Complete Replication Cycle Research Articles

Segregation into and replication of plasmid deoxyribonucleic acid in chromosomeless segregants of Escherichia coli.

The progeny cells of Escherichia coli strain P678-54, which normally do not contain deoxyribonucleic acid (DNA), were found to carry DNA when the parental cell carried colicin factor E1 (Col E1). The DNA found in the normally DNA-less segregants was shown to be Col E1 DNA, which is present primarily as a covalently closed circular molecule that can undergo more than one complete cycle of replication.

Elektronenmikroskopische Untersuchungen über den Infektionszyklus von Influenza A-Viren in Ehrlich-Aszites-Tumor-Zellen

The complete replication cycle of influenza A virus was studied by means of the electron microscope in Ehrlich ascites tumor cells kept <i>in vitro. </i>Adsorption of virus in the cold followed by incubation at 36°C produced synchrony of early infection. Using high multiplicities of infection, specific phagocytosis of viral particles could be shown. Twenty-five to 90 min after penetration the virus disintegrated within phagocytic vesicles. Events during the eclipse phase could not be observed. Morphological changes in the nucleus 6 h after infection (at a time when budding of viral particles at the cellular periphery was already taking place) seemed to be phenomena of secondary nature. The budding process was studied by thin sectioning and freeze etching, which allowed a re-interpretation of the viral structure.

Interferon response during Coxsackie B-3 infection in mice. I. The effect of cortisone.

The adverse effects of corticosteroids on resistance to viral infections have been recognized for a long time [1, p. 108; 2; 3]. In order to shed light on the mechanisms responsible for this enhanced susceptibility, studies were conducted to determine the effect of cortisone on interferon, an antiviral factor, the importance of which in resistance to viral infections is becoming widely accepted [4]. In the initial studies, Newcastle disease virus (NDV) was employed as the inducer of interferon [5]. NDV, which does not go through a complete cycle of replication in the mouse and which produces no pathology unless given in toxic amounts [6, 7], was thought to be particularly suitable for this study, since the ancillary effects of the steroid on viral replication per se would be circumvented. It was found that pretreatment of mice with cortisone prior to induction of interferon by NDV led to a significant suppression of the response to interferon. Titers of interferon in the cortisone-pretreated mice were 10-fold lower than those in the control animals at the time of the maximal inter-

Differential replication of male and female X-chromosomes in Drosophila

The replication patterns of larval salivary gland chromosomes of D. hydei and D. melanogaster were studied by autoradiography with tritiated thymidine injected in mid third instar larvae. The male X chromosome showed a different replication behavior in comparison to that of the female X chromosome and autosomes. It is concluded that the male X chromosome finishes its replication earlier than the female X chromosome. Moreover, the time needed for a complete replication cycle of individual identical replication units was found to be shorter in the male than in the female X chromosome. Although the whole X chromosomes behave different there were no differences observed in the sequence of the discontinuous labeling patterns of the two types of X chromosome. One autosomal replication unit was observed which showed a different replication behavior in males and females. The possible origin of the differential behavior of the two X chromosomes is discussed in terms of their difference in degree of polyteny.

The normal DNA replication cycle. II

Certain predictions made by the theory developed in our first paper have been examined in autoradiographic experiments. First, individual cells were found to continue DNA synthesis for very different periods after protein and RNA synthesis had been inhibited; this agrees with the assumption that, in the absence of protein and RNA synthesis, DNA replication runs to completion and then stops. Secondly, it was found that if a fraction of the cells of a culture is immune to thymineless death a similar fraction is unable to synthesize DNA under conditions of inhibited protein and RNA synthesis; this agrees with our assumption that an immune cell is one that has finished a replication cycle and not yet taken the steps necessary to initiate a new one. It was further predicted that under certain experimental conditions the cells would go through a complete replication cycle when transferred from a thymine deficient to a thymine containing medium. The actual experiment showed that individual cells in fact produced a quantity of DNA which corresponded to a single replication cycle. When 5-bromouracil was substituted for thymine it was found that the DNA synthesized in the absence of protein and RNA synthesis was hybrid with respect to bromouracil substitution.