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It was not recognized that chromosomes contain one immensely long DNA molecule until Kleinschmidt et al. published electron micrographs of a lysed bacterial cell with DNA spilling out that contained no apparent breaks and, indeed, not even any free ends ( 1 ). We now know that before replication, each of our chromosomes contains a single DNA molecule of immense length. If the DNA of an average human chromatid were a wire with a diameter of 2 mm, then this wire would be 50 km long. The vast length of chromosomal DNAs poses a number of fundamental problems. For example, how are they packaged during cell division into cylindrical threadlike chromatids? On page 672 of this issue, Terakawa et al. ( 2 ) describe an activity associated with a protein complex called condensin that has the potential to answer this. Remarkably, it might also explain chromosome segregation in bacteria as well as the mechanism that regulates enhancer-promoter interactions during development.

著者人数: 22名, Accepted: 2011-08-02, 資料番号: SA1003195000

When a major computer chip manufacturer brings a new integrated circuit to the market, its competitors often rush to reverse-engineer the chip to understand how it works. They shave off thin layers of the semiconductor material while imaging each newly revealed surface under an electron microscope. By tracing the wires in the electron micrographs, computer engineers assemble the wiring diagram of the chip, which allows them to eventually deduce its function. Similarly, neuroscientists hope to gain insights into the way the nervous system generates behavior by reconstructing its wiring diagram. Like the solid-state wires in computer chips, biological “wires” that carry electrical signals are tens of nanometers in diameter, requiring electron microscopes and ultrathin sectioning to image them. On page 437 of this issue, Jarrell et al. ( 1 ) use this approach to reconstruct the nervous system in the tail of a male Caenorhabditis elegans roundworm.

Identified reticulospinal neurons whose giant axons were severed after spinal cord transection were filled with horseradish peroxidase. Whole mounts and serial-section light and electron micrographs show axon regeneration across the spinal lesion and the formation of new synapses. Normal swimming activity returns in the spinally transected animals, although the regenerated synapses are in atypical regions of the spinal cord.

A new virus-like particle has been found in Drosophila. Thus far it has been detected only in electron micrographs of certain cells capable of division, such as those in larval imaginal disks, cell lines derived from imaginal disks, cells from a genetically controlled brain tumor, and adult gut cells. It appears to be slightly elliptical in shape, about 37 millimicrons by 45 millimicrons, and occurs in both the nucleus and cytoplasm. From the evidence it is suggested that the particle is a new virus.

Electron micrographs of positively stained preparations of nonmembranous ghosts prepared from HeLa cell nuclei have revealed the presence of an array consisting of rodlike and annular structures interconnected by strands sensitive to deoxyribonuclease. This array is believed to be responsible for the spherical shape of nuclei that are free of membrane. In addition, a configurational change in this array may be associated with the cyclic dissolution and reformation of the nuclear envelope that accompanies mitosis in mammalian cells.

Ribosomes, subunits and polysomes, have been isolated from cells of ripening (senescent) fruit. Sedimentation in sucrose gradients, sedimentation constants, and electron micrographs confirm the physical resemblance of fruit ribosomes to those from other living tissues.

Fourier analysis of electron micrographs has been accomplished underptimum conditions with a gas laser as the light source for an optical diffractometer.

Electron micrographs of serial sections through a meiotic prophase nucleus of the mycetozoan Labyrinthula sp. show that there are nine separate and distinct synaptinemal complexes. Since each complex represents a set of paired homologous chromosomes, it follows that the haploid chromosome number of this protist is nine.

Replicating circular molecules of colicin factor E1 have been isolated from minicells produced by a colicinogenic derivative of Escherichia coli P678-54. The replicating structures observed in electron micrographs are circular molecules with two branched points. Several structures found among the replicating molecules are interpretable as replicating rolling circles.

With some knowledge of perspective and by use of a perspective chart it is possible to construct an accurate three-dimensional illustration depicting the interrelation of tissue components from electron micrographs of serial sections of biological materials. Tracings of pertinent structures from electron micrographs are overlaid with a grid plane and subsequently redrawn on a perspective grid plane. The new tracings are then connected to one another in the most logical manner to form an illustration in perspective.

Some of the features revealed by the rotation technique of examining electron micrographs may be artifacts. To avoid misinterpretation, "rotated" photographs should be compared with originals. The criterion of reinforcement of structures in the "rotated" photographs is not enough, and it cannot be presumed that all the structures reinforced are actually present in the object under study. Further tests for radial symmetry in the arrangement of elements in a pattern should be made.

Electron micrographs of vertebrate and invertebrate smooth muscle indicate that the myofilaments are oriented obliquely to the long axis of the muscle fibers containing them and insert along the sides of the fibers. As a result, a greater proportion of the contractile elements are in parallel with one another and a smaller proportion are in series than would be possible if the myofilaments were strictly parallel to the fiber axis. From this ultrastructural organization it is possible to predict several well-known, but previously unexplained, physiological properties of smooth muscle.

Polyuridylic acid was deposited on carbon films in the presence of the thallous salt of mercury-p-dihydroquinone-O,O-diacetic acid under conditions leading to attachment of one of these molecules to each nucleotide. In electron micrographs of the stained polynucleotide chain, detail was obscured by the noise resulting from the substrate. Optically obtained autocorrelation functions of the distribution of density along the strands revealed peaks suggesting a structural regularity of 8 angstroms; this was interpreted as the internucleotide spacing.

Electron micrographs of purified replicating form DNA of the bacteriophage øX-174 show that it has ring structure. The circular property may explain both the abnormally high sedimentation coefficient and the great resistance of this DNA to irreversible heat denaturation. Intactness of the circle is required for the control of strand selection during transcription into complementary copies of RNA.

Electron micrographs of living specimens of the various developmental stages of the insect Tribolium confusum have been obtained with a scanning electron microscope. In most cases the specimens resumed their normal activity after being examined with the electron microscope and under went metamorphosis into the next stage.

Many intact axial units, with attached basal plates, are found in spermatozoa of Childia groenlandica negatively stained with phosphotungstic acid. Electron micrographs show a total of nine doublet microtubules, confirming observations on sectioned material, where nine peripheral doublets, but no single central ones, occurred. Living spermatozoa move by waves progressing along a double undulating membrane.

Combined tracings from electron micrographs of serial sections of leaf tissue infected with tobacco etch virus show that one type of cytoplasmic inclusion, when sectioned in different planes, can produce configurations which have been interpreted as being two distinct types of inclusion bodies.

Contrary to earlier reports, testes of "sex ratio" Drosophila pseudoobscura males have only half as many sperm per bundle as testes of normal males do. This fact, determined from electron micrographs, indicates that the functional pole hypothesis is not applicable to the action of "sex ratio."

Two-dimensional signal averaging has been applied to dark field electron micrographs of molecules of 2,3,4,5-tetraacetoxymercurithiophene. Only the mercury atom images are seen in single micrographs. However, in the composite image, resulting from photographic superposition of 64 individual images, the sulfur atom in the molecule is clearly revealed.