ο ο
.C Q.
2 to ζ LU οβ
Ο
Alexander Rich with his model for the left-handed helix form of DNA
kind of staggered array instead of a true helix. Hence, earlier x-ray anal ysis of synthetic DNA (containing alternating adenosine and thymine) did not find left-handed helices. Longer molecules probably can slip into a left-handed helical configura tion, Rich says. For instance, Struther Arnott, an x-ray crystallographer at Purdue University, has studied long synthetic fibers of DNA having the same C-G sequence as the small molecule studied at MIT. Though the structural analysis of the longer molecule is not so complete, it's con sistent with that for the smaller mol ecule—a left-handed helix. The left-handed DNA helix differs in several important ways from the usual right-handed helix. For in stance, instead of straddling the middle of the molecule, the base pairs are stacked around the long axis of the molecule, according to Rich. Thus, the mass of the molecule is more toward the perimeter than toward the center, as in a righthanded helix. However, the bases still form what has come to be called ''Watson-Crick" pairs. To get into the left-handed helix, the G residues in the DNA molecule must assume the unusual syn con formation (in contrast to the usual anti) around the bond between the sugar and nucleotide base. This causes the ring structure of the sugar to pucker, in turn, conferring an un usual conformation on the phosphate that links one nucleotide unit in the DNA molecule to the next. Overall, the sugar-phosphate backbone seems to zigzag along as it curls around the extended DNA molecule—instead of forming a smooth spiral as in a righthanded helix. Hence, Rich has nick
named the new form of DNA "Z," which is short for zigzag. The righthanded helix is known as Β DNA. A random nucleotide base se quence probably does not form a left-handed helix, Rich says, because an alternating sequence of residues in syn and anti conformations is re quired. But Rich does believe it possible for segments of DNA to flip into a Z configuration within a longer stretch of DNA remaining in a right-handed helix. "In doing this, there would be some 'kinking' between the Β and Ζ segments," Rich says. Thus, the Ζ segment would jut out a bit from the rest of the molecule. "This is all speculative," Rich ad mits, meaning that no one has yet seen such kinks of Ζ DNA amidst natural Β DNA. But then, no one has really looked for any. Certain indirect evidence from circular dichroism ex periments is consistent with there being patches of Ζ DNA within Β DNA, he notes. "It's fairly obvious where we should proceed along chemical lines," he says. "It's a little more subtle deciding where to go to pursue the biology." Still, the chemistry embodied in the new Ζ DNA structure carries some highly provocative biological
implications. Perhaps the most in triguing is that the left-handed helix may render some of the nucleotide bases more accessible to incoming chemicals, such as mutagens and carcinogens. For instance, one of the target genes in the familiar Salmo nella-based bacterial test for muta gens devised by biochemist Bruce Ames has a "hot spot" that is partic ularly sensitive to such chemicals. The sequence of that hot spot in cludes an alternating string of C's and G's, Rich says. Rich cites other, altogether differ ent observations that might bear on the biological importance of Ζ DNA. Many eukaryotes (higher organisms) contain short stretches of DNA hav ing an alternating C-G sequence. "Many studies [by others] indicate that, when cytosines are methylated, the gene is inactivated," Rich says. He speculates that such C-G containing DNA might flip into the Ζ form. Meanwhile, the MIT group is crystallizing and analyzing several other synthetic DNA molecules to verify and extend their current find ings. "Any novel discovery is doubted at first," Rich says, "even by the discoverers. Therefore we have to be that much more careful." Jeffrey L. Fox, C&EN Washington
NSF study finds gains for women scientists The end of the year and the decade seems to have prompted several looks at the position of women in the sci entific community. And most of the signs seem to indicate a moderate, but genuine, improvement in the status of women scientists, relative to men, since the beginning of the decade. The National Science Foundation, for instance, has re-examined its classic study of scientifically trained women in the U.S. that was published in 1977. NSF finds that some of the study's calculations were in error and some of its conclusions invalid. Chief among the changes to the report, "Women and Minorities in Science and Engineering," probably is the finding of the portion of scien tifically trained women that are em ployed in the labor force. The original study found that only 53% of the women trained in science were in the labor force in 1974. This was consid ered a particularly startling figure, because 64% of all women with four years or more of college were in the labor force in 1974. Thus, the original NSF data seemed to show that women trained in science were more apt not to be working than women trained in other fields. The recalculation of the data re verses this picture. NSF's revised
data show 84.5% of women trained in science were working in 1974. Simi larly, women comprised 8.0% of the scientific labor force in 1974, rather than the 5.8% first calculated. Their unemployment rate also changed markedly: from 1.8% in the original calculations to 3.9%. Because the first study showed so few scientifically trained women in the labor force, NSF commissioned
Salary differences between men, women increase with experience Men
Women
GOVERNMENT 1-2 3-5 6-10 10
Y e a r s of e x p e r i e n c e
$23,400 25,100 29,800 37.500
$23,200 23.900 27,400 31.700
0.9% 4.8 8.0 15.4
INDUSTRY 2 or less 3-5 6-9 10-14 15-19 20-24
$21,000 23,000 26,800 30,300 33.100 35,400
$19,500 20,900 25.400 28.200 27.200 28,400
7.1% 9.1 5.2 6.9 17.8 19.8
Différence
N o t · : Industry data are for 1977; government data for new hires between 1974 and 1978; all are for Ph.D. scientists and engineers. Source: National Research Council
