breaks are repaired, 1.e., restituted
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an aberration. This restitutional type of repair reduces the number of ends
available for interaction within a cell. Thus, 17 in a low intensity or dose
Practionation experiment, appreciable restitution occurs before all breaks are
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in the cell, then the total number present concurrently will be lower and
fewer two-break aberrations will be induced.
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The ease of study of two-bruak aberrations by cytological means has
allowed us to carry out experiments that have mada restitutional repair at
present the best understood of all genetic repair processes. We no lcriger need
to talk vaguely in terms of restitution which simply describes the repair,
but now know something about the chomical requirements of the repair process
itself.
The information regarding the repair process was first gained from
dose fractionation experiments in which we were able to separate effects on
breakage from those on rejoining.' In these experiments, a first dose was
given to induce breaks and then a second dose was given at a time after that
ordinarily required for breaks from the first doee to restitute. If after
the first dose various metabolic inhibitors were administered and if the milletine
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process inhibited was necessary for repair, then restitution would not occur.
Thus, when the second dose was given, all of the effects (breaks) induced.
by the first dose would still be extant and there would be no fractionation
effect. Under these circumstances, the yield of two-break aberrations would be
proportional to the square of the dose administered. If, however, restitutional
repair had occurred between the two doses, the yield would have simply been
the sum of those induced by the two separate doses.
Experiments performed in G, cells of Vicia faba seed showed that
cellular respiration and ATP production were necessary for rejoining to contar
48 might be expected since new chemical bonds have to be synthesized when a
broken chromosome 18 repaired. It was further found that the "repair system"
was itself radiation sensitive and that at higher doses repair was inhibited for
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longer times and thus recovery took longer.
We wondered about the nature of the bonds formed. Because chromosomes
consist of protein and nucleic acids, it seemed worthwhile to see if the
synthesis of any or all of these components was involved in the repair. In
Table 1 we see the results we obtained earlier on the effects of chloramphenicol
on the rejoining of breaks. The compound had no effect on the yield induced
after doses of 600r (In vacuo) in G, cells of soaked seeds of Vicia faba,
nor did it have an effect when given before a dose of 300r. When, however,
it was administered between the doses, it kept the breaks from the first dose
from rejoining before the second dose was given. The breaks of the two doses
were thus in the cell concurrently and could interact with one another to
form aberrations.
Since these experiments were performed in G, cells, DNA synthesis was
not thought to occur. This was confirmed in experiments in which we found
that tritiated thymidine was not incorporated into the cells at the time the
experiments were performed. We also found RNA was not affected by the
chloramphenicol treatment by finding that it did not change the incorporation
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of HP Into the nucleic acids. At the concentrations used (300 micrograms/
m2-) chloramphenicol was found only to effect a 30% reduction in Incorporation
of labeled amino acids into the cell's total protein. We have subsequently
checked only nuclear proteins by incubating the cells with tritiated lysine,
extraction the nuclei by the method of Mattingly and making and oradiographs
of nuclei uncontaminated with cytoplasmic proteins. Counts of the grains in
the audoradiographs showed that there 18 a 50% inhibition of incorporation
of labeled amino acids into nuclear protein and therefore presumably a toy
Inhibition of protein synthesis ( .
(1963)
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Because of the effect of chloramphenicol outlined above, and because
11461/
rejoining could occur throughout the cell cycle (even in stages where DNA
synthesis does not normally occur) we postulated that the bonds formed in the
repair of chromosome breaks were proteinacious and not in nucleic acid. We
recognized, however, that the studies on protein inhibition might only have
indicatod that synthesis of an enzyme (protein) was necessary. The formal
possibility still existed that the repair occurred in DNA but required so
little DNA synthesis that it could not be picked up in our experiments with
tritiated thymidine. Boll and I'then performed experiments with 5-florodeoxye-
uridine (FUAR), a compound that Inhibits DNA synthesis in bacterial systems.
