Embryonic stem (ES) cells have the ability to maintain pluripotency and self-renewal during in vitro maintenance, which is a key to their clinical applications. ES cells are a model in developmental biology studies due to their potential to differentiate in vitro. Understanding critical pathways of pluripotency, self-renewal, and differentiation during early embryonic development is important for the evaluation of the therapeutic potential of ES cells because of their ability for tumor transformation due to genetic and epigenetic instability acquired during in vitro culture maintenance. Single tandem repeats are sequences of DNA that have been implicated in the deregulation of gene expression in different human conditions. Understanding the origin of repetitive sequence instability and functions in the genome allow characterization of early genomic instability signals in ES cell pluripotency, differentiation, and tumor transformation pathways. The hypothesis of this study was that genetic stability, in repetitive sequences, located near embryonic developmental genes is responsible for pluripotency, self-renewal, differentiation, and chromatin assembly and could be a signal for adaptation, differentiation, or transformation of ES cells in vitro. Our result showed instability in specific repetitive sequences which increased during ES cell passages and embryoid body differentiation in vitro. ES cells displayed significant mean frequencies of genomic instability in repetitive regions that lead to ES cells pluripotency, self-renewal maintenance, or cell lineage specialization. The present study reports potentially biomarkers for identifying accumulation of genomic instability in specific genes that may contributes to adaptation of ES cells and could be the switch that initiates early ES cell lineage commitment in vitro. Determining genetic and epigenetic modifications, including single tandem repeat instability, gene expression changes, and chromatin modifications, is essential for elucidating possible molecular mechanisms of genomic instability and determining novel molecular characterization for diagnostic purposes to ensure ES cell stability and integrity that could potentially lead to use of ES cell derivatives that could then be a safe source needed for regenerative medicine applications

Megasatellites are polymorphic tandem repetitive sequences with repeat-units longer than or equal to 1000 base pairs. The novel algorithm Megasatfinder predicts megasatellites in the human genome. A structured method of analysing the algorithm is developed and conducted. The analysis method consists of six test scenarios. Scripts are created, which execute the algorithm using various parameter settings. Three nucleotide sequences are applied; a real sequence extracted from the human genome and two random sequences, generated using different base probabilities. Usability and accuracy are investigated, providing the user with confidence in the algorithm and its output. The results indicate that Megasatfinder is an excellent tool for the detection of megasatellites and that the generated results are highly reliable. The results of the complete analysis suggest alterations in the default parameter settings, presented as user guidelines, and state that artificially generated sequences are not applicable as models for real DNA in computational simulations.

<p>Megasatellites are polymorphic tandem repetitive sequences with repeat-units longer than or equal to 1000 base pairs. The novel algorithm Megasatfinder predicts megasatellites in the human genome. A structured method of analysing the algorithm is developed and conducted. The analysis method consists of six test scenarios. Scripts are created, which execute the algorithm using various parameter settings. Three nucleotide sequences are applied; a real sequence extracted from the human genome and two random sequences, generated using different base probabilities. Usability and accuracy are investigated, providing the user with confidence in the algorithm and its output. The results indicate that Megasatfinder is an excellent tool for the detection of megasatellites and that the generated results are highly reliable. The results of the complete analysis suggest alterations in the default parameter settings, presented as user guidelines, and state that artificially generated sequences are not applicable as models for real DNA in computational simulations.</p>

