Inhibition of mammalian DNA replication by dichlorobenzimidazole riboside

We have investigated the effect of the nucleoside analogue 5,6-dichloro-1-β- -ribofuranosyl-benzimidazole (DRB) on DNA synthesis in the L-929 line of mouse fibroblasts. Earlier studies have shown that this compound selectively inhibits the synthesis of a major fraction of nuclear heterogeneous RNA (hnRNA). At 60–75 μM, DRB decreases the rate of nuclear hnRNA synthesis by two-thirds and prevents the appearance in the cytoplasm of almost all poly(A)-containing messenger RNA (mRNA). At similar dose levels, DRB inhibits the overall rate of DNA synthesis as measured by thymidine incorporation by only 20% (after correction for decreased thymidine uptake into total cellular material). Analyses of density gradients of BUdR and [³H]thymidine-substituted DNA and of autoradiograms of extended fibers of [³H]thymidine-labeled DNA confirm this inhibition and suggest that DRB acts on DNA solely by reducing the rate of replication fork movement. The apparent size of replication units is unchanged by drug treatment. DRB also inhibits the transport of thymidine into cells. As determined by kinetic studies of thymidine uptake, transport is inhibited competitively by drug treatment. Several other aspects of thymidine metabolism are not affected by DRB. It does not alter the size of the total dTTP pool, and there is no breakdown of template DNA in DRB-treated cells.     

Membrane movements and fluidity during rotational motility of a termite flagellate. A freeze-fracture study

Freeze-fracture electron microscopy was used to examine the structure of a region of plasma membrane that undergoes continual, unidirectional shear. Membrane shear arises from the continual clockwise rotation of one part (head) of a termite flagellate relative to the rest of the cell. Freeze-fracture replicas show that the lipid bilayer is continuous across the shear zone. Thus, the relative movements of adjacent membrane regions are visible evidence of membrane fluidity. The distribution and density of intramembrane particles within the membrane of the shear zone is not different from that in other regions of the cell membrane. Also, an additional membrane shear zone arises when body membrane becomes closely applied to the rotating axostyle as cells change shape in vitro. This suggests that the entire membrane is potentially as fluid as the membrane between head and body but that this fluidity is only expressed at certain locations for geometrical and/or mechanical reasons. Membrane movements may be explained solely by cell shape and proximity to rotating structures, although specific membrane-cytoskeletal connections cannot be ruled out. The membrane of this cell may thus be viewed as a fluid which adheres to the underlying cytoplasm/cytoskeleton and passively follows its movements.     

Effects of autoclave-induced carbohydrate hydrolysis on the growth ofBeta vulgaris cells in suspension

Media of de Greef & Jacobs (1979) were autoclaved either with all the nutrient components in a single vessel (medium 1) or with the following components in separate vessels: FeNa–EDTA+CaCl₂ (medium 2), FeNa–EDTA+NaH₂PO₄ (medium 3) or sucrose (medium 4). Medium 5 was prepared by autoclaving FeNa–EDTA+NaH₂PO₄ and sucrose in two separate vessels. It was found that the dry mass yield of cell suspensions ofBeta vulgaris was lowest in medium 1, followed by media 2 and 3. There was no significant difference among media 3, 4, and 5.The plot of dry mass yield of the cell suspensions against the rates of cyanide-initiated oxygen consumption which indicate the extent of carbohydrate hydrolysis of the media during autoclaving, indicated the presence of a threshold rate of about 17–20 nmol ml^–1 min^–1. Dry mass yield of the suspensions decreased rapidly when the rate exceeded this value.For media with glucose as the source of carbohydrate, the rate of cyanide-initiated oxygen consumption exceeded the threshold value by a factor of 1.5 to 2, depending on the volume of the media autoclaved.Abbreviations FeNa-EDTA ferric monosodium ethylenediamine-tetraacetic acid     

Effects of diphenyl ether herbicides on porphyrin accumulation by cultured hepatocytes

Several diphenyl ether herbicides, such as acifluorfen methyl, have been previously shown to cause large accumulations of the heme and chlorophyll precursor, protoporphyrin, in plants. Lightinduced herbicidal damage is mediated by the photoactive porphyrin. Here we investigate whether diphenyl ether herbicides can affect porphyrin synthesis in rat and chick hepatocytes. In rat hepatocyte cultures, protoporphyrin, as well as coproporphyrin, accumulated after treatment with acifluorfen or acifluorfen methyl. Combination of acifluorfen methyl with an esterase inhibitor to prevent the conversion of acifluorfen methyl to acifluorfen resulted in a greater accumulation of porphyrins than caused by acifluorfen methyl or acifluorfen alone. In vitro enzyme studies of hepatic mitochondria isolated from rat and chick embryos demonstrated that protopor-phyrinogen oxidase, the penultimate enzyme of heme biosynthesis, was inhibited by low concentrations of acifluorfen, nitrofen, or acifluorfen methyl with the latter being the most potent inhibitor. These findings indicate that diphenyl ether treatment can cause protoporphyrin accumulation in rat hepatocyte cultures and suggest that this accumulation was associated with the inhibition of protoporphyrinogen oxidase. In cultured chick embryo hepatocytes, treatment with acifluorfen methyl plus an esterase inhibitor caused massive accumulation of uroporphyrin rather than protoporphyrin or coproporphyrin. Specific isozymes of cytochrome P450 were also induced in chick embryo hepatocytes. These effects were not observed in the absence of an esterase inhibitor. These results suggest that diphenyl ether herbicides can cause uroporphyrin accumulation similar to that induced by other cytochrome P450-inducing chemicals such as polyhalogenated aromatic hydrocarbons in the chick hepatocyte system.     

