scispace - formally typeset
Search or ask a question

Showing papers on "Metaphase published in 1969"


Journal ArticleDOI
TL;DR: A bifunctional highly fluorescent quinacrine mustard was shown to give chromosome breaks preferentially localized to heterochromatic chromosome regions (identifiable by cold treatment) in the M-chromosome.

300 citations


Journal ArticleDOI
TL;DR: Great and reproducible differences in the uptake of the fluorescent reagents by different chromosome regions were observed and a good correlation was found between regions reacting with several different fluorescent compounds and heterochromatin as defined by cold treatment.

264 citations


Journal ArticleDOI
TL;DR: In this paper, the onset of the first mitosis after doses of 500 rads was delayed as expected from previous studies of the age dependence of "mitotic delay," and the interval between this mitosis and the next was indistinguishable from that for unirradiated control cells, while the subsequent two generations were again prolonged, on the average, though not so severely as was the irradiated generation.

130 citations


Journal ArticleDOI
01 Dec 1969-Virology
TL;DR: The results are taken to indicate that the virus initiates chromosomal DNA synthesis but that infection eventually results in extensive destruction of the cell genome, and thus the death of the cells.

85 citations


Journal ArticleDOI
TL;DR: Points raised include consideration of a possible homologue of the so‐called centrosome of pennate diatoms and certain aspects of the numerical relation between chromosomes and spindle tubules.
Abstract: SUMMARY A spindle precursor is shown to be a relatively constant feature in non-dividing cells of Lithodesmium undulatum and its structure is described and illustrated. It is a small somewhat rectangular body closely pressed to the nuclear envelope and composed of a series of parallel plates of semi-opaque material placed edgeways on a square ground plan about 0·6 μm in length and breadth. The plates at each end are the densest and the next most dense marks the centre. During prophase in the spermatogonia the spindle proper is laid down between the precursor and the underlying nuclear envelope. It consists of tubules which increase in size and number as prophase advances, the spindle poles being marked by extensions from the terminal plates of the precursor, the rest of which ultimately breaks down, thereby permitting rapid elongation of the spindle itself to take place. After breakdown of the nuclear envelope, the spindle sinks into the mass of chromosomes, as previously described with the light microscope in other diatoms. The fine structure of the metaphase spindle is described and illustrated, special attention being paid to the number and arrangement of the component microtubules, both in a normal cell and a giant cell. Anaphase is not examined but a few stages of telophase are added, including some stages of cleavage. The results are discussed in a preliminary way pending completion of a study of meiosis. Points raised include consideration of a possible homologue of the so-called centrosome of pennate diatoms and certain aspects of the numerical relation between chromosomes and spindle tubules.

84 citations


Journal ArticleDOI
TL;DR: The results are consistent with the interpretation that the amount of satellite DNA in each chromosome is proportional to the total DNA of the chromosome and that it is not limited to a fixed amount per chromosome.

81 citations


Journal ArticleDOI
TL;DR: The G1, S and G2 periods of N. plumbaginifolia were found significantly different from those of the hybrid derivative, but the duration of the mitotic cycle did not show a significant difference.
Abstract: The duration of the mitotic cycle with its component phases in Nicotiana plumbaginifolia (pbg, 2n = 20) and a hybrid derivative of N. tabacum (tbc, 2n = 48 and N. plumbaginifolia was determined from the root tips. A 30 minute pulse label of H3-thymidine was employed for the autoradiographic detection of the labeled prophases.The time intervals, for the total generation time (T), postsynthetic phase (G2), prophase, synthetic phase (S), and the combined presynthetic phase, metaphase, anaphase, telophase (G1 + M + A + T) in N. plumbaginifolia were estimated as 11.0, 3.4, 1.3, 5.7 and 0 6 hours, respectively. In the hybrid derivative the corresponding estimates were 9.0, 1.4, 0.8, 3.8 and 3.0 hours.The G1, S and G2 periods of N. plumbaginifolia were found significantly different from those of the hybrid derivative, but the duration of the mitotic cycle did not show a significant difference. On the basis of the distribution of heterochromatin and its late replication in N. plumbaginifolia chromosomes, possible...