Orientador: Michel Eduardo Beleza Yamagishi / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Biologia / Made available in DSpace on 2018-08-15T17:21:19Z (GMT). No. of bitstreams: 1 Herai_RobertoHirochi_D.pdf: 3625854 bytes, checksum: 3f19d10a9b0bb7f77091197cd302f66e (MD5) Previous issue date: 2010 / Resumo: A grande quantidade de dados biológicos gerados recentemente permitiu verificar que os genomas são repletos de seqüências repetitivas (SR), como microsatélites e elementos genéticos móveis, altamente improváveis de ocorrer estatisticamente se os genomas fossem gerados a partir de uma distribuição aleatória de nucleotídeos. Tal comprovação motivou a classificação de tais seqüências e também a construção de diversas ferramentas de bioinformática, além de mecanismos de armazenamento baseados em sistemas de gerenciamento de bancos de dados (SGBD) para permitir localizá-las e armazená-las para posterior estudo. Entretanto, foi com a comprovação biológica da importância das SR, como no mecanismo de interferência por RNAi (SR reversa complementar), que as SR despertaram maior interesse por parte da comunidade científica. Atualmente, já há fortes evidências que associam as SR com fenômenos biológicos bastante interessantes, como o processamento de RNA por cis-splicing e a formação de transcritos quiméricos, freqüentes em organismos inferiores e muito raro em organismos superiores. Tais tipos de transcritos podem ser gerados a partir de trans-splicing ou, como conjecturamos nesse trabalho, pela transposição de elementos genéticos móveis (como por exemplo transposons ou retrotransposons). Em virtude disso, este projeto propõe a construção de metodologias de Bioinformática, disponibilizadas na WEB, para detectar transcritos quiméricos em genomas de organismos, tanto em versões draft ou em alta qualidade, e também estudar as SR que ocorrem no locus gênico dos transcritos envolvidos na formação de uma seqüência quimérica. As ferramentas propostas permitiram identificar, a partir de bibliotecas de transcritos de full-length cDNA, tanto de humanos quanto de bovinos, novos transcritos quiméricos provenientes de células de tecidos normais, e que não seguem splice-sites canônicos na região de fusão dos transcritos envolvidos. Além disso, as seqüências encontradas apresentam uma elevada taxa de concentração de pares de SR do tipo reverso complementar no locus gênico dos dois transcritos que formam a seqüência quimérica. As ferramentas propostas podem ser utilizadas para outros organismos e direcionar trabalhos experimentais para tentar comprovar em bancada novos transcritos quiméricos, tanto em organismos inferiores quanto em superiores / Abstract: The recent availability of a huge amount of biological data allowed to know about the high concentration of repetitive sequences (SR) like microsatellites and genetic mobile elements in different genomes. Repetitive sequences are improbable to occur statistically if genome data were generated by a random distribution of nucleotides. Such observation motivated the classification of repetitive sequences, and the construction of several bioinformatics tools. Furthermore, several mechanisms to store repetitive sequences, which are based on data base management systems (DBMS) were proposed and created. They can be used to search for specific sequences to make a posteriori study. However, it was with the biological confirmation of the importance of repetitive sequences, like by the RNA interference (reverse complement, or inverted repeat) mechanism, that the scientific community gained more interest by such sequences. Actually, there is strong evidence that associates the repetitive sequences with some interesting biological phenomena, like in RNA processing by cis-splicing, and in chimeric transcript formation mechanism. This last one is very frequently in inferior organism, but rare in superior organisms. Such types of transcripts can be generated by trans-splicing, or like conjectured in this work, by the retrotransposition of mobile genetic elements (like transposons or retrotransposons). In this way, this work proposed the construction of several Bioinformatics methodologies, available in the WEB, to detect new evidences of chimeric transcripts in genomes of different organisms, both in draft genome and in high quality genome assemblage. We also studied repetitive sequences in gene loci of the involved transcripts in a chimeric sequence formation. The proposed tools allowed us to identify, using a full-length cDNA databank, new chimeric transcript candidates in human and in bovine genome. They are from cells of normal tissues, and do not follow canonical splice-sites in the fusion region of the involved transcripts. Moreover, it was possible to show that the detected sequences have high concentration pairs of reverse complement type of repetitive sequences in gene loci of the two involved transcripts, which originated a new chimeric transcript candidate. The created bioinformatics tools can be used in other organisms in addition to the one used in this work, leading to the proposition of new experimental work to try to prove in vivo new chimeric transcripts, both in superior organism and in inferior organism / Doutorado / Bioinformatica / Doutor em Genetica e Biologia Molecular

