Tritiated uridine induced chromosome aberrations in relation to heterochromatin and nucleolar organisation in Microtus agrestis L

Insect sex chromosomes, XI. 3H-TdR induces random aberrations in the X chromosome(s) of Gryllotalpa fossor (Orthoptera)

The pattern of titrated thymidine (3H-TdR), a direct precursor of DNA, induced aberrations on the X chromosome of Gryllotalpa fossor was examined. 3H-TdR produced aberrations randomly distributed over the entire length of the X chromosome; breaks were observed in both the eu- and the heterochromatic arms of the X chromosome in both the sexes. Since the eu- and the heterochromatic arms cannot be distinguished cytologically in this insect, the presence of aberrations on both arms of the same X chromosome in the male and damage to both X chromosomes in the female indicate that both euchromatic and heterochromatic regions (facultative or constitutive) are equally liable to aberrations induced by H-TdR. This is in contrast to the non-random induction of aberrations by 3H-UdR, which causes chromosome damage due to the proximity of the labeled RNA to the DNA template during transcription.

Can 3H-uridine-induced sister chromatid exchange be used as a measure of transcriptional activity of chromosomes?

In order to test the validity of using 3H-uridine-induced chromosome damage as a measure of the transcriptional activity of chromosomes, experiments were performed to examine the mechanism by which 3H-uridine induces sister chromatid exchanges. DON and B14FAF28 Chinese hamster cells exposed to 3H-uridine showed a dose-dependent increase in SCE frequency, whereas unlabelled uridine produced no increase. 3H-uridine labelling in the presence of increasing concentrations of unlabelled uridine eliminated this increase in a concentration dependent manner. Competition between 3H-uridine and other unlabelled pyrimidines (thymidine and cytidine) was equally as effective; however, unlabelled hypoxanthine had no effect on 3H-uridine-induced SCEs. This suggested that 3H-uridine caused SCEs after interconversion into deoxycytidine followed by incorporation into DNA. Significant incorporation of 3H into DNA was confirmed by direct comparisons of label in the DNA and RNA fractions, and about 90% of this label was found to be in the cytosine fraction of hydrolysates. We therefore conclude that 3H-uridine produces chromosome damage because of the incorporation of a tritiated conversion product into DNA, rather than by exposure of transcriptionally active DNA to 3H-uridine-labelled RNA. We suggest therefore, that the results of earlier experiments, in which 3H-uridine chromosome damage has been used to assay the transcriptional activity of chromosomes, may need to be reinterpreted.

Insect sex chromosomes. VI. A presumptive hyperactivation of the male X chromosome in Acheta domesticus (L.)

The functional status of the X chromosome in Acheta domesticus has been analysed at the whole chromosome level on the basis of (1) 3H-thymidine autoradiography, (2) 5-BrdU/AO fluorescence microscopy (3) in vivo 5-BrdU incorporation and (4) 3H-UdR induced aberrations. The rationale of these techniques in relation to the functional aspect of the X chromosome is that the inactive X chromosome would (1) show asynchrony in DNA synthesis, (2) show differential fluorescence, (3) respond differentially to in vivo 5-BrdU treatment and (4) the active X chromosome would show aberrations when treated with 3H-Uridine. From the results, it appears that the X chromosomes in both male (XO) and female (XX) somatic cells of Acheta are euchromatic (active). Further, the single X in the male is transcriptionally as active as the two X chromosomes in the female. In other words, the single X in the male is hyperactive when compared with the single X in the female. From this it is inferred that the male X chromosome is differentially regulated in order to bring about an equalization of it's gene product(s) to that produced by both Xs in the female. Drosophila melanogaster has a comparable system of dosage compensation. Thus, Acheta is yet another insect showing evidence for an X chromosome regulatory mechanism of dosage compensation. Additionally, it is surmised that sex determination in Acheta is based on an autosomes/X chromosome balance mechanism.

Differential sensitivity of muntjac lymphocyte chromosomes to mitomycin C, bromodeoxyuridine and hydroxylamine at different cell-cycle stages

Quantitative and qualitative analyses were made of aberrations induced by 3 hitherto well-known mutagens, mitomycin C (MC), 5-bromodeoxyuridine (BUdR and hydroxylamine hydrochloride (HA), in muntjac chromosomes, during different stages of the cell cycle. The sensitivity to MC was increased in G1, reached its maximum in early S and was considerably decreased in late S and G2 stage treated cells. BUdR induced maximal aberrations when given during the synthetic phase and the cells in G1 and G2 were least affected. The sensitivity of the cells to HA in terms of induced chromosomal aberrations increased as they moved through the cell cycle, i.e. more damage was observed in cells treated in late S and G2 stage than in those treated at G1 and early S stages. While there were defined patterns of cell-cycle stage-dependent sensitivity for all 3 chemicals, the chromosomal sites being preferentially affected by each were found to be specific and invariant at different stages. Thus, it is presumed that the functional state of such "preferred sites" at one or other stage of the cell cycle is the factor responsible for the stage-dependent sensitivity of a cell towards these chemicals.