We found that it did not prevent breaks in G, cells from rejoining (Table 2).
When it was administered between two doses of x-rays, the yields were the
same as in the controls, i.e., simply the sum of the yields produced by the
two individual doses. That the compound can inhibit DNA synthesis in Vicia
faba was shown by Bell in experimento in which she treated actively rowing
lateral roots that had cells in S. She found (Table )) that sudoradiographs
of FUAR treated roots showed a decrease in incorporation of tritiated
deoxyscypidine into the DNA of the cells.
We are then left with the fact that repair occurs in those portions oť
the cell cycle in which DNA synthesis does not occur and that DNA synthesis
inhibitors do not prevent repair whereas protein synthesis inhibitors do.
We thus, st111 favor the interpretation that the bonds formed in chromosome
rejoining are proteinacious. Although an undetectable amount of DNA synthesis
cannot be completely ruled out as the cause of repair, to argue that it is
responsible for the rejoining seen 18 to argue in the face of experiments that
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By Irradiating cells in G, and inhibiting repair, we have been able to
obtain some information about the chemical nature of the repair processes.
There are, however, certain restrictions on the dose fractionation method that
must be kept in mind when performing experiments of this sort. One such
Kloor
restriction 18 that we have to be certain of the agents used to induce the
chromosome breaks act randomly throughout the genome and exhibit uniform
sensitivity throughout that portions of the cell cycle being studied. These
two restrictions so far have been mot only in studies with radiation on
cell in GZ:
The reasons for this are that cells throughout G, exhibit uniform
sensitivity to radiation. Thus, if the radiation in addition to bumbong
chromosomes also induced mitotic delay or mitotic diversions, the results
will not be affected. Such effects on mitosis will, however, appreciably
change the results if cells from S and Ga (wherein chromatid aberrations are
induced) are studied because these stages of the cell cycle exhibit variable
sensitivity. In general, cells close to mitosis are more añositive than
auch liknya
those farther away. If mitotic reversion underlays occur, then a population
2.10!
of more sensitive cells might be at metaphase and thus scored when compared to
cells that were not delayed as much. Such phenomenon can frequently be
Literarios.com
misleading and confound the experiments (see Wolff, 1964).
When chemical mutagens are used as the breaking agents, the results
also can be confounded because these compounds usually are effective
- Actu
primarily in S and occasionally in G2 (Evans, 1964). They also usually cause
mitotic delays. In addition, they do not affect the genome randomly. For
instance, It has been found that 8-ethoxycaffeine induces breaks in Vicia
faba cells preferentially in the regions around the secondary constrictions
núdzvienen MB)
of the two M chromosomes ; B-propriolactone seems to affect the centrameric
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regions of some of the S chromosomes; maleic hydrazide breaks chromosomes
only at S and , furthermore, If It doesn't induce breakage at the first cycle
of synthesis, the compound remains in the cell and, con induce breaks during
the second replication following its application. “All of these properties
,
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of chemical mutagens make it very difficult to interpret experiments in
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which fractioned doses of the mutagens are administered or even in which one
dose of mutagen plus one dose of x-rays are given.
2. ital.. 1196)
Mermers and Swanson reasoned that different chemical mutagens would
have different chemical activities and thus would induce chromosome breaks
of different chemical natures. They hoped that by performing fractionation
experiments and seeing if breaks from one treatment could interact with breaks
from another, they could find which breaks were chemically similar. From
the known chemical activities of the mutagens, then, it would theoretically
be possible to gain further insight into the nature of the chemical bonus
involved in chromosome breakage and rejoining. Unfortunately, what was found
was that none of the breaks induced by different chemical mutagens Interacted
quantitatively with one another whereas they all reacted with x-ray breaks.
Since the many agents may affect chromosomes in different parts of the cycle
it is not surprising (with hindsight) that they did not find interaction.
The fact that x-rays could produce breaks that would interact with the breaks
from various other mutagens is consistent with what we might expect since
x-rays cause breaks throughout the cell cycle. Thus, x-ray breaks can be
induced in cells that are being affected by the mutagen.
