Upload
john
View
213
Download
0
Embed Size (px)
Citation preview
artists have also utilized explosives in their work, but the results have generally been unpredictable. Now Evelyn Rosenberg, a sculptor working at the New Mexico Institute of Mining and Technology, has harnessed the energy of explosives to create predetermined bas-reliefs in brass, copper and stainless steel.
Rosenberg's first attempts were disappointing, she wrote recently in Leonardo, a journal of technology in the arts. Sometimes the explosives blew holes through the metal or flung the materials into the air. After 80 trials she was finally able to manipulate such variables as the thickness of the metal and the energy of the explosion
The power of plastic explosives can be harnessed to sculpt metal bas-reliefs called detonographs
EXPLOSIVES fused and molded stainless steel, brass and copper to form this four-foot-wide bas-relief titled Forces and Signs. Evelyn Rosenberg created it for Gaitersburg Junior High School in Maryland. The photograph is by Jerry Goffe.
20 SCIENTIFIC AMERICAN February 1989
to create sculptures, which she calls detonographs. Rosenberg has now produced 70 detonographs, seven of which hang above the entrance to the New Mexico Museum of Natural History in Albuquerque.
To create a detonograph, Rosenberg first sculpts a bas-relief in plaster to form a mold. She covers the mold with a three-by-four-foot panel of brass, copper or stainless steel. On the metal panel she sometimes places thin colored metallic foils and such objects as cloth, string or leaves. At a blast site in Socorro, N.Mex., a techniCian covers the layers with Du Pont's (-1 Detasheet, a plastic explosive composed of pentaerythritol tetranitrate, cellulose nitrate and a plastic bipder.
When the technician detonates this explosive sandwich (generating a 20-foot fireball), the explosion drives the metal panel into the mold and thereby reproduces the bas-relief in metal in the instant before the mold shatters. The tremendous pressure of the explosion clads the panel with the metal foils, which provide colored accents; the cloth, string or leaves are driven into the panel at 6,800 meters per second and provide texture by leaving their imprint before disintegrating in the flames. To complete the detonograph Rosenberg cleans and polishes the panel.
Although works similar to detonographs are made by die-stamping, casting at a foundry or repoussage (in which soft metal sheets are shaped by hammering), Rosenberg maintains that her method is more suitable for individual works of art, is less timeconsuming and is relatively inexpensive. Her method also makes it possible to clad one kind of metal with another, something that cannot be accomplished employing the other techniques. She comments: "My expression as a sculptor is reinforced by the magic, primitive quality of explosive art." -Russell Ruthen
BIOLOGICAL SCIENCES
POU! Goes the Homeobox Developmental DNA sequences are found in puzzling places
The homeobox, once thought to be the holy grail of developmental biology, has recently become
a trickier cup to drink from. The homeobox is a short segment of DNA found in genes that appear to control key events in the embryonic develop-
© 1989 SCIENTIFIC AMERICAN, INC
The Tandy lOOOTL
T he Tandy 1000 TL is a powerful computer for personal and business use. Its 80286 microprocessor gives you extraordinary speed and processing power. Plus, the 1000 TL comes with MS-DOS and the DeskMate Graphical User Interface built in, so you can be up and running in seconds, using plain-English commands.
DeskMate features ten applications that let you write reports and letters, prepare budgets, file, draw colorful pictures, create and play back songs and more. Plus, there's PC-Link� an online information service.
You also get the latest in computeraudio technology. When you use DeskMate's sound editor, you can record and edit voice, music or any analog source onto diskettes.
T he 640K Tandy 1000 TL comes with a 311z" disk drive and has room for an additional 311z" and 51/4" drive. A parallel printer adapter, RS-232 serial port, two joystick ports, a clock/calendar and five expansion slots are all standard. You also get a lOI-key enhanced keyboard for the ultimate combination of power, ease of use and affordability.
Tandy Computers: Because there is no better va1ueT� MS-OOS/Reg. TM Microsoft Corp. PC-Link/Reg. TM Quantum Computer Services.
286 power with MS--DOS® and DeskMate®
built in.
T he new generation Tandy 1000 TL. From the best-selling family of PC Compatibles made in America.
.. - - -----. • Send me a 1989 RSC·20
• Computer Catalog
• Mail to: Radio Shack, Dept. 89-A-921 • 300 One Tandy Center. Fort Worth, TX 76102
• Name • I
Address
• City __________ _
• State ZIP • a.;h';' ______ .I ladle IhaeK The Technology Store™
A DIVISION OF TANDY CORPORATION
© 1989 SCIENTIFIC AMERICAN, INC
ment of a very wide range of species. It has been assumed that homeobox proteins (the proteins that are encoded by such genes) function by binding to DNA and thereby controlling gene expression, but hard evidence was lacking.
Now several groups report conclusive evidence that homeobox proteins do bind to DNA and regulate the transcription of RNA molecules (the first step in gene expression). Yet the new findings were made in an unexpected context: not in the differentiating cells of an embryo but in the mature tissues of several mammalian species. It appears that some homeobox proteins determine the fate of mature cells rather than embryonic ones. Thus while the mechanism of the homeobox has been put on a sounder footing, its overall role has been considerably complicated.