The most common structural platform in biology, the βα-repeat classes of proteins, are represented by the (βα)8TIM barrel topology and the α/β/α sandwich, CheY-like topology. Previous studies on the folding mechanisms of several members of these proteins have suggested that the initial event during refolding involves the formation of a kinetically trapped species that at least partially unfolds before the native conformation can be accessed. The simple topologies of these proteins are thought to permit access to locally folded regions that may coalesce in non-native ways to form stable interactions leading to misfolded intermediates. In a pair of TIM barrel proteins, αTS and sIGPS, it has been shown that the core of the off-pathway folding intermediates is comprised of locally connected clusters of isoleucine, leucine and valine (ILV) residues. These clusters of Branched Aliphatic Side Chains (BASiC) have the unique ability to very effectively prevent the penetration of water to the underlying hydrogen bond networks. This property retards hydrogen exchange with solvent, strengthening main chain hydrogen bonds and linking tertiary and secondary structure in a cooperative network of interactions. This property would also promote the rapid formation of collapsed species during refolding. From this viewpoint, the locally connected topology and the appropriate distribution of ILV residues in the sequence can modulate the energy landscapes of TIM barrel proteins. Another sequence determinant of protein stability that can significantly alter the structure and stability of TIM barrels is the long-range main chain-side chain hydrogen bond. Three of these interactions have been shown to form the molecular underpinnings for the cooperative access to the native state in αTS. Global analysis results presented in Chapter II and Chapter III, suggest that the off-pathway mechanism is common to three proteins of the CheY-like topology, namely CheY, NT-NtrC and Spo0F. These results are corroborated by Gō-simulations that are able to identify the minimal structure of kinetically trapped species during the refolding of CheY and Spo0F. The extent of transient, premature structure appears to correlate with the number of ILV side chains involved in a large sequence-local cluster that is formed between the central β-sheet and helices α2, α3 and α4. The failure of Gō-simulations to detect off-pathway species during the refolding of NT-NtrC may reflect the smaller number of ILV side chains in its corresponding hydrophobic cluster. In Chapter IV, comparison of the location of large ILV clusters with the hydrogen exchange protected regions in 19 proteins, suggest that clusters of BASiC residues are the primarily determinants of the stability cores of globular proteins. Although the location of the ILV clusters is sufficient to determine a majority of the protected amides in a protein structure, the extent of protection is over predicted by the ILV cluster method. The survey of 71 TIM barrel proteins presented in Chapter V, suggests that a specific type of long-range main chain-side chain hydrogen bond, termed “βα hairpin clamp” is a common feature in the βα-repeat proteins. The location and sequence patterns observed demonstrate an evolutionary signature of the βαβ modules that are the building blocks of several βα-repeat protein families. In summary, the work presented in this thesis recognizes the role of sequence in modulating the folding free energy landscapes of proteins. The formation of off-pathway folding intermediates in three CheY-like proteins and the differences in the proposed extent of structure formed in off-pathway intermediates of these three proteins, suggest that both topology and sequence play important and concerted roles in the folding of proteins. Locally connected ILV can clusters lead to off-pathway traps, whereas the formation of the productive folding path requires the development of long-range nativelike topological features to form the native state. The ability of ILV clusters to link secondary and tertiary structure formation enables them to be at the core of this cooperative folding process. Very good correlations between the locations of ILV clusters and both strong protection against exchange and the positions of folding nuclei for a variety of proteins reported in the literature support the generality of the BASiC hypothesis. Finally, the discovery of a novel pattern of H-bond interactions in the TIM barrel architecture, between the amide hydrogen of a core ILV residue with a polar side chain, bracketing βαβ modules, suggests a means for establishing cooperativity between different types of side chain interactions towards formation of the native structure. See Additional Files for copies of the source code for the global analysis program and the cluster analysis program.

