Accurate replication of the genome is essential for reproduction in all cells. However, even under normal conditions, the replication machinery may face a variety of impediments that can prevent it from completing its task. The mechanism by which cells overcome these hurdles is likely to vary depending upon the nature of the obstacle. Both UV irradiation and inactivation of replicative proteins in DnaB can inhibit the progression of the DNA replication machinery. However, the mechanism by which replication recovers following UV irradiation is different from the mechanism of recovery following the inactivation of the replicative proteins. Previous results show that following UVinduced damage in Escherichia coli, the replication fork is maintained and protected from extensive degradation by RecF, RecO, and RecR until replication can resume. By contrast, replication does not recover following inactivation of the replication protein DnaB, and the nascent DNA is extensively degraded irrespective of whether RecF is present. In this study, we verified DNA replication arrest by monitoring the total DNA accumulation and rate of DNA synthesis following UV-induced DNA damage and inactivation of thermosensitive replication alleles, such as dnaB266. We measured the amount of nascent DNA degradation, allowing us to determine how the newly synthesized strand of DNA is affected following replication fork arrest. Our data indicate that following inactivation of DnaB266, the replication fork is not maintained and is subject to extensive degradation. The degradation that occurs after DnaB266 inactivation is partially reduced in the absence of RecF-O-R, RecJ, and ExoI, suggesting that DNA processing by these enzymes occurs after DnaB arrest. In addition, two-dimensional agarose gel analysis revealed that unique structural intermediates accumulated following inactivation of DnaB266. These observations indicate that the recovery of replication when impeded by DNA lesions, such as those produced by UVirradiation, is maintained and processed through mechanisms that do not resemble the events occurring when replication proteins are inactivated.

