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Biomedical subjects

N Hardman

Publications and source records attributed to N Hardman.

At least 55 records · Page 3Linked to original sources
PubMed1980

Sequence organisation in nuclear DNA from Physarum polycephalum. Interspersion of repetitive and single-copy sequences.

Nuclear DNA from Physarum polycephalum is shown to contain three sequence components by reassociation kinetic analysis; a foldback component consisting of 6% of the DNA, a component with the properties of repetitive sequences comprising 31% of the DNA, and a majority component containing 63% of the DNA which reassociates with the kinetics characteristic of single-copy sequences. The complement of repetitive sequences is comprised of about 80 families of repeated elements, each containing approximately 1800 repeats per family. On average, these sequences are 6.4% richer in guanine and cytosine than total Physarum nuclear DNA. The repetitive sequences within a single family appear not to be identical, since on denaturation and annealing they give rise to collections of heteroduplexes less stable than native DNA. It is calculated that these duplexes are about 10% mismatched on average. Hydroxyapatite binding of DNA fragments of different sizes containing reassociated repeated elements demonstrates that these sequences are interspersed with single-copy sequences in a large portion of the Physarum genome. These observations are confirmed by direct examination of reassociated DNA using the electron microscope. In this manner it is shown that repetitive sequence elements possess a wide spectrum of lengths averaging 590 nucleotide residues, and they are separated by intervening segments of DNA about 930 residues in length.

Base Sequence↗
PubMed1980

Utilization of polyadenylate mRNA during growth and starvation in Physarum polycephalum.

The effect of growth on the efficiency of utilization of poly(A)-containing mRNA for translation has been investigated in microplasmodia of Physarum polycephalum. Measurement of the relative proportions of poly(A)-rich mRNA in polysomal and post-polysomal fractions isolated by sucrose density gradient centrifugation reveals that newly synthesized poly(A)-rich mRNA is present in increasing proportions in the polysomal region during exponential growth. However, the proportion of long-lived poly(A)-rich mRNA observed in actively-translating polysomes declines as starvation approaches. The ribonuclease content and morphology of the microplasmodia were monitored during growth and starvation in an effort to related this phenomenon to the onset of spherulation.

Microscopy, Electron↗
PubMed1980

DNA replication in Physarum polycephalum. Analysis of replicating nuclear DNA using the electron microscope.

DNA has been isolated from Physarum polycephalum nuclei obtained from macroplasmodia at different stages in the mitotic cycle, and examined using the electron microscope. Putative replicating structures were identified, the majority of which contained clusters of 2--37 'microbubbles', each microbubble corresponding to a segment of DNA 100--5000 nucleotides long. The microbubble-containing structures are unstable in the formamide hyperphase used to prepare specimens for electron microscopy, possibly due to dissociation of newly replicated nascent DNA fragments from the parental DNA template during manipulation. The microbubble clusters present in early S-phase DNA extent over segments averaging 16400 nucleotide residues, and are separated by non-replicated regions of DNA varying in length from 10000 to 50000 nucleotides. It is suggested that each microbubble cluster may represent a 'replicon', and that many 'replicons' in Physarum DNA may contain several sites for the initiation of DNA synthesis that are active during S-phase.

Cell Nucleus↗
PubMed1979

Microbubbles in replicating nuclear deoxyribonucleic acid from Physarum polycephalum.

Clusters of microbubbles, represent probable sites of newly initiated DNA synthesis, were identified in nuclear DNA from Physarum polycephalum by using the electron microscope. Their presence is associated specifically with S-phase. Each microbubble corresponds in size to a replicating segment of DNA about 100-5000 nucleotide residues in length. The DNA structures containing microbubbles are metastable, and revert to native DNA in the presence of moderate concentrations of formamide used to prepare samples for electron microscopy. It is suggested that each cluster of microbubbles may correspond to a unit of replication (a replicon) in Physarum DNA.

Cell Nucleus↗
PubMed1979

Organisation of inverted repeat sequences in hamster cell nuclear DNA.