The fact that fractionated x-rays and various chemical mutagen doses
seem to produce breaks that interact, on the surface, seems to indicate that
the original hypothesis in regard to the different mutagens Inducing breaks
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of different chemical bonds might not be tenable. We should remember, however,
that the properties of these compounds causing mitotic delays makes it
very difficult to be certain that the chromatid aberrations seen at any given
time after treatment always come from cells in the same portion of the cell
cycle and thus cells that have the same sensitivity to treatment (see Riegerythinchreine
1963).
Studies with chemical mutagens are further complicated by the knowledge
... : 19616
that the chromosome breaks do not rejoin randomly even after irradiation.
There are but a limited number of places or sites within the nucleus where the
chromosome strands come close enough to one another to rejoin and form an
aberration iſ broken. If a mutagen preferentially affects only certain
regions of the chromosomes, then to get an interaction of breaks, these regions
have to be within sites and, more specifically, within sites at that time in
the cell cycle at which the mutagen is effective.
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The early experiments on repair mechanisms had indicated that intragenic
mutations differed from intergenic mutations in that the former were irreparable.
As outlined earlier, this was almost a corollary of their one-hit nature.
Among the intergenic mutations too, however, there was a class of aberrations
that were independent of intensity and so thought to be irreparable. These
were the one-break as opposed to two-break aberrations. Recently, however,
we have had to modify our thinking in regard to the reparability of one-hit
events both for point mutations and gross chromosome rearrangements.
62)
Waltgen first performed studies on UV Induced mutagenesis in E. coli.
that showed that the final fixation of the mutation did not occur at the time
of irradiation. Although the mutations increased linearly with dose, the
post-irradiation environment of the bacterial cells affected the numbers of
recoverabir mutations. This was not caused by selection and waitgen
postulated that the initial damage inflicted upon the gene was a pre-mutational
damage that was converted to a final mutation at some later time. According
to this concept, at least a part of the action of radiation on genes 18 to
cause a one-hit latent effect. This later effect can be dissipated or
repaired without giving rise to a mutation.
Kimball soon found that a similar repair occurred for one-hit/810w
growth and lethal mutations induced in paramecium by x-rays. His work, as
Witkin
well as that of what they can, indicated that the period of latency could exist until
the cell synthesized DNA. From that point on, all damage that was not repaired
was converted to true mutation. There experiments all indicated that the
pre-mutagenic lesion is in DNA; the nature of the repair process that dissipates
the lesion, however, is still elusive.
:
Although experiments with mature mouse sperm fail to show any dose
intensity effects, experiments by Russell showed that fewer mutations were included
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spermatogonia when the intensity of x-rays was lowered from 90r/min to
Str. indi 1963)
C.009r/min. The frequency did not drop at a lower rates. A similar
intensity effect was found when mouse ovcités were irradiated. Russell has
ruled out cell selection as being the cause of his intensity effect and he
favors the interpretation that the decreased frequency of mutation at lower
intensities is the result of true intracellular repair processes. He has
also found, however, that dose fractionation does not give the same qualitative
results in spermatogonia as does lowering the intensity, 1.e., when the dose
was fractionated, the yield went up instead of down. This has indicated that
there may be shifts in cell sensitivity to radiation.much as we know occur
for chromosome breakage throughout S and Gg. Thus, if the dose is not given
instantaneously, the yield could either go up or down according to the
sensitivity of the genes at the moment irradiated. Such a rapid change in
(1961)
sensitivity has been observed by Tacima et alin'studies on induced mutagenesis
in silkworms. Thus, although the results on mammals and silkworms in the main
fit the concepts of intracellular repair of premutagenic damage, effects
caused by changes in stage sensitivity remain to be ruled out.