80 citations


Journal ArticleDOI
TL;DR: The decrease in protein synthesis which occurs in mammalian cells during cell division is associated with significant disaggregation of polyribosomes, and whether messenger RNA survives this disaggregation was investigated in synchronized HeLa cells.
Abstract: The decrease in protein synthesis which occurs in mammalian cells during cell division is associated with significant disaggregation of polyribosomes. For determining whether messenger RNA survives this disaggregation, the reformation of polyribosomes was investigated in synchronized HeLa cells as they progressed from metaphase into interphase in the presence of 2 µg/ml Actinomycin D. The persistence of messenger during cell division was evidenced by: (1) a progressive increase in the rate of protein synthesis in both treated and untreated cells for 45 min after metaphase; (2) reformation of polyribosomes, as determined by both sucrose gradients and electron microscopy, within 30 min after the addition of Actinomycin D to metaphase cells; (3) the persistence of approximately 50% of the rapidly labeled nonribosomal RNA which had associated with polyribosomes just before metaphase; (4) the resumption of synthesis, following cell division, of 6 selected peptides in Actinomycin-treated cells.

79 citations


Journal ArticleDOI
TL;DR: Anaphase was characterized in both fungi by separation of chromosomes to poles established by the centrioles, and in F. oxysporum anaphase separation of chromosome separation was observed in vivo.
Abstract: Vegetative nuclei of fungi Ceratocystis fagacearum and Fusarium oxysporum were studied both in the living condition with phase-contrast microscopy and after fixation and staining by HCl-Giemsa, aceto-orcein, and acid fuchsin techniques. Nucleoli, chromosomes, centrioles, spindles, and nuclear envelopes were seen in living hyphae of both fungi. The entire division process occurred within an intact nuclear envelope. Spindles were produced between separating daughter centrioles. At metaphase the chromosomes became attached to the spindle at different points. In F. oxysporum the metaphase chromosomes were clear enough to allow counts to be made, and longitudinal splitting of the chromosomes into chromatids was observed. Anaphase was characterized in both fungi by separation of chromosomes to poles established by the centrioles, and in F. oxysporum anaphase separation of chromosomes was observed in vivo. Continued elongation of the spindles further separated the daughter nuclei. Maturing daughter nuclei of both fungi were quite motile; and in C. fagacearum the centriole preceded the bulk of the nucleus during migration. The above observations on living cells were corroborated by observations on fixed and stained material.

78 citations


01 Jan 1969
TL;DR: The spatial relationships between the homologous pairs of chromosomes in the normal human colcemid-treated metaphase plate were tested by two different mathematical approaches.
Abstract: The spatial relationships between the homologous pairs of chromosomes in the normal human colcemid-treated metaphase plate were tested by two different mathematical approaches: (a) determination of the distances between the centromeres of the homologous chromosomes compared to the mean distance of all centromeres of the mitosis in question; (b) measuring the distances of the different chromosomes from the center of the mitosis.

75 citations


Journal ArticleDOI
TL;DR: Aspects of fertilization encompassing the meiotic events of the maternal chromatin, the formation of the polar bodies, and the development of the female pronucleus have been investigated in the lamellibranch, Mytilus edulis.
Abstract: Aspects of fertilization encompassing the meiotic events of the maternal chromatin, the formation of the polar bodies, and the development of the female pronucleus have been investigated in the lamellibranch, Mytilus edulis. The cromosomes of the mature egg of Mytilus edulis at the time of fertilization are organized on the first metaphase plate of meiosis. The first meiotic metaphase figure, oriented normal to the egg's cortex, consists of two asters, each of which contains paired centrioles, microtubules, and some smooth endoplasmic reticulum. Subsequent to sperm incorporation, the first polar body is produced by a process akin to cytokinesis following the movement of the chromosomes at anaphase. Later, the metaphase plate of the second meiotic apparatus is formed. Morphologically, the second metaphase figure is similar to the first; however, each aster appears to contain only one centriole. The second polar body, produced in the same manner as the first, remains associated with the zygote via a cytoplasmic bridge which persists up to the time of the first cleavage. Both polar bodies contain the same cellular inclusions as the zygote; however, only the second polar body has its chromatin delimited by a perforated nuclear envelope. After the formation of the second polar body, the maternal chromatin is organized into chromosomal vesicles. The chromosomal vesicles later fuse to form the female pronucleus.

