Search PubMed⌕ Search

Biomedical subjects

R T Johnson

Publications and source records attributed to R T Johnson.

At least 253 records · Page 14Linked to original sources
PubMed1975

The induction of DNA synthesis in the chick red cell nucleus in heterokaryons during the first cell cycle after fusion with HeLa cells.

Induction of DNA synthesis in embryonic chick red cells has been examined during the first and second cell cycles after fusion with HeLa cells synchronized in different parts of G1 and S-phase. The data indicate that: (i) the younger the embryonic blood the more rapidly the red cells are induced into DNA synthesis; (ii) the greater the ratio of HeLa to chick nuclei in the heterokaryon, the more rapidly the induction occurs; (iii) DNA synthesis in the chick nucleus can continue after the HeLa nucleus has left S-phase and entered either G2 or mitosis; (iv) the induction potential of late S-phase HeLa is somewhat lower than that of early or mid S-phase cells; (v) less than 10% of the chick DNA is replicated during the first cycle after fusion and only a small proportion (15%) of the chick nuclei approach the 4C value of DNA during the second cycle after fusion; (vi) the newly synthesized DNA is associated either with the condensed regions of the nucleus or with the boundaries between condensed and non-condensed regions; (vii) the chick chromosomes at the first and second mitosis after fusion are in the form of PCC prematurely condensed chromosomes); they are never fully replicated and are often fragmentary; (viii) DNA synthesis in the chick nuclei is accompanied by an influx of protein (both G1 and S-phase protein) from the HeLa component of the heterokaryon.

Amino Acids↗
PubMed1975

Perturbation of mammalian cell division. II. Studies on the isolation and characterization of human mini segregant cells.

A method is described for the isolation, according to size, of mini segregants produced by the abnormal cleavage of reversibly arrested mitotic HeLa cells. Many of these mini segregants contain small amounts of DNA, as judged by Feulgen staining and chromosome analysis. After fusion with mitotic HeLa cells, the interphase chromosomes of the mini segregants are seen as either monovalent or bivalent prematurely condensed chromosomes (PCC), some of which are damaged. A proportion of isolated mini segregants synthesize DNA, RNA and protein. Fusion of mini segregants with interphase HeLa cells gives rise to cells with 'hybrid' karyotypes.

Cell Division↗
PubMed1974

Analysis of interphase chromosome damage by means of premature chromosome condensation after X- and ultraviolet-irradiation.

Sendai virus-mediated fusion between mitotic and interphase mammalian cells causes the rapid condensation of the interphase chromosomes into distinct structures, a process termed premature chromosome condensation. This phenomenon has been used to assess the immediate action of x-rays and ultraviolet light on the chromosomes of HeLa cells irradiated in the G1 phase of the life cycle. X-irradiation produces fragmented chromosomes; but even the most finely chopped fragments retain the condensed morphology characteristic of the premature chromosome condensation of unirradiated G1 cells. For doses up to about 1800 rads, the increase in the number of fragments is linearly related to the dose. One mean lethal dose (about 100 rads) yields a net increase of 10-15 fragments per G1 cell, which is considerably larger than previous estimates based on scoring of mitotic chromosomes. Incubation of irradiated cells produces a rapid (within 2 hr) reduction in the number of fragments, indicative of a rejoining process. The decrease in the number of pieces is not accompanied by unscheduled DNA synthesis detectable by radioautography. G1 chromosomes of cells irradiated with UV light in G1 phase are not appreciably fragmented but are elongated and attenuated so that they resemble the premature-chromosome-condensation chromosomes of unirradiated S-phase cells. Both the degree of "S-like" character attained by the G1 chromosomes in a cell and the percentage of the cell population displaying the G1 --> S transition increase with the dose and incubation time after irradiation. Thus, in contrast to the immediate manifestation of damage from x-rays, the maximum induction of the "S-like" state does not occur until about 2 hr after irradiation. The "S-like" chromosomes are capable of unscheduled DNA synthesis. We suggest that the difference in chromosome morphology found after UV- and x-irradiation underlies the reason why the former, but not the latter, induces unscheduled DNA synthesis in G1 cells.

Cell Fusion↗
PubMed1974

Visna virus infection of American lambs.

Random-bred fetal and 4-week-old American lambs, inoculated intracerebrally with visna virus, developed a persistent infection in the brain and sometimes in the lung. The pathologic changes present in these lambs were similar to the early lesions of visna in Icelandic sheep, thus providing a possible model for the study of virus-induced demyelinating disease.

Animals↗
PubMed1974

Age dependence of viral expression: comparative pathogenesis of two rodent-adapted strains of measles virus in mice.

The pathogenesis of two rodent-adapted strains of measles virus was studied in 1- to 2-day-old suckling and 4-week-old weanling BALB/c mice. Both the mouse-adapted Edmonston (MAEd) strain and the hamster-neurotropic (HNT) strain caused necrotizing giant-cell encephalitis with a 90 to 100% mortality after intracerebral inoculation into suckling mice. After intracerebral inoculation into weanling mice, MAEd virus caused fatal disease in 20% of the mice; HNT virus caused fatal disease in 30%, but an additional 35% of these mice developed disease and then recovered. Even when mice were moribund there was little histological evidence of disease in weanling mice inoculated intracerebrally with either strain of virus. Fluorescent-antibody staining showed extensive measles virus antigen in the suckling mouse brain and focal areas of measles virus antigen in the weanling mouse brain. Infectious virus was recovered easily from the brains of suckling mice by plaquing on Vero cells, but no infectious virus could be recovered similarly from weanling mice. However, virus could be recovered by intracerebral inoculation of weanling mouse tissue homogenates into suckling animals. The immune response appeared to play no role in the recovery from infection or in these age-related differences in disease. It appears that maturation of the cells of the mouse central nervous system converted the production of measles virus from the infectious form in the suckling mouse to a primarily defective infection in the weanling mouse.

Age Factors↗