Yields of one-hit chromos me aberrations too seem to be affected by
repair. Brewen has found the yield of chromatid aberrations induced in the
corniplepithelium of Chinese hamster increases if a post- irradiation
treatment with protein synthesis inhibitors 18 administered. This has been
interpreted as being a similar phenomenon to that observed above in the dose
fractionation experiments with protein synthesis inhibitors; that 18, that
repair requires protein synthesis and that if repair 18 Inhibited, less
restitution will occur and the yield of aberrations will increase. In
experiments on Gy cells (chromosome aberrations) or Vicia faba, however, we
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yield. .
have never found inhibition to increase the yield after but a single dose
which has led us to postulate that it is the number of breaks in the cell
concurrently and not the time that they remain open that affects the yield. "
in finale carrossenestorid
Brewen has shown that when S and Gg cells (chromatid aberrations) of
Vicia faba are studied the apparent difference is a reflection of the fact that
S and G, are not uniformly sensitive to the radiation. Thus, he thinks that
in Vicia, at least, protein synthesis inhibitors affect mitotic rates so that
at any given time after x-irradiation, a different population of cells are
1 Thischenne in
scored if an inhibitor was given. An arrival of cells at metaphase could
increase the yield of chromatid aberrations as we have found to be the case
10.! .Opin/76.31
when far-red radiation is given with x-rays." Yleid from G, cells which show
uniform sensitivity to x-rays would not be subject to changes caused by
mitotic delays and reversion.
Although we feel confident that post-irradiation treatments after a
single dose do not affect the yield of chromatid aberrations in Vicia faba
root tips, Brewen's original interpretation of his results in Chinese hamster
might still be valid. Kirby-Smith has found, for instance, that a low dose
1
of ultraviolet radiation to dry Tradescantia pollen grains that are all in
the same stage can greatly increase the yield of chromatid breaks induced by
very low doses of x-rays. He has postulated that the reason for the synergism
is that "UV results in the inactivation or inhibition of chromosome rejoining
mechanisms in accordance with the ideas by Wolff". Under such circumstances
X-ray induced primary breaks that ordinarily restitute become available for
aberratior: formation.
Bailey and I habien ) have performed an experiment on chromatia
pollen grains
aberrations induced in Tradescantia tuletospores that is consistent with this
interpretation. We irradiated with either x-rays or ultraviolet radiation
RE
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castruction
dry Tradescantia pollen grains and then søwed these on agaflides with and
without chloramphenicol (to inhibit protein synthesis). Post-Irradiation
treatment with chloramphenicol increased the yield of x-ray but not of UV
induced chromatid aberrations. If chloramphenicol inhibits rejoining, as it
does in Vicia, then we find that such an inhibition has increased the yield
after a single dose much as was found in Brewen's experiment on hamster cells.
With Tradescantia pollen grainş, however, there 18 no question of differential
stage sensitivity and mitotic delays affecting the results because the cells
are all in the same stage when x-rayed. This experiment also slowed no
effect of chloramphenicol after ultraviolet radiation which is to be expected
1f Kirby-Smith was correct in postulating that w Itself inhibits the repair
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th chancesinin artists
Conclusions
fudendo!
In summary, let me say that contrary to early results in radiation
t amantate the same
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genetics, results are now being obtained that indicate intracellular repair
moine dimentica
the maintained in this content
2
of one-hit genetic lesions, capucccur. In some of these cases, however, other
explanations have not/unequivocally been miled out. For instance, we must make
sure that stage sensitivity differences coupled with mitotic delays and
reversions and even cell selection do not obscure the results.
Our position is somewhat clearer in regard to two-hit intergenic
mutations. These have long been known to undergo restitutional repair. By
the use of dose fractionation techniques, we have been able to separatetty
effect repair from breakage and by inhibiting various metabolic processes
between two doses of radiation, have been able to gain some insight into
cheroneal
Which of the processes the involved in repair.
care
.
bertandandi
Wh
Bailey, P. C. and S. Wolff 1964 A comparison of x-ray and ultraviolet-
induced aberrations in pollen tube chromosomes of Tradescantia.