Made available in DSpace on 2016-08-10T10:38:29Z (GMT). No. of bitstreams: 1 Emilia Oliveira Alves Costa.pdf: 2624347 bytes, checksum: 633de56159a2dded5fb51871dbe9fd21 (MD5) Previous issue date: 2010-01-12 / The Brazilian radiological accident that occurred in 1987, in Goiânia, it was a terrible radiation episode. As a consequence, hundreds of people were contaminated due to the Cesium-137 radiation. Recently, many studies had shown that genome instabilities, such as, mutations, chromosomal aberrations, micronuclei formation and microsatellite instability and a delay on cellular death are usually reported on mammal cells exposed to ionizing radiation, being considered as a manly risk to humans. Mutations can be spontaneous, and the occurrence is dependent on the organism, or, induced, being associated to mutagenic exposition. Ionizing radiations are an example of physical and mutagenic agents that could harm the cell repair and could cause the development of many types of cancer. The evaluation of the biological effects of the ionizing radiation, in somatic and germline cells, with a consequent determination of the radio-induced mutations, it is extremely important to estimate the genetic risks, manly in population exposed to radiation. The analyses of repetitive DNA sequences have been demonstrated that such sequences are prone to high rates of spontaneous mutations. The minisatellites and microsatellites have been used to demonstrate the induction of germline mutation rates on mouse, humans, among others organisms. The aim of the present study was to analyze the frequency of microsatellite alterations to determine the mutation rates occurred in germ cells of the parents exposed to the ionizing radiation of the Cesium-137. The studied group was constitute of 10 families of individuals accidentally exposed to Cesium-137 and by the control group constituted by 645 healthy individuals who carried out paternity tests on 2009. We found only one mutation of paternal origin in the D8S1179 locus on the exposed group, being the mutation rate of 0.003. In the control group, we found 01 mutation on D16S539 loci and on D3S1358; 02 mutations on Penta E locus; 04 mutations on D21S11 locus and 03 mutations on FGA locus, comprising a total of 11 mutations and a mutation rate of 0.0009. In such context, we did not find significant differences (p= 0.15), indicating a possible exposure effect on the mutation rates of the STR loci, in the group accidentally exposed to Cesium-137. / O acidente radiológico de Goiânia em 1987 foi um grave episódio de contaminação por radioatividade ocorrido no Brasil. Como conseqüência foram contaminadas centenas de pessoas através das radiações emitidas pelo césio 137. Recentemente, vários estudos têm mostrado que instabilidade no genoma, como por exemplo, mutações, aberrações cromossômicas, formação de micronúcleos, instabilidade de microssatélites e atraso na morte celular são comumente relatadas em células de mamíferos expostos à radiação ionizante, sendo consideradas como o principal fator de risco em humanos para o câncer. As mutações podem ser espontâneas, sendo a freqüência de ocorrência dependente do organismo, ou ainda, induzida, podendo ser ocasionadas pela exposição a agentes mutagênicos. As radiações ionizantes são exemplos de agentes físicos e mutagênicos que podem levar ao comprometimento dos mecanismos de reparo celular e ao desenvolvimento de diversos tipos de câncer. Avaliar os efeitos biológicos da radiação ionizante, em células somáticas e germinativas, com conseqüente determinação da taxa de mutações radioinduzidas, é extremamente importante para a estimativa de riscos genéticos, principalmente em populações expostas à radiação. Análises de seqüências repetitivas de DNA têm demonstrado que estas seqüências são sujeitas a altas taxas de mutações espontâneas. As seqüências minissatélites e microssatélites têm sido usadas para demonstrar satisfatoriamente a indução de mutação germinativa em camundongos, humanos, dentre outros organismos. O objetivo do presente estudo foi analisar a freqüência de alterações em microssatélites para determimar a taxa de mutações ocorridas em células de linhagem germinativa dos progenitores expostos à radiação ionizante do Césio-137. O grupo exposto foi constituído por 10 famílias do grupo 2 e o grupo controle por 645 indivíduos provenientes de exames de vínculo genético realizados em 2010. Foi encontrada um mutação de origem paterna no locus D8S1179 com uma taxa de mutação de 0,003. No grupo controle, foram encontradas 01 mutação no loci D16S539 e no loci D3S1358; 02 mutações no locus Penta E; 04 mutações no locus D21S11 e 03 mutações no locus FGA, compreendendo um total de 11 mutações e uma taxa de mutação de 0,0009. Nesse contexto, não foram encontradas diferenças estatisticamente significativas (p= 0,15) indicando o efeito da exposição e taxa de mutação em locos STR no grupo acidentalmente exposto ao Césio-137.

