PubMed Health⌕ Search

Biomedical subjects

H C Macgregor

Publications and source records attributed to H C Macgregor.

At least 37 records · Page 2Linked to original sources
PubMed1980

The development of lampbrush chromosome-type transcription in the early diplotene oocytes of Xenopus laevis: an electron-microscope analysis.

Oocytes of Xenopus laevis in pachytene and early diplotene of meiosis have been studied using the Miller spreading technique. Transcription first appears in germinal vesicles 25-40 micrometers in diameter, when the oocyte is in early diplotene. Transcription at this stage consists of arrays of short RNP transcripts, irregularly distributed along the DNP axis. Linear regression analysis has shown that many of these arrays are transcription units (Type I) with the transcripts having a common point of origin. The lengths of these early transcription units (mean = 7.06 +/- 5.06 micrometers), calculated from the linear regression data, are comparable to the lengths of transcription units from later stages, including Dumont stage 3. The polymerase granules of Type I transcription units are widely and irregularly spaced, having a mean spacing of 505 +/- 748 nm. More advanced transcription units (Type II, mean length = 8.72 +/- 3.77 micrometer) are usually found in the same chromosome set as the Type I units. Type II transcription units have closer and more regularly spaced polymerase granules than Type I transcription units (mean spacing = 92 +/- 49 nm). Both Type I and II transcription units have comparatively short RNP transcripts, the mean values for the slopes of their regression lines being 0.1336 and 0.1440 respectively. By the time the germinal vesicles are about 50-60 micrometers in diameter the transcription units have a quite different morphology (Type III). The lengths of the Type III transcription units are comparable to the Type I and II units, the mean length being 6.34 +/- 4.03 micrometers. The spacing of the polymerase granules in the Type III units is closer and more regular than the earlier stages (70 +/- 40 nm). Another significant difference between Type III and Types I and II transcription units is a decrease in the foreshortening of the Type III RNP transcripts. The mean slope of the regression lines for Type III transcription units is 0.2439. The morphological appearance of the Type III transcription unit is virtually identical to that of the transcription units from Dumont stage 3 oocytes, both with respect to the length and the spacing of the polymerase granules. However, the transcripts in Type III transcription units are still more foreshortened than those of Dumont stage 3 oocytes, having mean regression slopes of 0.4728. From the data obtained in the present study, it has been concluded that the pattern of lampbrush-type transcription is virtually fully established by the time most germinal vesicles are about 50 micrometers in diameter.

Animals↗
PubMed1979

In situ hybridization of highly repetitive DNA to chromosomes of Triturus cristatus.

Highly repetitive DNA of C0t 0--0.2 was purified from whole DNA of Triturus cristatus carnifex, labelled by nick translation, and in situ hybridized to RNA transcripts on the loops of lampbrush chromosomes and to the DNA of mitotic chromosomes from intestinal epithelium from the same species. The labelled DNA bound to 20--30 loops on the long arms of lampbrush bivalent 1, a pair of loops near the centromere on bivalent 10, and a number of other loops most of which were localized in pericentric regions. In mitotic preparations the same labelled DNA bound to the heteromorphic regions of the long arms of both chromosomes 1, and to the centromeric regions of all chromosomes. Centromeric labelling was light on chromosomes 4 and particularly clear on the 3 shortest chromosomes of the set. The heavy labelling of the heteromorphic arms of chromosome 1 is discussed in relation to several other peculiar properties of these arms, including their extraordinary lengths, their Giemsa banding patterns, and the absence of meiotic crossing over. It is suggested that insofar as the results with DNA/DNA hybridization and mitotic chromosomes match those obtained with the DNA/RNA-transcript hybridization and lampbrush chromosomes, confidence in the latter technique may be increased accordingly.

Animals↗
PubMed1976

Interspecific "common" repetitive DNA sequences in salamanders of the genus Plethodon.

Intermediate repetitive sequences of Plethodon cinereus which comprised about 30% of the genomic DNA were isolated and iodinated with 125I. About 5% of the 125I-repetitive fraction hybridized with a large excess of DNA from P. dunni at Cot 20. About half of the 125I-DNA in the hybrids was resistant to extensive digestion with S-1 nuclease. The average molecular size of the S-1 nuclease-resistant fraction was about 100 nucleotide pairs. The melting temperature of the S-1 nuclease-resistant fraction was about 2 degrees lower than that of the corresponding fraction made with P. cinereus DNA. These results are taken to indicate the presence in the genomes of P. cinereus and P. dunni of evolutionarily stable "common" repetitive sequences. The average frequency of repetition of the common repetitive sequences is about 6,000 X in both species. The common repetitive fraction is also present in the genomes of other species of Plethodon, although the general populations of intermediate repetitive sequences are markedly different from one species to another. The cinereus--dunni common repetitive sequences could not be detected in plethodontids belonging to different tribes, nor in more distantly related amphibians. The profiles of binding of the common repetitive sequences to CsCl or CS2SO4-Ag+ density gradient fractions of P. dunni DNA suggested that these sequences consisted of heterogeneous components with respect to base compositions, and that they did not include large amounts of the genes for ribosomal RNA, 5S RNA, 4S RNA, or histone messenger RNA. In situ hybridization of the 3H-labelled intermediate repetitive sequences of P. cinereus to male meiotic chromosomes of the same species gave autoradiographs after an exposure of seven days showing all 14 chromosomes labelled. The pattern of labelling appeared not to be random, but was impossible to analyse on account of the irregular shapes and different degrees of stretching of diplotene and prometaphase chromosomes. In situ hybridization of the same sequences to meiotic chromosomes from P. dunni gave autoradiographs after 60 d exposure in which all chromosomes were labelled. These heterologous in situ hybrids can only have involved the "common" repetitive sequences.