Dec. 24, 1979 C&EN
15
Distribution of jobs in industry differs by sex Men
Basic research 7%
Management, other
9%
Women
Management, other 5% Development 11%
Management of R&D 14%
Note: Data are for doctoral mathematicians, physical scientists, and engineers in 1977. Source: National Research Council
the Scientific Manpower Commission to take a closer look at women scientists to find out why those who were not working were not. The result was recently released in a four-part study of the female scientific work force. One part looks specifically at women chemistry and engineering graduates of the past 15 years from 10 schools. Altogether 249 engineering graduates and 338 chemistry graduates were studied in 1978. The study found 75% of these women employed, although about 10% of them were working in fields outside science and engineering and about 10% were working only parttime. Of those not working, 13% were students, 10% chose not to work because of family or other concerns, and 2% were seeking employment. The study found that women with children usually still continued to work—64.4% of the mothers were working—and that this likelihood increases with increasing education—91% of mothers with doctorates were working. Half of the chemistry graduates and one quarter of the engineering graduates had breaks in their careers of six months or more. Of those out of the labor force when the study was conducted, 80% intended to return to work. The survey also asked the women what factors, if any, they believed had had a negative impact on their career development. For chemists, the items most frequently mentioned were having young children at home and other family demands on their time. Engineers mentioned these factors less often, more frequently listing geographical restrictions on jobs and unsatisfactory job opportunities. Farther down the list—mentioned 17% of the time by the chemists and 25% by the engineers—was sex, race, or age discrimination. 16
C&EN Dec. 24, 1979
Recommendations coming from this study include providing more opportunities for part-time employment for women and training and updating programs to help women re-enter the work force after breaks in their careers. NSF, through its Women in Science Program, has been experimenting with retraining programs aimed at getting women with degrees in science who are not working back into the scientific work force. Four years into the program, NSF has just completed an evaluation of this project. It finds two things: There is a great demand for such programs, and some that it has sponsored have been highly successful. Altogether, NSF has sponsored 33 projects of this type since 1976. Of these, the most successful, measured by the rate of employment of graduates, are those that train women to work in specialized fields where there is current high demand for personnel. These fields include areas such as engineering and computer science, says Alma Lantz, a research scientist with Denver Research Institute who headed the analysis project. Good projects, Lantz also finds, have close ties with industry, including internships or co-op programs and industrial advisory boards that help design the curriculum. A very successful program at the University of Dayton, for example, retrains bachelor's-level chemists to be chemical engineers. It is a 12month intensive program. Of the 70 students that have gone through the program so far, all but one have found jobs. A number of factors that were thought might influence success in the programs were found not to. Among these are marital status, previous work experience, and problems
adjusting to what Lantz calls onthe-job hassles. NSF is not the only part of government currently examining how women in science are faring. The President's Office of Science & Technology Policy has just published its report on progress during the 1970's of women scientists employed by industry and government. This is a companion study to one issued last spring on women scientists in academia. That first study found that women were becoming more common in tenure-track positions at universities, but that there were still substantial differences between men and women in salaries or achievement of tenure (C&EN,May7,page6). The picture is not greatly different either in government or in industry, the new report finds. In the federal government the number of women scientists and engineers increased 50% from 1974 to 1978, a period when overall federal employment of scientists and engineers increased only 16%. Women scientists were promoted to higher grades and to management positions at a faster rate during that period than were men. But women were typically hired at lower grades and lower pay than men, so that women at the bachelor's and master's levels with the same educational background were typically hired at 20% lower salary than men. For Ph.D. scientists, though, this salary difference was much less. In industry, the proportion of women scientists is about half their portion of the available labor force. For example, women comprise about 6% of the doctoral chemists, but only about 3% of the doctoral chemists working in industry. Women tend to have different work in industry than do men. They are half as likely to be involved in management. Within R&D, women concentrate in basic research whereas men are more likely to be found in development. Perhaps partly because of these job differences, women scientists in industry earn less than men at all stages of their careers. Women with doctorates at mid-career typically earn $4500 per year less than men do; roughly the same salary difference is found in government. The study concludes that very real gains have been made in both government and industry in reducing the differences in the jobs of men and women scientists. Nevertheless, even newly trained women can expect to earn less and advance more slowly than similarly trained men even after a decade of mandated equal opportunity. Rebecca Rawls, C&EN Washington