Journal ArticleDOI
TL;DR: It is suggested that the formation of wall rings, characteristic of cell division in Oedoyonium, represents an adaptation of more usual processes of algal mucilage secretion.
Abstract: Development of new processing techniques has enabled a light-and electron-microscopic study of cell division in Oedoyonium to be made; many unusual or unique features are described. A series oflight micrographs confirms the sequence of events described by earlier workers. Splitting of the cell wall at the ring has been observed in vivo; the rupture is often quite violent, and subsequent elongation is rapid for a short while. At the ultrastructural level, the premitotic movement of the spherical nucleus towards the wall ring coincides with the appearance of some nearby micro-tubules. The preprophase nucleus enlarges and becomes flattened and spindle-shaped as a sheath of microtubules envelopes it. The nuclear membrane at each pole is very drawn out, appearing as two closely apposed membranes undulating into the cytoplasm; between the membranes is a dense amorphous material, and around them are longitudinally oriented microtubules. During prophase, chromatin condensation accompanies nucleolar dispersion. Microtubules start appearing within the nuclear membrane which remains essentially intact throughout subsequent division. At prometaphase, intranuclear microtubules increase and some are attached to diffuse scattered kinetochores. By metaphase, many bundles of microtubules are attached to paired kinetochores, which have a complex, layered structure. Separated kinetochores are visible at anaphase. At telophase, the spindle becomes very elongated, and many interzonal microtubules appear. A granular and heavily staining "midbody" is found between the nuclei, and this later disperses. The daughter nuclei then come very close together, and between them is found a complex of vesicles and microtubules. A septum is then formed across the cell, evidently by these microtubules (accompanied by the vesicles) pushing out the tonoplast across the vacuole till partitioning is accomplished. The nuclei then separate prior to wall splitting. The ring formed at one end of the cell has a highly characteristic structure with two lips of toughened material at its outer edge attached to the outer wall; splitting of the older wall occurs between these lips. A proliferation oflarge, electron-transparent golgi vesicles was apparent at this stage; these were apparently being discharged into the vacuole, and so may have been associated with a possible build-up of turgor pressure within the cells. The soft material of the ring is drawn out between the lips during cell elongation. The septum (still composed of vesicles and microtubules) moves up the cell like a diaphragm, eventually reaching the bottom lip of the extended ring structure. Coalescence of vesicles then occurs in the septum to form the new transverse wall attached to the lower lip from the ring. The extensible material of the ring becomes the outer layer of the cell (mucilage?); the cell wall proper is evidently formed as a thinner layer on the inside edge of this diffuse material. It is therefore suggested that the formation of wall rings, characteristic of cell division in Oedoyonium, represents an adaptation of more usual processes of algal mucilage secretion.

Journal ArticleDOI
TL;DR: Young wheat seedlings were centrifuged; premitotic and dividing cells of the leaf epidermis were examined, and some evidence suggests that the subsidiary cell nucleus and cytoplasm at telophase are primarily responsible for the formation of the new, highly curved cell wall.

Journal ArticleDOI
TL;DR: Optically controlled serial sections for electron microscopy show that the mid-bodies seen in light micrographs and synaptinemal complexes seen in electronmicrographs are the same structure.
Abstract: In chiasmatic meiosis of mosquitoes, ascomycetes and lilies the synaptinemal complex (SC) disassociates from the bivalent before metaphase I. Conversely, in the achiasmatic meiosis of Bolbe nigra, the SC remains associated with the bivalent during first metaphase. Light microscopy reveals mid-bodies between disjoining half-bivalents during early first anaphase in Bolbe. Optically controlled serial sections for electron microscopy show that the mid-bodies seen in light micrographs and synaptinemal complexes seen in electron micrographs are the same structure. Electron micrographs indicate that the SC breaks transversely at a point corresponding to the chromosomal kinetochore during anaphase I as the chromatin and the SC begin to separate. During telophase I, SC remnants are at the poles with the chromosomes or between poles. Presently, the evidence is inadequate to state whether the SC serves alternately or simultaneously as a biological contrivance for conjunction and crossing-over or singly as a device for one of these phenomena.

