Early Life Forms Research Articles

Moral Pluralism and the Debate over Research on Embryonic Tissue

Debates about embryo and human embryonic stem (ES) cell research reveal deeply divided judgments about the status of early forms of human life and are often occasions for polemics rather than respectful dialogue. It is a merit of these two books that they engage their subjects thoughtfully and earnestly, even though they take very different approaches. Green describes his work as a philosophical memoir (p. xv) of his experiences as a member of the 1994 NIH Human Embryo Research Panel. His reflections on that tenure, as well as chapters about human cloning and ES cells, provide a highly engaging critique of what he deems an inappropriate politicization of such debates, especially by conservative religious groups (thus the vortex of his title). The Holland-Lebacqz-Zoloth edited volume, with its twenty chapters from nineteen contributors, provides a broader overview, with four sections devoted, respectively, to the state of current ES cell research, ethical issues, religious perspectives, and policy concerns. These books are especially interesting for the issues of policy ethics that they engage. First is the central, apparently unresolvable issue of the moral status of the human embryo, which raises questions about which perspectives should govern appropriate pluralistic policy. Second, how are we to assess claims about possible scientific breakthroughs, and how should we weigh them against other less quantifiable values and perspectives? Finally, what are the implications of so-called reason and deliberative democracy for embryo and ES cell debates? More generally, how should we craft genuinely respectful public policy on controversial matters that involve collective funding? Too often, emphases on individual liberty and social utility tend to dismiss other concerns as merely symbolic, parochial, or epistemologically privileged. Yet all forms of public discourse, including appeals to individual rights and preference satisfaction, are deeply symbolic in their own right as particular (and often quite minimalistic) depictions of human flourishing. These questions may be less intriguing to many observers than musings about tomorrow's scientific miracles. Yet issues of policy ethics are of fundamental importance because the framework for public dialogue and policy choice we establish now may well set the tone and substance of public debates about reproductive and regenerative medicine for the foreseeable future. Consider how an appropriate pluralism might address the core question in dispute--the moral status of embryonic human life. Green and several contributors to the stem cell volume (Elliot Dorff, Laurie Zoloth, Margaret Farley, Erik Parens, Gilbert Meilaender, and Ted Peters) explore this issue in detail. In comparing and contrasting their perspectives, two questions arise. What interpretive paradigms best frame their differing views and, more pointedly, what, if anything, does for human embryos entail? For a significant minority, including traditional Roman Catholics and others, respect for embryos follows from seeing them as early and quite vulnerable forms of human life and therefore as human subjects worthy of protection. It is not entirely clear what that perspective suggests for the fate of spare embryos, but one can surmise either of two recommendations: that such embryos should be adopted by other infertile couples or, in ways analogous to the ethics of end of life care, that such subjects be allowed to die (that is, that life-suspending treatment be discontinued) but not killed for non-therapeutic reasons. For others who see embryos as very early forms of human pre-personal or even pre-individuated life, the language of respect is more difficult to interpret. It clearly cannot mean that embryos are deemed human subjects. Instead, it may suggest parallels with the paradigm of tissue donation, wherein donor consent is a central requirement. …

Stripped Down to the Bare Essentials

EVOLUTION Coupling the belief that early forms of life arose under inhospitable conditions (high temperatures and low oxygen) to the reasonable assumption that simpler organisms are more ancient than complex ones would place Methanopyrus kandleri near the base not only of the “black smoker” chimneys in the deep ocean but also of the archaeal tree. Nevertheless, having sequenced the genome of this organism, which is capable of growing on H2 and CO2 at temperatures above 100°C, Slesarev et al. conclude just the opposite. A substantial fraction of the M. kandleri proteins can also be found in two other archaeal methanogens, and this trio consistently forms a cluster in trees of archaeal species when analyzed on the basis of genes and operons. Unlike its relations, however, M. kandleri shares only a small percentage of its proteome with bacteria, which the authors suggest may reflect having had fewer opportunities for lateral transfer in its extreme environment, and its present impoverished state (in terms of gene number and signal transduction pathways) appears to be a consequence of life as a minimalist.— GJC Proc. Natl. Acad. Sci. U.S.A. 99 , 4644 (2002).