The homeobox was first identified in 1983 as a common region in several genes that control the division of the embryo of the fruit fly Drosophila into segments and help to determine the segments' fate (which segment becomes a leg as opposed to becoming a wing, for example). Since that discovery many additional homeobox-containing genes have been found in Drosophila, and similar DNA segments have been implicated in the embryonic development of worms, frogs and mice. In all these cases genes containing the homeobox seem to serve as "master" genetic elements, interacting with a cascade of other genes to send a cell down a specific pathway of development.
The three mammalian genes that have recently been shown to include the homeobox act in a different context. Each of these genes encodes a "transcription factor," which, by binding to a speCific stretch of DNA, influences the rate at which messenger RNA molecules are transcribed. The transcription factors are known as OCT-I, OCT-2 and Pit-I, and each acts in a characteristic range of tissues. At least two of the transcription factors-OCT-2 and Pit-I-serve to determine the final identity of a particular set of mature tissues.
OCT-2 is made in immune-system cells known as B cells; it causes them to produce large quantities of antibodies as they develop into the mature type called the plasma cell. The transcription factor has its effects by triggering immunoglobulin genes, which encode the components of antibodies. It had been known that OCT-2 acts by binding to DNA In a recent paper in Nature a group led by Robert
G. Roeder of Rockefeller University reports that they have cloned the gene for OCT-2 and shown that it contains a homeobox. This result, according to the investigators, is "surprising but significant."
Pit-I, on the other hand, has its effects in the pituitary, where it apparently causes two closely related cell populations to make their characteristic hormones: growth hormone and prolactin (which sustains lactation). Now two groups at the University of California at San Diego School of Medicine (one led by Michael Karin, the other by Michael G. Rosenfeld) report in Cell that they have cloned the Pit-I gene. Both groups agree that the gene for Pit-I includes a homeobox.
The discovery that the homeobox functions in mature cells was not the end of the surprise. Flanking the homeobox in each of the three newly characterized genes was another common segment of DNA The homeobox contains 180 nucleotides coding for 60 amino acids; the new region is slightly larger, including enough DNA to encode 75 amino acids. Because it was discovered in the genes for Pit and OCT and also in the roundworm Caenorhabditis elegans in a gene called unc-86 (which has a similar role in specifying the type of mature tissues), the newly discovered conserved region has been named POU (pronounced "pow").
Although the discovery of the combination of POU and the homeobox is intriguing, it clearly raises as many questions as it answers. Among the more significant puzzles are: What is the common thread between the effects of such genes in early development and in the terminal differentiation of mature tissues? What are the evolutionary relations between these two different functions? Of even greater immedi.ilte interest is the question of how the homeoboxPOU combination works at the level of molecular detail.
The simple answer to the last question would seem to be that each homeobox protein binds to a unique
DNA sequence, thereby triggering a unique gene or combination of genes. Unfortunately the simple answer appears to be false. Unlike OCT-2 and Pit-I, the OCT-I protein is "ubiquitous," or present in not one but many cell types; it triggers the release of proteins synthesized in many different types of cell. Furthermore, the type-specifying protein OCT-2 and the ubiquitous protein OCT-I bind to the same DNA sequence. A simple model in which the identity of a cell results
22 SCIENTIFIC AMERICAN February 1989
from a unique match between a homeobox protein and a particular piece of DNA no longer holds. If the new results are confirmed, then it would seem that the grail holds a beverage of a larger quantity and a lesser potency than once was thought. -John Benditt
A Breed Apart Finicky flies lend credence to a theory of speciation �at is the origin of species?
Conventional wisdom holds that a new species can arise
when members of the same species are separated by geographic barriers; the barriers prevent interbreeding between the two populations, and heritable differences are established as the populations adapt to their different environments. Since Darwin's time, however, it has been proposed that species can also originate "sympatrically"; that is, some members of a population may become reproductively isolated from the rest of the population when no physical barriers to mating exist. Now entomologists may have found the first genetic evidence that sympatric speciation does in fact take place.
The evidence comes from studies of a fruit fly called Rhagoletis pomonella. The insect is a parasite of the hawthorn tree and its fruit, which is commonly called the thorn apple. The life cycle of the hawthorn fly revolves around the thorn apple: the fruit serves as a rendezvous for courtship and mating, the females lay their eggs in it and the larvae feed on it.
At least some of them do. About 150 years ago R. pomonella began to infest apple trees as well as hawthorns. Fly populations diverged into two socalled host races: one that prefers apples and one that prefers hawthorns. A former classmate of Darwin's named Benjamin Walsh first recognized the new apple-fly race and suggested that the differential host preferences could lead to speciation.
In the early 1960's a Harvard University graduate student named Guy L . Bush set out t o discredit Walsh's theory by demonstrating that apple and hawthorn flies were not becoming two different species. But as Bush learned more about Rhagoletis he began to doubt his own premise. Generally organisms are considered separate species if they do not interbreed. Bush realized that if the host fruit provides the site for Rhagoletis mating, breeding between apple and hawthorn flies
© 1989 SCIENTIFIC AMERICAN, INC