Diss. (sammanfattning) Stockholm : Karol. inst., 2003. / Härtill 4 uppsatser.

In this study, the reassociation kinetics of codling moth (Cydia pomonella) was performed in order to characterize the molecular composition of the codling moth (Cydia pomonella) genome and to isolate highly repetitive and unique DNA components. The effectivity of distinct repetitive DNA fractions as a specific competitor in fluerescence in situ hybridization was tested.

The recent explosion of DNA sequence information has provided compelling evidence for the following facts. (1) Simple repetitive sequences-microsatellites and minisatellites occur commonly in the human genome and (2) these repetitive DNA sequences could play an important role in the regulation of various genetic processes including modulation of gene expression. These sequences exhibit extensive polymorphism in both length and the composition between species and between organisms of the same species and even cells of the same organism. The repetitive DNA sequences also exhibit structural polymorphism depending on the sequence composition. The functional significance of repetitive DNA is a well-established fact. The work done in many laboratories including ours has conclusively documented the functional role played by repetitive sequences in various cellular processes. Structural studies have established the sequence requirement for various non-B DNA structures and the functional significance of these unusual DNA structures is becoming increasingly clear. The structures that were characterised earlier purely from conformation point of view have aroused interest after the recent realisation that these structures could be formed in vivo when cloned in a supercoiled plasmid. The discovery of novel type of dynamic mutations where intragenic amplifications of trinucleotide repeats is associated with phenotypic changes causing many neurodegenerative disorders has provided the most compelling evidence for the importance of simple repeats in the etiology of these disorders. Secondary structures adopted by these simple repeats is a common causative factor in the mechanism of expansion of these repeats. This realisation prompted many investigations into the relationship between the DNA sequence, structure and molecular basis of dynamic mutation. Many experimental evidences have implicated paranemic DNA structures in various biological processes, especially in the regulation of gene expression. Earlier work done in our laboratory on the structure function relationship of repetitive DNA sequences provided experimental evidence for the role of paranemic DNA structure in the regulation of gene expression. It was demonstrated that intramolecular triplex potential sequences within a gene downregulate its expression in vivo (Sarkar and Brahmachari (1992) Nucleic Acids Res., 20, 5713-5718). Similarly the effect of cruciform structure forming sequences on gene expression was also documented. Sequence specific alterations in DNA structures were studied in our laboratory using a variety of biophysical and biochemical techniques. An intramolecular, antiparallel tetraplex structure was proposed for human telomeric repeat sequences (Balagurumoorthy, et al., (1994) J. Biol. Chem., 269, 21858-21869). The telomeric repeats are not only present at the end of chromosomes but they are also present at many interstitial sites in the human genome. Database search reveals that the human telomeric sequences as well as similar sequences with minor variations are present at many locations in the human genome. Telomeric repeats are GC rich sequences with the G rich strand protruding as a 3' end overhang at the end of chromosomes. When human telomeric repeats are cloned in a supercoiled plasmid, the C rich strand adopts a hairpin like conformation where as the G-rich strand extrudes into a quadruplex structure. However, the biological significance of these structures in vivo still remains to be elucidated completely. The role of a putative tetraplex DNA structure in the insulin gene linked polymorphic region of the human insulin gene in vivo in the regulation of expression of the insulin gene has been suggested. In this context, we have addressed the question whether the telomeric repeats when present within a gene affect its expression in vivol If so, what would be the possible mechanism? An attempt has been made to understand the effect of presence of telomeric repeats within a gene on its expression. The