II Influence of protein synthesis inhibition. Rad. Bot. (in press).
sind eiendommentarii recente
Bell, S. and S. Wolff 1964 Studies on the mechanism of the effect of
fluorodeoxyuridine on chromosomes. Proc. Natl. Acad. Sci. US 51
195-202.
sen
Brewen, J. G. 1963 Dependence on frequency of x-ray-induced chromosome
aberrations on dose rate in the Chinese Hamster. Proc. Natl. Acad.
Sci. US 50 322-329.
Evan, H. J. and D. Scott 1964 Influence of DNA synthesis on the production
of chromatid aberrations by x-rays and maleihydrazide in Vicia faba.
Genetics 49 17-38.
ami
ERAAN
t
Kihlman, B. A. 1963 Aberrations induced by radiomimetic compounds and
their relations to radiation-induced aberrations. In, Radiation-
Induced Chromosomal Aberrations, ed. S. Wolff, Columbia U. Press,
New York, pp. 3.00-122.
Klien
Kimball, R. F. 1963 The relation of repair to differential radiosensitivity
in the production of mutations in Paromecium. In, Repair from Genetic
Radiation Damage, ed. F. K. Sobels, Pergamon Press, Oxford, pp. 167-178.
.
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Kirby-Smith, J. S. 1963 Effects of combined UV and x-radiation on
chromosome breakage in Tradescantia pollen. In, Radiation-Induced
Chromosomal Aberrations, ed. S. Wolff, Columbia U. Press, New York,
pp. 203-214.
Lea, D. E. 1946 Actions of Radiations on Living Cells. Cambridge U.
Press.
حمضحسسنسند
Mattingly, Sr. d. 1963 Nuclear protein synthesis in Vicia faba.
Expti. Cell Res. 29 314-326.
Merz, J., C. P. Swanson, and N. S. Cohn 1961 Interaction of chromatid
breaks produced by x-rays and radiomimetic compounds. Science 133
703-705.
Rieger, R. and A. Michollis 1963 on the time period during which
chemically induced chromatid 'breaks are available for interaction.
Expti. Cell Res. 31 202-205.
Russell, W. L. 1963 The effect of radiation dose rate and fractionation
on mutation in mice. In, Repair from Genetic radiation Damage, ed.
F. H. Sobels, Pergamon Press, Oxford, pp. 205-217.
Scott, D. and H. J. Evans 1964 On the nonrequirement for deoxyribonucleic
acid synthesis in the production of chromosome aberrations by
8-ethoxycaffeine. Mutation Research 1 246-156.
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Tozima, Y., s. Kondo, and T. Sado 1961 Two types of dose-rate dependence
of radiation induced mutation rates in spermatogonic and oogonia of
the silkworm. Genetics 46 1335-1345.
*
Witkin, E. M. 1963 "Dark Repair" of mutations induced in Escherichia
coli by ultraviolet light. In, Repair from Genetic Radiation
Damage, ed. F. H. Sobels, Pergamon Press, Oxford, pp. 151-161.
Wolff, s. 19612 Radiation Genetics. In, Mechanisms in Radiobiology,
ed. M. Emerce and A. Forssberg, Academic Press, New York, pp. 419-475.
Wolff, s. 1957 Recent studies on chromosome breakage and rejoining.
In, Advances in Radiobiology, ed. G. C. de Hevesg, A. G. Forssberg,
and J. D. Abbott, Oliver and Boyci, itd. Edinburg, pp. 463-469.
Wolff, s. 1960 Radiation studies on the nature of chromosome breakage.
Am. Nat. XCIV 85-93.
Wolff, s. 19616 Some postirradiation phenomena that affect the
induction of chromosome aberrations. J. Cell and Comp. Phys. 58
suppl. 1, 151-162.
1
Wolff, S. and K. C. Atwood 1954 Independent x-ray effects on chromosane
breakage and reunion. Proc. Natl. Aca. Sci. US 40 187-192.