Influence of the estrous cycle on heterosexual aggression in two strains of rats (S3 and WEzob)

Two strains of rats (S₃ and WEzob), which show different levels of aggression in the laboratory, were tested in repeated heterosexual confrontations. Daily 15-min observations were made of the interactions between a female throughout a complete estrous cycle and the same male partner. In both strains the topography of aggression was similar in males and females, but the frequency of specific parameters varied. Males showed more offensive and females more defensive patterns. The overall level of aggression was very low on the day of estrous, when the female was sexually receptive. There were no differences in any elements of female or male behavior between the other 3 days of the cycle. The results support previous conclusions from single-sex encounters that in rats there is no sexual dimorphism in the ability to show aggression.     

Ciliary reversal without rotation of axonemal structures in ctenophore comb plates

We have used a newly discovered reversal response of ctenophore comb plates to investigate the structural mechanisms controlling the direction of ciliary bending. High K+ concentrations cause cydippid larvae of the ctenophore Pleurobrachia to swim backward. High-speed cine films of backward-swimming animals show a 180 degree reversal in beat direction of the comb plates. Ion substitution and blocking experiments with artificial seawaters demonstrate that ciliary reversal is a Ca++-dependent response. Comb plate cilia possess unique morphological markers for numbering specific outer-doublet microtubules and identifying the sidedness of the central pair. Comb plates of forward- and backward-swimming ctenophores were frozen in different stages of the beat cycle by an "instantaneous fixation" method. Analysis of transverse and longitudinal sections of instantaneously fixed cilia showed that the assembly of outer doublets does not twist during ciliary reversal. This directly confirms the existence of radial switching mechanism regulating the sequence of active sliding on opposite sides of the axoneme. We also found that the axis of the central pair always remains perpendicular to the plane of bending; more importantly, the ultrastructural marker showed that the central pair does not rotate during a 180 degree reversal in beat direction. Thus, the orientation of the central pair does not control the direction of ciliary bending (i.e., the pattern of active sliding around the axoneme). We discuss the validity of this finding for three-dimensional as well as two-dimensional ciliary beat cycles and conclude that models of central-pair function based on correlative data alone must now be re-examined in light of these new findings on causal relations.     

Reaction of poliovirus RNAs with antibodies to double-stranded RNA demonstrated by an immunochemical binding assay.

An immunochemical binding assay was used to investigate the reactivity of radioactively labeled viral RNAs from poliovirus-infected cells with antibodies to the synthetic double-stranded RNA, poly(I)-poly(C). A RNase-free antibody-containing serum fraction was employed. Poliovirus replicative form reacted with the antibodies to poly(I)-poly(C) as well as or better than poly(I)-poly(C). Poliovirus replicative intermediate reacted with the antibodies to a greater extent than poliovirus single-stranded RNA, but both were less reactive than replicative form. The use of the immunochemical binding assay with sucrose-gradient fractions demonstrated that for both poliovirus single-stranded RNA and replicative form the peak of reactivity with the antibodies was coincident with the peak of radioactive material precipitated by trichloroacetic acid. The proportion of replicative intermediate that reacted with the antibody increased in sucrose-gradient fractions containing the more slowly sedimenting RI RNA.     

The cultural evolution of fertility decline

Cultural evolutionists have long been interested in the problem of why fertility declines as populations develop. By outlining plausible mechanistic links between individual decision-making, information flow in populations and competition between groups, models of cultural evolution offer a novel and powerful approach for integrating multiple levels of explanation of fertility transitions. However, only a modest number of models have been published. Their assumptions often differ from those in other evolutionary approaches to social behaviour, but their empirical predictions are often similar. Here I offer the first overview of cultural evolutionary research on demographic transition, critically compare it with approaches taken by other evolutionary researchers, identify gaps and overlaps, and highlight parallel debates in demography. I suggest that researchers divide their labour between three distinct phases of fertility decline—the origin, spread and maintenance of low fertility—each of which may be driven by different causal processes, at different scales, requiring different theoretical and empirical tools. A comparative, multi-level and mechanistic framework is essential for elucidating both the evolved aspects of our psychology that govern reproductive decision-making, and the social, ecological and cultural contingencies that precipitate and sustain fertility decline.