Hamster cell nuclear DNA is shown to contain inverted repeat (foldback) sequences, in some respects similar to the foldback fraction in DNA from other animal cell types. Using electron microscopy the majority of foldback duplexes are shown to be located in simple hairpin-like DNA structures, formed from individual pairs of complementary inverted repeated sequences 50--1000 nucleotides in length, in some cases arranged in tandem, and in other cases separated by intervening sequences, up to 16000 nucleotide residues long. In addition, a novel class of foldback structure, referred to as 'bubbled hairpins' is reported, which appear to be formed from clusters of inverted repeat sequences that are separated from adjacent clusters of complementary inverted repeats by large intervening sequences which vary in length from 5000 to over 20000 nucleotide residues. Due to the special pattern of distribution of these latter inverted repeat sequences, 'bubbled hairpins' are observed only in long foldback DNA. Evidence is presented that the distribution of foldback sequences in hamster cell DNA is highly ordered. The lengths of the intervening single chains in foldback structures appear to vary non-randomly. This gives rise to a localised periodic pattern of organisation that is believed to be a consequence of regular alternating arrangements of foldback and non-foldback sequences in the segments of DNA from which foldback structures are derived.

Animals↗
PubMed1979

Characterisation of ribosomal satellite in total nuclear DNA from Physarum polycephalum.

The distinctive properties of satellite DNA molecules containing the genes for ribosomal RNA in Physarum polycephalum permits their identification in total, unfractionated nuclear DNA in the foldback form, after denaturation and fast annealing. Using the electron microscope the location and properties of three characteristic regions containing tandemly-repeated, inverted sequences have been investigated. At least two additional regions, also containing tandem repeats, are shown to be present and located towards each end of the rDNA molecule, at a site adjacent to the segment coding for the 26 S rRNA. All the regions which contain tandem repeats are composed of sequences which, within experimental error, appear to share a common unit repeat length of about 90 nucleotides.

Base Sequence↗
PubMed1979

Distribution of inverted repeat sequences in nuclear DNA from Physarum polycephalum.

Inverted repeat sequences, capable of forming stable intra-chain foldback duplexes, are shown using electron microscopy to be located in over 90% of fragments of nuclear DNA from Physarum polycephalum. A statistical treatment of the data indicates that, on average, foldback sequence foci are spaced every 7,000 nucleotides and that they are distributed uniformly amongst the DNA chains. The majority of inverted repeat sequences give rise to the simple types of foldback structure observed in DNA from other eukaryotic species, but a significant proportion of the DNA fragments also contain novel foldback structures with a more complex appearance, referred to as 'bubbled' hairpins. The latter structures appear to be formed by the annealing of several distinct segments of homologous inverted repeat sequence, each separated by interspersed non-foldback sequences of variable sizes up to 15,000 nucleotides in length. The size, both of the foldback duplexes and of the intervening single-chain segments of DNA, are not random. Instead, they appear to form a regular, arithmetic series of lengths. These observations suggest that the different segments of Physarum DNA from which foldback structures are derived contain nucleotide sequences that share a highly ordered and unform pattern of structural organisation. These regular units of organisation in Physarum DNA in some cases extend over distances up to 50,000 nucleotides in length.

Base Sequence↗
PubMed1978

The effects of acridine orange on deoxyribonucleic acid in Escherichia coli.

1. Acridine Orange inhibits growth of Escherichia coli K12 when incubated at pH 7.9, but not at pH 7.4.2. At a non-permissive temperature for DNA polymerase I, Acridine Orange inhibits growth of a temperature-sensitive strain and also increases the rate of elimination of the F'-Lac plasmid. 3. DNA isolated from cells treated with Acridine Orange under conditions that inhibit growth contains material of low molecular weight, which is absent from DNA isolated from cells treated under conditions in which growth is not impaired. 4. Cells incubated with Acridine Orange at both pH 7.4 and 7.9 suffer degradation of DNA, as shown by loss of labelled DNA from the acid-insoluble fraction, which is not observed with untreated cells at either pH. 5. The results suggest that elimination of the F'-Lac plasmid by Acridine Orange requires inactivation of repair processes.

Acridines↗
PubMed1978

Periodic organisation of foldback sequences in Physarum polycephalum nuclear DNA.