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Wolff, S. and H. E. Luippold 1958 Metabolism and chromosome break
rejoining. Science 122 231-232.
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comissions
Wolff, S. and H. E. Luippold 1960 On the apparent synergistic effect
of far-red and x-rays in the production of chromatid aberrations.
In, Progress in Photobiology, Proceedings of the 3rd International
Congress on Photobiology. ed. B. Chr. Christensen and B. Buchmann,
Elsevier Publishing Co., Amsterdam, pp. 457-460.
Wolff, s. 1964 Radiation effects as measured by chromosome damage.
ستفتحسسيسية
In, Radiation Cellular Biology. Proceedings of the 18th Annual
Symposium on Fundamental Cancer Research. University of Texas
Press, Austin (in press).
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Fig. 1 - Linear relation of visible mutation to dose of x-rays
(data of Timof eef-Ressovsky and Delbruck. ZIAV 71 322
1936).
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TABLE 1
The effect of protein synthesis inhibitors on chromosome break rejoining
Interval
Expected aberracion
yield
Dose 1
(r; in
vacuo)
Dose 2
(r)
Twochie
aberration yield
per 100 cells
observed
Time
Treatment
Wich
rejoining
Without
rejoining
600
600
75 min. H,0
75 mio. . Chlorom.
75 min. H,0
75 min. Chloram.
75 min. H,O
75 min. Chloram.
300
300
10.0 * 1.8
9.3 # 1.8
9.7 11.9
10.3 +1.9
25.9 #2.4
37.3 * 3.6
07
600
300
300
19.7
19.7
39.3
39.3
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TABLE 2
..
Effect of Chloramphenicol on Incorporation of
H-Lysine into Nuclei of Vicia faba Root Tips
Ave No. Grains
Dose (~)
Chloramphenicol
Nucleus
On
1
23.4
+
12.8
I
21.7
.
1000
+
14.3
1
Out
-
TABLE Y 3
RESULTS OF X-RAY DOSE FRACTIONATION STUDIES WITH VARIOUS TREATMENTS BETWEEN DOS ES
Observed no.
1080
in vacuum
DOSO II
in air
90 min treatment
betweon doses
Expected yiold
ir the two
fractions aro
additive
2-bit
aberrations
per 100 cells
Expected
yield with
complete
interaction'
aberrations
600
*000
100
Coro
sove
33.3
12.3
300
· novo
Dono
Doa
600
300
45.0
45.6
85.9
22.7
sont
300
16.7
DOO
20.0
00.0
73.1
300
.
39.7
38.4
10-5 Muridine
10-5 Muridine
20-5 Muridine
20-5 M FUAR +
10-5 M uridine
10-5 N FUAR +
10-5 Muridine
2005 M FUAR +
10-5 Muridine
35.7
00.0
300
13.7
20.0
00.0
300
51.7
49.4
93.5
*300 cells were scored for all treatments.
tmethod of Wolff and Atwood 25, (Vyield 1 + Vyield 2)2.
TABLE W
Erreot of fuar on the Incorporation of in-deoxyoytidine
into DNA of Vioia faba Root Tips

.21
.
1
Tabloul. Effoct of chloramphenicol on X-ray- and UV-induced oborrarlons In pollon
tube chromosomos of Tradescantia paludosa
Exposure
Inhibitor
prosent
Por cont No. chromatid
colls dolotions por
normal 100 colls
No. isos
chromatld
breaks per
100 calls
Total
aberrations
por 100
colls
X-ray (r)
100
None
ST00
100
200
None
Chloramphenicol
None
100
B
68
6
10
21
13
18
28
17
28
29
200
Nono
200
Chloramphonicol
43
R11
UV (ergs/cm2)
5 X 105 None 92
5x105 Chloramphenicol
Chloramphenicol % 96
1x106
74
!
3
i
22
to
.33
.
Nono
1 X 106
Chloramphenicol
81
1,3
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