details of these studies have been presented in Chapter 2 of this thesis. Contrary to telomeric repeats which provide stability to the chromosomes, recently expansion of a GC rich dodecamer repeat upstream of cystatin B gene (chromosome 21q) has been shown to be the most common mutation associated with Progressive Myoclonus Epilepsy (EPM1) of Unverricht-Lundberg type. Two to three copies of the repeat (CCCCGCCCCGCG)n are present in normal individuals whereas the affected individuals have 30-75 copies of this repeat. The expression of cystatin B gene is reduced in patients in a cell specific manner. The repeat also shows intergenerational variability. The exact mechanism of expansion of this repeat is not known. In the case of trinucleotide repeat expansion, it is shown that the structure adopted by the repeat plays an important role in the mechanism of expansion and that some of the secondary structures adopted by trinucleotide repeats could be inherently mutagenic conformations. In order to understand the mechanism of expansion EPM1 dodecamer repeat, the work reported in this thesis was carried out with the following objectives. • To understand the structure of G rich and C-rich strands of EPM1 repeat. • To understand the variations in the structure with the increase in the length and its possible implications in the mechanism of expansion of EPM 1 repeat. Studies aimed with these objectives are presented in chapters 3, 4 and 5 of the thesis. Chapter 1 provides a general introduction to repetitive DNA, the various structures adopted by repetitive DNA sequences in the genome, the functional significance of the various simple repetitive DNA sequences in the genome has been presented. An account of trinucleotide repeat expansion and associated disorders, non-trinucleotide repeat expansions and associated disorders has been presented. The various non B-DNA structures adopted these repeats and their implications in the mechanism of expansion have been discussed. Chapter 2 describes in frame cloning of human telomeric repeats d(G3T2A)3G3 in the N-terminal region of β-galactosidase gene. The effect of such repeat Sequences on transcription elongation in vivo has been studied using E.coli as a model system. The 3.5 copies of human telomeric repeat sequences were cloned in the sense strand of plasmid pBluescriptllSK+ so as to create plasmid clone pSBQ8 and in the template strand of plasmid pBluescriptHKS+ so as to create clone pSBRQ8. One dimensional chloroquine gel shift assay indicated presence of an unwound structure in pSBQ8 and pSBRQ8. β-galactosidase activity assay suggested downregulation of the gene in vivo. In the case of plasmid pSBQ8 the difference in β-galactosidase activity was approximately 6 fold as compared to the parent plasmid pBluescriptIISK+ whereas in the case of pSBRQ8 the difference in β-galactosidase activity was approximately 8 fold as compared to the control pBluescriptIIKS+. The analysis of β-galactosidase transcript showed that full length transcript was formed in the case of pSBQ8. Full length transcript was not formed in the case of pSBRQ8. We propose that in the case of pSBQ8 the gene expression is inhibited in steps subsequent to transcription elongation. In the case of pSBRQ8, we propose that quadruplex structure may be formed by the template strand at the DNA level thereby blocking transcription elongation step. Chapter 3 describes studies aimed at understanding the structure of G-rich strand (referred to as G strand) of Progressive Myoclonus Epilepsy (EPM1) repeat. The sequence of the G strand of dodecamer EPM1 repeat is d(GGGGCGGGGCGC)n. Oligoucleotides containing one (12mer), two (24mer) and three(36mer) were synthesised. These oligonucleotides are referred to as dG12, dG24 and dG36 respectively. Structural studies were carried out using CD spectroscopy, UV melting, non-denaturing gel electrophoresis and chemical and enzymatic probing. The G strand oligonucleotides showed enhanced gel elecrophoretic mobility in the presence of monovalent cations KCl and NaCl. Oligonucleotide dG12 also showed retarded species on non-denaturing gel in the presence of 70mM KCl indicating intermolecular associations. Oligonucleotides dG24 and dG36 predominantly formed intramolecular structures which migrated anomalously faster than the expected size. The CD spectrum for dG12 showed an intense positive band at 260nm and a negative band at 240nm in the presence of KCl indicative of an intermolecular, parallel