Thesis (PhD)--Stellenbosch University, 2001. / ENGLISH ABSTRACT: The factors which control radiosensitivity are of vital importance for the understanding of cell inactivation and for cancer therapy. Cell cycle blocks, total induced DNA damage, DNA repair, apoptosis and chromatin structure are likely to playa role in the responses leading to cell death. I have examined aspects of irradiation-induced G2/M blocks in DNA damage and repair. In HT29, L132 and ATs4 cells the total amount of induced DNA damage by isodoses of 4.5 Gy, 5 Gy and 2 Gy was found to be 14 %, 14 % and 12 % respectively. Most of the DNA repair was completed before the G2/M maximum and only 3 % of DNA damage remains to be restored in the G2/M block. The radiosensitivity in eleven cell lines was found to range from SF2 of 0.02 to 0.61. By FADU assay the undamaged DNA at 5 Gy was found to range from 56% to 93%. The initial DNA damage and radiosensitivity were highly correlated (r2=0. 81). After 5 Gy irradiation and 12 hours repair two groups of cell lines emerged. The group 1 cell lines restored undamaged DNA to a level ranging from 94 % to 98 %. The group 2 cell lines restored the undamaged DNA to a level ranging from 77 % to 82 %. No correlation was seen between residual DNA damage remaining after 12 hours repair and radiosensitivity. In CHO-K1 cells chromatin condensation induced by Nocodazole was found to marginally increase the radiosensitivity as shown by the change of the mean inactivation dose (D) from 4.446 to 4.376 Gy. Nocodazole also increased the initial DNA damage, induced by 5 Gy, from 7 % to 13 %. In xrs1 cells these conditions increased the radiosensitivity from D of 1.209 to 0.7836 Gy and the initial DNA damage from 43 % to 57 %. Disruption of chromatin structure with a hypertonic medium was found to increase radiosensitivity in CHO-K1 cells from D of 4.446 to 3.092 Gy and the initial DNA damage from 7 % to 15 %. In xrs1 cells these conditions caused radiosensitivity to decrease from D of 1.209 to 1.609 Gy and the initial DNA damage from 43 % to 36 %. Repair inhibition by Wortmannin increased the radiosensitivity in CHO-K1 from a D of 5.914 Gy in DMSO controls to a D 3.043 Gy. In xrs1 cells repair inhibition had no effect on radiosensitivity. Significant inhibition of repair was seen in CHO-K1 at 2 hours (p<0.0001) and at 20 hours (p=0.0095). No inhibition of repair was seen in xrs1 cells at 2 hours (p=0.6082) or 20 hours (p=0.6069). While DNA repair must be allocated to the post-irradiation period, the G2/M block seen in p53 mutants reaches a maximum only 12 hours post-irradiation when most of the repair is completed. As the G2/M block resolves and cells reenter cycle 28 hours after the G2 maximum it appears that repair processes cannot be the only reason for the G2IM cell cycle arrest. At low doses of irradiation initial DNA damage correlates with radiosensitivity. This suggests that the initial DNA damage is a determinant for radiosensitivity. Repair of DNA double-strand breaks by the non-homologous end joining (NHEJ) mechanism, identified by inhibition with Wortmannin, was shown to influence residual DNA damage and cell survival. Both the initial DNA damage and DNA repair were found to be influenced by chromatin structure. Chromatin structure was modulated by high salt and by Nocodazole, and has heen identified as a parameter which influences radiosensitivity. / AFRIKAANSE OPSOMMING: Die faktore wat betrokke is in die meganisme van stralings-sensitisering is van hoogs belang vir die begrip van sel inaktiveering en kanker terapie. Sel siklus blokke, totale geïnduseerde DNS skade, DNS herstel, apoptose en chromatien struktuur is moontlike rol vertolkers in die sellulêre response wat ly tot seldood. Ek het die aspekte van stralings-geïnduseerde G2/M blokke in DNS skade en DNS herstelondersoek. Die hoeveelheid geïnduseerde DNS skade, deur ooreenstemmende stralings-dosisse, in HT29, L132 en ATs4 selle is 14 %, 14 % en 12 %. Meeste van die DNS herstel is klaar voordat die G2/M maksimum beryk word en net 3 % DNS skade blyoor om herstel te word in die G2/M blok. Die stralings-sensitiwiteit in elf sel lyne varieer tussen 'n SF2 van 0.02 en 0.61. Deur die gebruik van die FADU metode is gevind dat die onbeskadigde DNS na 5 Gy bestraling varieer tussen 56 % en 93 %. Die totale geïnduseerde DNS skade en stralings-sensitiwiteit was hoogs gekorreleer (r2=0.81). Na 5 Gy bestraling en 12 ure herstel kan die sel lyne in twee groepe gegroepeer word. Die groep 1 sellyne herstel die onbeskadigde DNS terug na 'n vlak wat varieer tussen 94 % en 98 %. Die groep 2 sel lyne herstel die onbeskadigde DNS terug tot op 'n vlak wat varieer tussen 77 % en 82 %. Geen korrelasie is gesien tussen oorblywende DNS skade en stralings-sensitiwiteit na 12 ure herstel nie. In die CHO-K1 sel lyn, chromatien kompaksie geïnduseer deur Nocodazole, vererger die stralings- sensitiwiteit soos gesien deur die gemiddelde inaktiveerings dosis (D) wat verlaag het van 4.446 tot 4.376. Nocodazole het ook die totale DNS skade verhoog van 7 % tot 13 %. Onder dieselfde kondisies, in die xrs1 sel lyn, is 'n verergering van stralings-sensitiwiteit (D) gesien van 1.209 tot 0.7836 en verhoog ONS skade van 43 % tot 57 %. Die ontwrigting van die chromatien struktuur deur die gebruik van hipertoniese medium het die stralings-sensitiwiteit (D) vererger in CHO-K1 selle van 4.446 tot 3.092. Die totale ONS skade is verhoog van 7 % tot 15 %. Onder dieselfde kondisies, in die xrs1 sellyn, verbeter die stralings-sensitiwiteit (D) van 1.209 tot 1.609 en die totale ONS skade verminder van 43 % tot 36 %. ONS herstel inaktiveering in die teenwoordigheid van Wortmannin het die stralings-sensitiwiteit (D) in CHO-K1 selle vererger van 5.914 in DMSO verwysings kondisies tot 3.043. Die ONS herstel inaktiveering in xrs1 selle het geen uitwerking gehaat op stralingssensitiwiteit nie. Noemenswaardige inaktiveering van ONS herstel is gesien in CHO-K1 selle na 2 ure (p<0.0001) en na 20 ure (p=0.0095). Geen inaktiveering is gesien in xrs1 selle na 2 ure (p=0.6082) of na 20 ure (p=0.6069) nie. TerwylONS herstel moet plaasvind na die bestralings periode, beryk die G2/M blok in p53 gemuteerde selle sy maksimum 12 ure na bestraling terwyl meeste van die ONS herstel alreeds voltooi is. Aangesien die G2/M blok eers 28 ure later begin sirkuleer moet die G2/M blok nog 'n funksie vervul anders as ONS herstel. By lae dosisse van bestraling korreleer die totale geïnduseerde ONS skade met stralings-sensitiwiteit. Dit dui daarop dat die totale ONS skade 'n bepalende faktor moet wees in stralings-sensitiwiteit. Die herstel van ONS skade deur die nie-homoloë eindpunt samevoeging (NHES) meganisme, geïdentifiseer deur inaktiveering deur Wortmann in, het 'n invloed op oorblywende ONS skade en sellulêre oorlewing. Beide die totale ONS skade en ONS herstel was beïnvloed deur die chromatien struktuur. Chromatien struktuur was gemoduleer deur hoë sout konsentrasies en deur Nocodazole, en is geïdentifiseer as a belangrike parameter wat stralings-sensitiwiteit beïnvloed.