Journal ArticleDOI
TL;DR: Quantitative and qualitative cytological observations strongly suggest movement of intact wall microtubules to the spindle at preprophase and then back again at telophase.
Abstract: Dividing cells of Spirogyra sp. were examined with both the light and electron microscopes. By preprophase many of the typical transverse wall micro-tubules disappeared while others were seen in the thickened cytoplasmic strands. Microtubules appeared in the polar cytoplasm at prophase and by prometaphase they penetrated the nucleus. They were attached to chromosomes at metaphase and early anaphase, and formed a sheath surrounding the spindle during anaphase; they were seen in the interzonal strands and cytoplasmic strands at telophase. The interphase nucleolus, containing 2 distinct zones and chromatinlike material, fragmented at prophase; at metaphase and anaphase nucleolar material coated the chromosomes, obscuring them by late anaphase. The chromosomes condensed in the nucleoplasm at prophase, moving into the nucleolus at prometaphase. The nuclear envelope was finally disrupted at anaphase during spindle elongation; at telophase membrane profiles coated the reforming nuclei. During anaphase and early telophase the interzonal region contained vacuoles, a few micro-tubules, and sometimes eliminated n ucleolar material; most small organelles, including swollen endoplasmic reticulum and tubular membranes, were concentrated in the polar cytoplasm. Quantitative and qualitative cytological observations strongly suggest movement of intact wall rnicrotubules to the spindle at preprophase and then back again at telophase.

Journal ArticleDOI
TL;DR: Ritonucleotide reductase activity was studied during the replication cycle of Colcemid synchronized Chinese hamster fibroblasts in culture to indicate that an inhibitor was not reponsible for the low levels of activity during the G1 phase.

Journal ArticleDOI
TL;DR: Human lymphocytes were grown in short-term tissue culture and were arrested in metaphase with Colcemid to induce relaxation of fiber packing and reveal certain underlying fiber arrangements.
Abstract: Human lymphocytes were grown in short-term tissue culture and were arrested in metaphase with Colcemid. Their chromosomes were prepared by the Langmuir trough-critical point drying technique and were examined under the electron microscope. In addition, some chromosomes were digested with trypsin, Pronase, or DNase. The chromosomes consist entirely of tightly packed, 240 ± 50-A chromatin fibers. Trypsin and Pronase treatments induce relaxation of fiber packing and reveal certain underlying fiber arrangements. Furthermore, trypsin treatment demonstrates that the chromatin fiber has a 25–50 A trypsin-resistant core surrounded by a trypsin-sensitive sheath. DNase digestion suggests that this core contains DNA.

Journal ArticleDOI
TL;DR: Mature mice were induced to ovulate by injections of PMS and HCG and their eggs were prepared, using an air-dry technique, for the examination of chromosomes during the first and second meiotic divisions, indicating that a certain length of time may be required for capacitation of mouse sperm.
Abstract: Mature mice were induced to ovulate by injections of PMS and HCG and their eggs were prepared, using an air-dry technique, for the examination of chromosomes during the first and second meiotic divisions Sixty percent of 72 eggs recovered from follicles between 10 and 85 hours before ovulation were at prophase, whereas 91%–64% of 139 eggs were at metaphase I from 7 to 3 hours before ovulation Anaphase I and telophase I were seen in 17 eggs 45–0 hours before ovulation The bivalents were clumped together in a single mass at the earliest stage of diakinesis Separate, extended bivalents which underwent contraction up to metaphase I were observed later Counts of bivalents in 82 eggs at diakinesis and metaphase I revealed no deviation from the haploid number (n = 20) The dyads in the first polar body were usually extended and somewhat diffuse in appearance, whereas those in the egg were not A total of 295 eggs were examined from 24 females inseminated 15 hours after ovulation At 3, 35, 45 and 55 hours after ovulation 3%, 20%, 26% and 43% of recovered eggs respectively were penetrated by sperm, indicating that a certain length of time may be required for capacitation of mouse sperm Approximately 90% of eggs with an intact sperm head (Type I) were at metaphase II and nearly 100% of eggs with a swollen sperm head (Types III and IV) were at telophase II Four distinct configurations of metaphase II, “spread,” “clumped,” “compact,” and “granular” were seen in 384 unfertilized eggs recovered from 36 females at various times after induced ovulation The haploid number of dyads was observed in all of 70 eggs counted at metaphase II in the “spread” configuration The proportion of unfertilized eggs with a “compact” metaphase II increased from 2% at 5–7 hours to 12% at 24–315 hours after ovulation, while the proportion of eggs with a “granular” metaphase II increased from 11% to 47% over the same period The second meiotic metaphase exhibited these degenerative changes mainly at the end of the fertilizable period of the egg