G quartet structure. The CD spectra of dG24 and dG36 showed 260nm positive peak, 240nm negative peak along with a positive band around 290nm. This is indicative of folded back structure. These findings support the results of non-denaturing gel electrophoresis of G strand oligonucleotides. The UV melting profiles suggested increase in the stability with the increase in the length. These structures were further characterised by PI nuclease and chemical probing using DMS and DEPC. The structural studies with G-rich strand of EPM1 dodecamer repeat showed that this repeat motif adopts intramolecularly folded structures with increase in the length of the repeat thereby favouring slippage during replication. Chapter 4 deals with the studies aimed at understanding the structure at acidic pH of C-rich strand (referred to as C strand) of Progressive Myoclonus Epilepsy (EPM1) repeat. The sequence of the C strand of dodecamer EPM1 repeat is d(CCCCGCCCCGCG)n. The C rich oligonucleotides are known to form a four stranded structure called i-motif at acidic pH involving intercalated base pairs. The i-motif consists of two parallel stranded, base paired duplexes are arranged in an antiparallel orientation. Since, the base pairs of one base paired duplex intercalate into those of the other duplex, the structure is called as i-motif. We have investigated structure of C strand of EPM1 repeat by circular dichroism (CD), native polyacrylamide gel electrophoresis and UV melting. Oligonucleotide dC12 showed two bands of which the major band was retarded on the native gel (pH 5.0) at low temperature suggesting that dC12 predominantly formed intermolecular structure, Oligonucleotides dC24 and dC36 migrated anomalously faster than the expected size indicating formation of compact, intramolecularly folded structures. Circular dichroism studies indicate that, all the oligonucleotides displayed an intense positive band near 285nm, a negative band around 260nm with a cross over at 270nm, This is a characteristic CD signature for an i-motif structure and reflects the presence of secondary structure due to formation of hydrogen bonded pairs between protonated cytosines. All the C strand oligonucleotides showed hyperchromism at 265nm, which is an isobestic wavelength for C protonation. Studies described in this chapter suggest an intramolecular i-motif structure for dC24 and dC36 and an intermolecular i-motif for oligonucleotide dC12. In addition, it was interesting to note that inspite of the presence of G residues, the stretch of C residues could adopt i-motif structure. Although these structures are formed at an acidic pH, it is indicative of formation of possible intramolecularly folded structure. Many reports have suggested the possibility of cytosine rich sequences adopting i-motif structure even at neutral pH. In order to test this possibility, structural studies were carried out on the C strand EPM1 oligonucleotides at pH 7.2 in the presence of 70mM NaCl. These studies have been described in Chapter 5. The investigations were done using CD spectroscopy, UV melting, native polyacrylamide gel electrophoresis, and chemical probing using hydroxylamine and PI nuclease. These studies indicate that all the C strand oligonucleotides form intramolecular, hairpin structure at physiological pH. All the three C strand oligonucleotides migrated anomalously faster on the native gel indicating the presence of a compact structure. The CD spectra at pH 7.2 showed a blue shift as compared to those at pH 5.0. This indicated absence of base pairs. The hydroxylamine chemical probing suggested presence of G-C Watson-Crick base pairs. The loop residues of the folded back hairpin structures were probed with PI nuclease. The C strand oligonucleotides showed possibility of formation of multiple hairpin structures with the increase in the length of the repeat. The propensity to form hairpin structures suggests a possibility of formation of slip loop structures during the replication process thereby promoting expansion of this repeat. Formation of folded back hairpin like structures is significant in terms of mechanism of expansion of this repeat. Chapter 6 is devoted to concluding remarks highlighting the significance of the experimental results presented in this thesis and their possible biological implications in the light of contemporary research.

Thesis (Ph. D.)--Uniformed Services University of the Health Sciences, 2005. / Typescript (photocopy).