Amphibians↗
PubMed1976

In situ hybridization of "nick-translated" 3H-ribosomal DNA to chromosomes from salamanders.

A technique is described for preparation of 3H-labelled DNA by nick-translation employing deoxyribonuclease I and DNA polymerase I. The labelled DNA can be obtained in high yield with specific activities of 10(6) cpm/mug or more. Ribosomal DNA, isolated from ovaries of young Xenopus laevis, and whole DNA from Plethodon cinereus were labelled in this way. The rDNA was used for in situ hybridization to meiotic chromosomes from P. cinereus, P. vehiculum and P. dunni. Autoradiographs of in situ hybrids were exposed for 5 to 10 days, by which time nucleolus organizer regions on the chromosomes of all 3 species were clearly and specifically labelled. In all cases, labelling was confined to a short region near the middle of the short arm of both halves of a medium length bivalent. It is concluded that nick-translation is a useful and altogether efficient method of labelling nucleic acids for subsequent use in experiments involving in situ hybridizations.

Animals↗
PubMed1975

Chromomere number and its genetic significance in lampbrush chromosomes.

The chromosomes of three species of salamander belonging to the genus Plethodon have been studied with regard to the absolute length of the haploid set of lampbrush chromosomes and the number and distribution of chromomeres per laploid set of lampbrush chromosomes. Each aspect has been considered in relation to the substantial difference in genome size between P. cinereus (C = 20 pg), P. vehiculum (C = 36.8 pg) and P. dunni (C = 38.8 pg). --Karyotype analyses carried out on unfixed preparations of lampbrush chromosomes showed that the absolute length of a haploid complement of lampbrush chromosomes from medium-sized yolky oocytes is much greater for P. vehiculum and P. dunni than for P. cinereus. Nonetheless, the relative dimensions of chromosomes are nearly identical in all three species. --Chromomers were counted along the whole length of the 14th (shortest) bivalent, and the total number of chromomeres in the haploid set of lampbrush chromosomes was determined by extrapolation on the basis of the known relative length of the 14th bivalent in the respective species. Chromomeres were also counted in measured segments of undamaged lampbrush preprations in which all chromosomes could be clearly identified. The average distance between chromomeres (chromomere distribution) was determined and the total chromomere number was estimated on the basis of measured total length of lampbrush chromosomes in a preparation. Chromomere distribution is the same for lampbrush chromosomes from all 3 species, and since P. vehiculum and P. dunni have longer chromosomes than P. cinereus, it is clear that the former 2 species have many more chromomeres (60-70%) per haploid set of lampbrush chromosomes. The term chromomere was used here in the descriptive sense to define a discrete granule of deoxyribonucleoprotein on the axis of a lampbrush chromosome.--These findings are discussed in relation to what is known of the molecular organization of eukaryotic genomes, and in relation to ideas on gene action and transcription in lampbrush chromosomes.

Animals↗
PubMed1975

In situ hybridization of ribosomal DNA labelled with 125iodine to metaphase and lampbrush chromosomes from newts.

Methods are described for in situ hybridization of ribosomal DNA from Xenopus laevis, labelled in vitro with 125iodine, to mitotic and lampbrush chromosomes from Triturus cristatus carnifex. The hybridization reaction was carried out in a mixture containing 50% formamide, 4 X SSC, 0.1 M KI, at 37 degrees C, or in 2 X SSC, 0.1 M KI at 65 degrees C. Autoradiographs of mitotic metaphases from 2 males showed labelling over the middle of the short arm of one chromosome IX in each metaphase. In some cases, a region near the end of a longer chromosome was also labelled. In a lampbrush preparations, labelling was confined to a region identified as about 53 units, near the middle of the short arm of both halves of bivalent IX. The usefulness of the technique and the significance of the labelling of only 1 of the 2 chromosomes IX in mitotic preparations are discussed.

Animals↗