Book ChapterDOI
TL;DR: Considerable amount of experimental evidence has provided results consistent with the observational appearance of half-chromatids, and the effects of incorporated isotopes are expressed in generations succeeding labeling.
Abstract: Publisher Summary Various observational evidences indicate that chromosomes are multistranded. These observations are obtained by light microscopic studies on organisms containing large chromosomes. The two half-chromatids separate quite far from one another and can be found interwound over great distances along their length. In favorable material the separation can be found even in prophase and metaphase. Considerable amount of experimental evidence has provided results consistent with the observational appearance of half-chromatids. The evidence falls into five different categories dealing with (1) the types of aberrations induced in late prophase or metaphase by radiation, (2) the types of aberrations induced by radiation at the end of G 1 before the chromosomes have replicated, (3) experiments in which chromosome structure has been unraveled by treatment with enzymes or other agents, (4) the distribution of labeled DNA among the chromosomes at mitoses subsequent to labeling, and (5) experiments in which the effects of incorporated isotopes are expressed in generations succeeding labeling.

Journal ArticleDOI
TL;DR: A fine structural analysis of apolar mitosis induced by chloral hydrate was made on Haemanthus katherinaeBak and the relation of ER to formation of MTs is evident and briefly discussed.
Abstract: A fine structural analysis of apolar mitosis induced by chloral hydrate was made on Haemanthus katherinae Bak. endosperm. Under the influence of chloral hydrate MTs disappear initially and then are formed de novo. Kinetochore fibers grow away from kinetochores and their formation is asynchronous for all chromosomes in the set and also for sister kinetochores. Bundles of MTs forming kinetochore fibers converge toward one of the poorly defined polar regions during formation of kinetochore fibers (metaphase) and in motionless kinetochores. Such MTs increasingly diverge when kinetochores move during anaphase. The relation of ER to formation of MTs is evident and briefly discussed. A continuous transition exists between NE and ER during formation and disintegration of the NE. Some theoretical aspects of these problems were also discussed.

Journal ArticleDOI
TL;DR: It would appear that hydroxyurea may prove a useful agent in the study of erythropoiesis because of its selective effect on DNA synthesis in the erythroid series.
Abstract: SummaryA single i.v. dose of hydroxyurea (0.9 g/kg) produced transient marrow erythroid depopulation and reticulocytopenia in the mouse. Within minutes after drug administration there was a virtually complete erradication of marrow DNA synthesis which persisted for approximately 2 hr. Erythroid cell cycle kinetics, estimated with colchicine, revealed an entry of erythroid cells into mitosis for 80 min followed by no further accumulation of metaphase forms until the seventh hour. Thereafter erythroid cells accumulated in mitosis at a normal rate. These findings are consistent with the hypothesis that hydroxyurea kills erythroid cells in DNA synthesis; the initial 80-min accumulation of erythroid mitoses represents the progression of G2 cells into mitosis. Unaffected cells in G1 at the time of drug administration require 7-8 hr to progress through DNA synthesis, G2, and enter mitosis. From these data it would appear that hydroxyurea may prove a useful agent in the study of erythropoiesis because of its sele...

Journal ArticleDOI
01 Aug 1969-Cancer
TL;DR: The DNA‐packing ratio, i.e., the length of DNA double helix per unit length of chromosomal fiber, appears to be much lower than in metaphase chromosomes derived from normal lymphocytes.
Abstract: Chromosomes of Burkitt's lymphoma cell line AL-1 were examined and measured by whole-mount quantitative electron microscopy after isolation by surface-spreading and critical point drying. The ultrastructure of the chromosomes is based entirely on twisted, irregularly folded fibers about 300 A in diameter. An acrocentric marker chromosome, exceeding all others in length, was seen to be connected to 13-15 chromosomes by common fibers. Dry mass values of 5 marker chromosomes from different metaphases ranged from 20.1 to 25.8 × 1O−13 g. Within each measured metaphase, the dry mass ratio of the marker to 13-15 chromosomes was 2.2 and of the marker to 21-22 chromosomes was 4.3. These ratios identify the marker in terms of dry mass as the missing number 2 chromosome. Average fiber dry mass was 11.2 × 10−16 g per micron. For one marker chromosome, total fiber length per chromatid was 1089 microns. Cross-sectional scans through the chromatids of this marker indicated 83 fiber equivalents in the centromere, 68 to 178 in the long arms, and 155 to 157 in the telomeres. On the assumption that the DNA content of the marker is equal to a chromosome 2, the DNA-packing ratio, i.e., the length of DNA double helix per unit length of chromosomal fiber, appears to be much lower than in metaphase chromosomes derived from normal lymphocytes.