It is increasingly recognised that individual risk of cancer may be related to genetically determined differences in the ability of cells to identify and repair DNA damage. Cell cycle based assays of chromosomal radiosensitivity provide the greatest power for discriminating differences in response to DNA damage and it has been suggested that individuals who are genetically susceptible to cancer show increased chromosomal radiosensitivity. The relationship between chromosomal radiosensitivity and early onset cancer was investigated in a population of Danish survivors of childhood and adolescent cancer and a control group comprising of their partners using the G₂ assay of chromosomal radiosensitivity. Heritability was also examined in the offspring. No significant differences in radiosensitivity profiles were found between partner controls and either the cancer survivors or offspring. However, when compared to the Westlakes Research Institute control population, significant differences were observed with the cancer survivors (P = 0.002) and offspring (P < 0.001), supporting an association of chromosomal radiosensitivity with cancer predisposition. Heritability studies suggested the majority of phenotypic variance of chromosomal radiosensitivity was attributable to a putative major gene locus with dominant effect. Since G2 chromosomal radiosensitivity indirectly measures the ability of cells to repair DNA damage induced by ionising radiation exposure, variants in DNA repair genes may explain inter-individual variation observed. Sixteen polymorphisms in nine genes from four DNA repair pathways were investigated. Genotype frequencies at the Asp148Glu polymorphism were associated with childhood cancer in survivors. Analysis of variance and FBAT analysis suggested significant associations at both the Thr241Met and Ser326Cys polymorphism sites with G₂ radiosensitivity, but neither remained significant after multiple-test adjustment. This study invites further exploration of the predictive capacity of G₂ chromosomal radiosensitivity in cancer predisposition. Clearly, further work is needed to correlate radiosensitivity with genetic polymorphisms, which may underlie cancer susceptibility and variation in radiosensitivity.

The effects of radiation on an operational amplifier were investigated through simulation. The μA 741 was simulated using Spice. Under normal conditions the 741 had the following properties: offset Voltage (Vos) of 0.8 mV, bias current (IB) of 27 nA, offset current (Ios) of 1 nA, and an open loop gain (A0.1.) of 112 dB. When exposed to neutron fluence of 5 x 10¹³ n/cm², these parameters changed to offset voltage of 45 mV, bias current of 1500 nA, offset current of 500 nA, and an open loop gain of 66 dB. A new circuit is proposed that provides improvements in the above parameters. The modified circuit gives a Vos of 3 mV, IB of 200 nA, Ios of 34 nA and A0.1. of 93 dB following exposure to a neutron fluence of 5 x 10¹³n/cm².

Application of power MOSFETs in spaceborne power converters was simulated by exposing devices to low-dose-rate ionizing radiation. Both radiation-hardened and nonhardened devices were tested with constant and switched gate biases during irradiation. In addition, some of the devices were under load. The threshold-voltage shifts were strongly bias dependent. The threshold-voltage shift of the nonhardened parts was approximately dose-rate independent, while the hardened parts exhibited significant dose-rate dependence. A pre-anneal dose-rate dependence was found for the interface-state buildup of the switched and positively biased devices, but the results for the switched devices were qualitatively different than those for the positively biased devices. The buildup of interface trapped charge was found to be the primary contributor to mobility degradation, which results in reduced drive capability and slower operation of the devices. These results indicate that new methods need to be utilized to accurately predict the performance of power MOSFETs in space environments.