Nuclear DNA from the slime mould Physarum polycephalum is shown to contain interspersed inverted repeat sequences, such that denatured fragments of DNA containing pairs of these sequences form intra-chain duplexes under appropriate conditions. The organisation and distribution of the nucleotide sequences responsible for the formation of foldback structures in Physarum DNA have been investigated using the electron microscope. The majority of foldback duplexes have sizes ranging up to 800 base pairs, and about 60-80% of DNA molecules 2.2 X 10(4) bases in length contain interspersed foldback elements. The size of individual foldback duplexes, and also the length of the intervening sequences which separate them, are non-random. The results can best be explained by a model in which separate foldback foci in Physarum DNA are spaced periodically at regular intervals. The regions containing foldback foci are thought to contain smaller, tandemly-arranged sequences of discrete sizes, in some cases related to other nucleotide sequences of a similar nature in the same locality in Physarum DNA.

Base Sequence↗
PubMed1977

Characterization of foldback sequences in Physarum polycephalum nuclear DNA using the electron microscope.

An examination of the foldback fraction of nuclear DNA from Physarum polycephalum has been carried out using the electron microscope. Results show that the inverted repeat sequences responsible for the formation of foldback DNA range from 150-3000 bases in length, with a number-average size of 340 bases. About one-half of the inverted sequences form looped structures with loop sizes averaging 1200 bases in length. The distance between adjacent foldback sequences is estimated to be in the range 100-1500 bases.

Cell Nucleus↗
PubMed1977

Characterization of foldback sequences in hamster DNA using electron microsocpy.

Foldback sequences in nuclear DNA from cultured Hamster fibroblasts (BHK-21/C13 cells) have been characterized by electron microscopy. One half of the structures observed when denatured hamster DNA is allowed to anneal in the range O less than Cot1 less than 1 x 10(-4) M sec result from the annealing of inverted sequences forming foldback DNA. The remainder have a probable bimolecular origin. arising from rapidly-annealing sequences of satellite-like complexity. The average length of the inverted sequences in the foldback molecules is about 0.9 kilobases. There is estimated to be about 42,000 such sequences (21,000 pairs) in the hamster genome, approximately 45% of which form looped structures with a mean loop length of 1.74 kilobases. Contrary to previous reports, binding of the renatured duplex molecules to hydroxyapatite results in a poor recovery of structures containing identifiable foldback sequences, due to preferential enrichment of the bound fraction with duplexes formed by intermolecular annealing.

Base Sequence↗
PubMed1976

Replication of the deoxyribonucleic acid of multiple-drug-resistance factor in Escherichia coli.

1. It was shown that a system previously described for labelling R-factor DNA during transfer to an irradiated recipient strain of Escherichia coli did not allow high selectivity in the incorporation of thymine into R-factor DNA. 2. Lack of selectivity was shown to be due to cross-feeding from recipient to donor strain. 3. An improved system using a nalidixic acid-resistant recipient strain is described in which incorporation of thymine into the DNA of donor cells is minimized by addition of nalidixic acid after completion of transfer of the plasmid during conjugation.

Conjugation, Genetic↗
PubMed1974

Formation of rings from segments of HeLa-cell nuclear deoxyribonucleic acid.

Duplex segments of HeLa-cell nuclear DNA were generated by cleavage with DNA restriction endonuclease from Haemophilus influenzae. About 20-25% of the DNA segments produced, when partly degraded with exonuclease III and annealed, were found to form rings visible in the electron microscope. A further 5% of the DNA segments formed structures that were branched in configuration. Similar structures were generated from HeLa-cell DNA, without prior treatment with restriction endonuclease, when the complementary polynucleotide chains were exposed by exonuclease III action at single-chain nicks. After exposure of an average single-chain length of 1400 nucleotides per terminus at nicks in HeLa-cell DNA by exonuclease III, followed by annealing, the physical length of ring closures was estimated and found to be 0.02-0.1mum, or 50-300 base pairs. An almost identical distribution of lengths was recorded for the regions of complementary base sequence responsible for branch formation. It is proposed that most of the rings and branches are formed from classes of reiterated base sequence with an average length of 180 base pairs arranged intermittenly in HeLa-cell DNA. From the rate of formation of branched structures when HeLa-cell DNA segments were heat-denatured and annealed, it is estimated that the reiterated sequences are in families containing approximately 2400-24000 copies.

Base Sequence↗