Journal ArticleDOI
TL;DR: The time and location of the pericentriolar transitions are consistent with their being intimately involved in the mechanics of chromosome separation, and it may be that the ER serves as a storage depot for some fraction of depolymerized microtubules.
Abstract: As the metaphase HeLa cell approaches anaphase, pericentriolar spindle tubules fragment and become encapsulated by a unit membrane. By early anaphase, the encapsulated forms appear to have expanded, giving rise to polar spherical aggregates. Some of these elements show ribosomes on their bounding membrane, and some of them localize on the condensed chromatin during reformation of the nuclear membrane. It thus is suggested that these elements are newly derived cisternae of the endoplasmic reticulum (ER). Similar transformations are seen in later anaphase in the interzonal region, and it may be that the ER serves as a storage depot for some fraction of depolymerized microtubules. The time and location of the pericentriolar transitions are consistent with their being intimately involved in the mechanics of chromosome separation.

Journal ArticleDOI
TL;DR: The results indicate that constriction length depends on the amount of genome and that the deletion is limited to the constricted segment, supporting the view that the constriction is the nucleolar organizer.
Abstract: Secondary chromosomal constrictions are thought to be the loci of the genome which code for ribosomal RNA synthesis. Their metaphase length could depend on nucleolar size or level of functional activity in interphase or on gene content. Wild-type frogs and a frog heterozygous for the Oxford nucleolar mutation were studied to determine which possibility is more probable. The mutant was studied because its single nucleolus is larger than wild-type nucleoli, it has only one constriction, half as many ribosomal genes, but produces the same amount of ribosomal RNA. The results indicate (1) that constriction length depends on the amount of genome (whereas others have shown nucleolar size to be related to level of activity) and (2) that the deletion is limited to the constricted segment, supporting the view that the constriction is the nucleolar organizer. Also, metaphase constrictions are longer than expected from their DNA content.

Journal ArticleDOI
TL;DR: DNA replication studies were carried out on metaphase plates showing satellite associations to determine the association of individual D chromosomes.
Abstract: DNA replication studies were carried out on metaphase plates showing satellite associations to determine the association of individual D chromosomes. In a total of 279 D chromosomes involved in satell

Journal ArticleDOI
TL;DR: It is concluded that the apparent asynchrony of replication of homologues is principally the result of different degrees of condensation of the chromatin in the late-S phase at the time of synthesis, and these differences are related to the proximity to the nuclear membrane.
Abstract: The peripheral location of H3-thymidine labelled chromosomes in spreads of colcemid blocked metaphases from human fibroblasts has been investigated in several karyotypes. Chromosomes show non-random distribution in relation to the periphery of the plate. Longer chromosomes are significantly more peripheral than smaller chromosomes. Those chromosomes which contain large amounts of late-synthesising DNA (Y, 18 and 13) are significantly more peripheral than early synthesising chromosomes of similar length (21–22, 19–20 and 14–15). These locations are compared with those from peripheral leucocytes obtained by other workers. When location and grain number of homologues of nos. 1, 2, 3, 16 and late-X are compared it was found that in late-S labelled cells, the more peripheral of each pair was significantly more heavily labelled; in early-S labelled cells the reverse was the case. Location and patterns of labelling between late-X homologues of XXXXY cells showed that peripheral location was related to more delay in DNA synthesis. These results are discussed in terms of peripheral and central interphase distribution of condensed chromatin. It is concluded that the apparent asynchrony of replication of homologues is principally the result of different degrees of condensation of the chromatin in the late-S phase at the time of synthesis, and these differences are related to the proximity to the nuclear membrane.

