Constriction Size Research Articles

Location of genes coding for 18S and 28S ribosomal RNA within the genome of Mus musculus.

Cytological detection of cistrons coding for 18S and 28S ribosomal RNA (rRNA) within the genome of Mus musculus inbred strain SEC/1ReJ was accomplished using the technique of in situ hybridization. Metaphase chromosome spreads prepared from cultured fetal mouse cells were stained with quinacrine-HCl and photographed. After destaining, they were hybridized to Xenopus laevis tritiated 18S and 28S rRNA, specific activity 7.5 X 10(6) dpm/mug. Silver grains clustered over specific chromosomes were readily apparent after 4 months of autoradiographic exposure. The identity of the labelled chromosomes was established by comparing the autoradiographs to quinacrine photographs showing characteristic fluorescent banding of the chromosomes in each metaphase spread. The 18S and 28S rRNA was found to hybridize to chromosomes 12, 18, and 16. Statistical analysis of the grain distribution over 26 spreads revealed that the three chromosomes were significantly labelled. Grains over these chromosomes were concentrated in an area immediately distal to the centromere, a region which in chromosomes 12 and 18 in this particular strain is the site of a secondary constriction. The relative size of the secondary constrictions, long and thus prominent on chromosome 12, obvious but shorter on 18, and indistinguishable on chromosome 16, correlated with the average number of grains observed over the centromeric region of these chromosomes, 2.5, 1.0, and 0.78, respectively.

The Influence Of Constriction Size OnThe Pulsating Flow In A Channel

The flow field in a channel with a constriction studied with the aim to better understand the flow in stenosed blood vessels. The flow was numerically simulated by solving the time-dependent incompressible Navier-Stokes equations. A pulsating incoming flow, resembling a cardiac cycle at the aortic root, specified at the upstream boundary. The influence of the size of a circular-arc constriction studied in the present work. It shown that even for a very small constriction size (7% of the height of the channel) large transverse variations are developed. At more severe constrictions, large vortices are generated; the strength and the number of vortices increase with the severity of the constriction. Yet, the propagation velocity is independent of the constriction size, because the vortices are convected downstream with the bulk-flow. The vortical flow structures have a prominent effect on the wall-shear. But in the constricted region, low shear stress found independent of the severity of the constriction.

Variation in the activity of nucleolar organizers and their ribosomal gene content.

In metaphase preparations from leucocytes of the toad, Bufo marinus, conspicuous secondary constrictions are present in the number 7 pair of chromosomes. These constrictions were considered to be the nucleolar organizers since they were associated with nucleoli during prophase. In 35 out of 60 individuals taken from natural populations, the homologous nucleolar organizers produced two equal-sized nucleoli and secondary constrictions (Group I animals). Pour animals (Group II) had only one very large secondary constriction in the majority of their metaphase preparations and an abnormally high frequency of cells containing one nucleolus. The remaining 21 animals (Group III) had unequal-sized constrictions in most of their metaphases but were more variable than the individuals of Groups I or II since they also had metaphases with two equal constrictions or only one constriction. The DNA from individuals of each group was hybridized with radioactive ribosomal RNA in order to correlate the size of nucleoli and constrictions with the amount of DNA (rDNA) homologous to ribosomal RNA. The two animals of Group II which were studied contained 0.056% of their genome homologous to ribosomal RNA a value considerably higher than those found for any of the animals of Groups I or III. These high values for rDNA coupled with the morphological appearance of the nucleolar homologues suggested a duplication of the nucleolar organizer in the homologue with the long constriction. The amount of rDNA in animals of Group I and III varied between 0.025 and 0.048% of the genome. Although the animals with unequal-sized constrictions (Group III) had generally lower contents of rDNA than those with equal-sized constrictions (Group I), the values overlapped between the two groups. Further evidence which correlates the size of nucleoli with the number of ribosomal RNA genes comes from studies with a “small nucleolar” mutant of the Mexican axolotl (Ambystoma mexicanum). Animals homozygous for this deletion were found to contain only 55% of the complement of rDNA present in the wild type. It is concluded that partial deletions and duplications of the nucleolar organizer as well as highly variable contents of rDNA are common in the genome of these amphibians.

Some aspects of the p-n junction second-breakdown mode

An analysis is presented of the p-n junction second-breakdown mode. Using the relationships between the potential and the temperature within a current constriction, explicit equations are derived for the second-breakdown terminal voltage, for both low- and high-power devices. An assessment of the constriction size is given, and some generality incorporated into the analysis by the introduction of ellipticity factors. Finally, a theory is developed which permits the approximate determination of mesoplasma-activation energy.