Commercial sunscreen products are used to protect the skin against hannful ultraviolet (UV) radiation that can induce skin cancer at high dosage. These products contain UV filters that can reflect, scatter or absorb UV light. The chemical UV filters responsible for the absorption of UV radiation can be photochemically modified and as a result reduce the efficacy of the sunscreen formulation. This study focused on the possible use ofplant polyphenols as potential stabilisers of photo-unstable sunscreen chemical absorbers. The photo-instability of some sunscreen absorbers results in radical formation; this prompted the use of the plant, Sutherlandia microphylla (Cancer Bush plant), as a potential photostabiliser. The Cancer Bush plant is used by the indigenous people of South Africa to treat AIDS and cancer. The radical scavenging properties of polyphenolic compounds present in the plant are possibly responsible for the plant's anti-tumour and anti-IDV properties. Therefore, these Cancer Bush polyphenols could possibly be used to photostabilise photo-unstable sunscreen absorbers. Potential polyphenolic photostabilisers from the Cancer Bush plant were extracted by means of various polyphenolic extraction methods. These extracts were analysed by gas chromatography (GC), high-performance liquid chromatography (HPLC), UV spectroscopy and gas chromatography-mass spectrometry (GC-MS). The phenolic content and the antioxidant activity ofthese extracts were investigated by means of the Folin-eiocalteu reagent (FCR) and the diphenylpicrylhydrazyl (DPPH) radical assays respectively. Polyphenols were also extracted from various Rooibos teas and compared with those extracted from the Cancer Bush plant. Both the Cancer Bush and various Rooibos tea extracts were found to contain simple phen~lics and potential polyphenolic compounds. The Cancer Bush extracts as well as the Rooibos tea extracts together with the specific polyphenols, epicatechin and rutin, were assessed for their ability to photostabilise sunscreen absorbers. The photostability of the chemical absorbers in the absence and in the presence of the polyphenol extracts was investigated by UV spectroscopy, by monitoring their absorption spectra during irradiation with solar-simulated radiation. These extracts inhibited the photodegradation of the absorber avobenzone. The photostability of avobenzone is solventIntroduction III dependent hence the investigations were carried out in three solvents, namely, cyclohexane, ethyl acetate and dimethylsulfoxide. Additionally, the cause ofthe instability ofavobenzone in these solvents was investigated by means of DV spectroscopy, HPLC and nuclear magnetic resonance spectroscopy. The oxygen dependency of the photo-instability of avobenzone was also determined. The photo-instability of avobenzone was found to occur as a result of photoisomerisation and!or photodegradation, depending on the solvent. Avobenzone photoisomerised extensively in dimethylsulfoxide and photodegraded appreciably in cyclohexane, whereas both processes occurred to a similar extent in ethyl acetate. Photoisomerisation only occurred in the presence of oxygen whereas photodegradation occurred irrespective of oxygen. The Cancer Bush and various Rooibos tea extracts as well as other polyphenols photostabilised avobenzone in ethyl acetate and dimethylsulfoxide but not in cyclohexane. This photostabilisation effect was potentially due to the radical scavenging ability of polyphenols which prevented the oxygendependent photoisomerisation, but not the oxygen independent photodegradation process from occurring. / Thesis (M.Sc.)-Chemistry-University of KwaZulu-Natal, 2005

A resonant frequency cavity was used to determine the dielectric properties of various preparations of Azotobacter vinelandii ATTC 12837. It was found that the bacteria investigated did interact with microwave radiation in the absence of free water. The data presented here indicate that bacteria demonstrate frequency specific dielectric properties. The techniques employed in these experiments may also be used to determine microwave spectra of other species of bacteria in different physiological stages.

This thesis represents one phase of a joint effort of research on the properties of liquids and solids. This work is concerned primarily with the microwave properties of solids. In this investigation the properties exhibited by conductor and semiconductor materials when they are subjected to electromagnetic radiation of microwave frequency are studied. The method utilized in this experiment is the perturbation of a resonant cavity produced by introduction of a cylindrically shaped sample into it.

The efforts addressed in this thesis refer to assaying the degradations in modern solar cells used in space-borne and/or nuclear environment applications. This study is motivated to address the following: 1. Modeling degradations in Si pn-junction solar cells (devices-under-test or DUTs) under different ionizing radiation dosages 2. Preemptive and predictive testing to determine the aforesaid degradations that decide eventual reliability of the DUTs; and 3. Using electrical overstressing (EOS) to emulate the fluence of ionizing radiation dosage on the DUT. Relevant analytical methods, computational efforts and experimental studies are described. Forward/reverse characteristics as well as ac impedance performance of a set of DUTs under pre- and post- electrical overstressings are evaluated. Change in observed DUT characteristics are correlated to equivalent ionizing-radiation dosages. The results are compiled and cause-effect considerations are discussed. Conclusions are enumerated and inferences are made with direction for future studies. / by George A. Thengum Pallil. / Thesis (M.S.C.S.)--Florida Atlantic University, 2010. / Includes bibliography. / Electronic reproduction. Boca Raton, Fla., 2010. Mode of access: World Wide Web.

Many experiments are being conducted to find compounds that offer radioprotection against radiation damage and that are also non-toxic. It is hopeful that in the future, research for this technology will benefit patients undergoing cancer treatment by reducing radiation damage to normal cells and therefore reducing short and long term side effects experienced from treatments. Hamster cells were irradiated at doses of 60 and 120 rad, with and without Sphingosine-1-Phosphate mixed in with their growth medium. Post irradiation, it was observed that the S1P molecule seemed to have a radioprotective effect by decreasing the amount of cell death compared to the amount of cell death that occurred with the absence of the molecule. The results of this experiment will sent to Dr. Jon Tilly at Massachusetts General Hospital. Dr. Tilly is currently researching S1P as a possible radioprotector. / Graduation date: 2003