Journal ArticleDOI
TL;DR: Findings indicated that some mitosis-related protein is synthesized during a short period prior to mitosis and is conserved and reutilized by daughter cells for their own subsequent divisions, consistent with previous findings on the effects of p-fluorophenylalanine.

Journal ArticleDOI
TL;DR: The experiments indicate that the human somatic chromosome is a multistranded structure that loosens in late G 1 to form functional chromatids.
Abstract: If cells are irradiated during most of the G 1 portion of the cell cycle, chromosome aberrations are produced. If, however, they are irradiated while in S or G 2 , chromatid aberrations are formed. The best estimate of when the transition occurs is late G 1 . That is, in both plant and animal cells, the chromosome splits into chromatids before DNA synthesis. This doubling of the chromosome conceivably could be caused by either a synthesis of chromosomal protein prior to S, or by a loosening of a multistranded struture. Experiments were carried out with phytohemagglutinin-stimulated human lymphocytes to test which of the two possible explanations cited above would be the more likely. Phytohemagglutinin-stimulated lymphocytes were cultured in the presence of 2·10 −3 M hydroxyurea and triated thymidine for 40 h. The hydroxyurea was used to inhibit DNA synthesis and to enrich the population of cells at the end of G 1 . Cells were then irradiated with 200 R of X-rays and recultured in the absence of tritium. When the cells reached metaphase, unlabeled cells were scored for the types of aberrations induced by the prior irradiation. Chromatid aberrations were found, indicating that in human cells, too, the split occurs in G 1 in the absence of DNA synthesis. Similar experiments in which the protein synthesis inhibitor cycloheximide was added after the cells were in culture for 16 h (before any DNA synthesis had occurred) and kept in until the fortieth hour showed that the transition occurred when protein synthesis was inhibited, too. The experiments indicate that the human somatic chromosome is a multistranded structure that loosens in late G 1 to form functional chromatids.

Journal ArticleDOI
TL;DR: Microspectrophotometric analysis of various developmental stages reveals that the total nuclear complement of the quiescent embryo is in G1, and that the initiation of DNA synthesis occurs at approximately the 1.5–2.0 mm. point in root development, in agreement with the value determined earlier.
Abstract: SUMMARYThis study of the duration of the mitotic cycle in the germinating onion shows a mitotic cycle at stability of 12.8 hours, with G1, S, G2, and M-periods of 1.5, 6.5, 2.4, and 2.3 hours, respectively. Phase durations are 0.8, 0.2, 0.1, and 1.2 hours for prophase, metaphase, anaphase, and telophase, in that order. When examined as a function of developmental stage, no significant variation is seen in the lengths of the above intervals. Microspectrophotometric analysis of various developmental stages reveals that the total nuclear complement of the quiescent embryo is in G1, (essentially all 2C), and that the initiation of DNA synthesis occurs at approximately the 1.5–2.0 mm. point in root development, in agreement with the value determined earlier.

Journal ArticleDOI
TL;DR: The fine structure of radiation-induced chromosomal aberrations in Potorous tridactylis (rat kangaroo) cells was examined in situ by electron microscopy and the interpretation of this evidence is consistent with the hypothesis that the chromosome is a multistranded structure.
Abstract: The fine structure of radiation-induced chromosomal aberrations in Potorous tridactylis (rat kangaroo) cells was examined in situ by electron microscopy. The observations on the structure of terminal deletions (acentric fragments), anaphase bridges and "gaps," sidearm bridges, and specialized regions, such as the nucleolus organizer, are discussed in detail. Conclusions based on these observations are the following: (a) damage is physically expressed only at anaphase; (b) a gap region is composed of two subunits, each of which is about 800-1000 A in diameter and may correspond to a half-chromatid structure; (c) the ends of acentric fragments are structurally similar to normal chromosome ends, except where the break occurs in a specific region such as the secondary constriction; (d) at metaphase the fragment and the main portion of the chromosome move as a single unit to the equator, and the two units are disconnected only at the onset of anaphase; (e) sidearm bridges appear to be exchanges, involving a subchromatid unit. The interpretation of this evidence is consistent with the hypothesis that the chromosome is a multistranded structure.