Non-random distribution of aberrations and identification with C- and G-bandings of the position of breakage points on Muntjac chromosomes induced by mitomycin c, bromodeoxyuridine and hydroxylamine

The analysis of chromosomes from muntjac after treatment of its lymphocyte cultures with 3 chemical mutagens having different base-pair affinities and modes of action, namely mitomycin C (MC), 5-bromodeoxyuridine (BUdR) and hydroxylamine hydrochloride (HA), with G- and C-band staining displayed non-random distribution of chemically specific damage points on them. The randomness of the involvement of each site on the chromosomes were examined by assuming an expected value calculated on the basis of its relative mitotic length. The observation revealed that a large fraction of MC-induced aberrations was preferentially located in the C-band positive constitutive heterochromatin, especially in the long "neck-like" centromeric region of the X-chromosome. On the chromosomal arms, the light G-bands were involved in aberrations either in proportion to or higher than that expected. When the cells were treated with BUdR, the dark G-bands on all the chromosomes of the complement were the preferred sites, displaying statistically significant higher numbers of aberrations. A single "hot-spot" for induced damage on 1 mid-q was also recorded. HA induced a very high frequency of damage in the secondary constriction regions of the chromosome pairs 1, X and Y2, and the frequency was slightly lower than this in the centromeres of 1, 2 and X chromosomes. The observation of specific distribution of damage points induced by the 3 chemicals lead to the suggestion that, though the effect of a chemical on chromosome segments depends on several factors, each being partially responsible for the end result, it is perhaps primarily depended by the chemical's base-pair affinity and mode of action.

Molecular mechanisms involved in the production of chromosomal aberrations. II. Utilization of Neurospora endonuclease for the study of aberration production by X-rays in G1 and G2 stages of the cell cycle

Chinese hamster ovary cells (CHO) were X-irradiated in G1 and G2 stages of the cell cycle and subsequently Neurospora endonuclease (NE) (E.C.3.1.4), an enzyme which is specific in cleaving single-stranded DNA, was introduced into the cells, after making the cells permeable by treatment with inactivated Sendai virus. With this treatment all classes of X-ray-induced chromatid aberrations increased in G2 cells, whereas in G1 cells an increase in chromosome type of aberrations was found, associated with a profound induction of chromatid type of aberrations as well. Duration of the availability of single-strand gaps for the action of NE has been studied in G2 cells following X-irradiation and the influence of different parts of the G2 stage on the type and frequencies of chromatid aberrations was discerned. While the increase in chromosome type of aberrations by NE in X-irradiated G1 cells has been interpreted as due to the conversion of DNA single-strand breaks or gaps to double-strand breaks by NE, the induction of chromatid aberrations in G1 has been assumed to be due to conversion of some of the damaged bases into strand breaks by NE. Biochemical evidence is presented for the conversion by NE of DNA single-strand breaks induced by X-rays into double-strand breaks using neutral sucrose gradient centrifugation.

The chromosomal localisation of satellite DNA in Ptyas mucosus (Ophidia, Colubridae)

Ptyas mucosus male DNA has a repetitious DNA satellite (p = 1.700 g cm-3) constituting 5% of the haploid genome. In situ hybridisation of radioactive complementary RNA (cRNA) has revealed that satellite sequences are located in the centromeric region of one pair of macrochromosomes and in the terminal region of 8 pairs of microchromosomes. These regions are constitutively heterochromatic as revealed by C-banding. The possibility of involvement of satellite rich microchromosomes in nucleolus organisation is discussed.

X-ray induced visible alterations in the giant chromosomes of Phryne cincta (Nematocera, Diptera): relation of radiation sensitivity to pronuclear chromosome structure

In order to induce chromosomal rearrangements, males were exposed to x-rays and then mated to non-irradiated females. The number of each type of structural alteration was determined by examination of the polytene chromosomes of the F1 progeny. -- A comparison of the results with similar studies made on Drosophila revealed a significantly greater sensitivity in Phryne. Parallel to that an extremely high frequency of small inversions was ascertained in Phryne, and the observed ratio of inversions to translocations was the inverse of that which would be expected from purely mathematical considerations based on the lengths of the different chromosomes. These facts allow the conclusion that the paternal pronuclear chromosomes in Phryne are highly spiralized. Besides, the kinetochore-to-translocation-breakpoint distance was measured in both of the chromosomes involved in each reciprocal translocation and the differences (kinetochore-break distance differences) were registered and from them the arrangement of the chromosomes in the pronucleus of Phryne deduced. The data obtained support the assumption of an ordered, polar-field type of orientation. In Drosophila, in contrast, the comparable data showed that the pronuclear chromosomes are not spiralized and are randomly arranged (Bauer, 1939). -- These results seem to indicate that a close correlation exists between the different radiation sensitivities of Drosophila and Phryne and the different states of spiralisation and arrangements of their chromosomes in the pronucleus stage. It is hypothesized that the influence of the maternal genome on the degree of spiralization of the paternal chromosomes could account for differences in the pronuclear chromosome structure of both species.

Constitutive heterochromatin and micronucleoli in the human oocyte at the diplotene stage

In the diplotene stage of the human oocyte, the processes of elaboration of the nucleolar material are amplified. The principal nucleoli are more voluminous but their relations with the secondary constrictions and the satellites of the D and G chromosomes are not modified. Numerous micronucleoli, frequently to the number of 15-20 this stage. The most remarkable point is their association to various segments of constitutive heterochromatin: centromeric regions, secondary constrictions of the C9 and probably of the A1 and E16. These observations reveal that the human oocyte at the diplotene stage shows an amplification of the ribosomal cistrons. This phenomenon is homologous, to a more reduced scale, of this described from the inferior vetebrates. Besides, the role of heterochromatin in the synthesis of nucleolar material without the intervention of the classic nucleolar organizers is suggested.