Planetary cratering: A probabilistic approach

I develop several statistical indices of cratering on planetary surfaces based on Poissonian probability. These cratering formulas, being both analytic and probabilistic, have advantages over numerical or nonprobabilistic approaches in ease of calculation, clarity of interpretation, and evident flexibility. Specific indices developed include the fraction of a planet's surface expected to be cratered N occasions over a given time interval, expected uncertainty in crater coverage, the depth and distribution of developed megaregolith, and the evolution in crater population. For instance, under current conditions, 15% of the Earth's surface should have been cratered one or more times in the past 3 billion years. In the median case, ejecta from these impacts would have blanketed the planet to a depth of 313 m. These indices, of course, depend upon asteroid flux and the minimum asteroid size imposed by atmospheric filtering. Analytical formulas make it simple to account for such variations by feeding in their history from current conditions to those present on the early Earth. If, as is thought, Earth's bolide flux rate has decreased by a factor of 10,000 from its formation until now, then 99.99% of the Earth's surface should have been cratered one or more times in the past 3 billion years. In the first 100 Ma of the early Earth, 90% of its the surface would have suffered more than 50 impacts, even considering the protection of the atmosphere. Ramifications of these bombardment statistics pertain to the survivability of crustal fragments and early life forms.

Cometary impacts into ocean: Thermodynamical equilibrium calculations of high-temperature O2 generation on the early Earth

The early Earth?s atmosphere apparently differed from the present atmosphere mainly in its lack of free O2, and this absence is believed to have been indispensable for the origin of early anaerobic life forms. One of the central problems in Earth science is to explain the apparent transition from the primitive atmosphere (free of O2) to the present atmosphere which contains 21% of the gas. Theoretical models suggest that the initial form of O2 in the Earth?s atmosphere may have been H2O, which was converted into atmospheric O2 mainly through photosynthesis. We have investigated an alternative (abiotic) method for the conversion of H2O to O2: a high-temperature shock generated during a cometary impact into an ocean (or on land). The calculations presented here show that 1% of the present level of O2 could have resulted from an icy 1.3x1016 kg comet entering the early (pre-oxygenic) Earth with a velocity of between about 11 and 30 km s-1.

Abstracts of the General Meeting of the NASA Astrobiology Institute. April 10-12, 2001. Washington, D.C., USA.

A is an exciting new discipline devoted to the study of life on Earth and elsewhere in the Universe—its origin, evolution, distribution, and future. This new area of multidisciplinary study is bringing together the physical and biological sciences to address some of the most fundamental questions of the natural world. The laws of physics appear to hold universally based on astronomical and other observations obtained over many years of study. Similarly, geologists assume that the underlying mechanisms that drive geology on Earth are the same on other planetary bodies. Biologists too have recently come to realize that the study of life itself cannot be fully understood unless viewed from a larger perspective than just our own Earth, prompting them to ask some of the most fundamental questions in all of Science: How did living systems emerge? How do habitable worlds form and how do they evolve? Does life exist elsewhere than on Earth? What are the potentials for survival and adaptation beyond our home planet? These questions are too large, however, to be answered by biologists alone. They require astronomers and physicists to describe the formation of life’s chemical precursors, to discover new planets, and to determine their habitability; chemists to understand the transition from molecular interaction to life itself; geologists to search for evidence of water and key minerals on other planets; paleontologists and evolutionary molecular biologists to look for and comprehend the earliest forms of life, as well as climatologists, planetary scientists, and researchers from nearly every field of science. And, of course, they require biologists to trace the evolution of life on Earth, to define the very limits which organisms can survive, and to understand the capacity of terrestrial life to adapt to the conditions of space. Together, these researchers and others, comprise the new discipline which is astrobiology. In 1998 NASA, recognizing the significance of this new area of research, established the NASA Astrobiology Institute (NAI). Serving as one of the major mechanisms by which NASA funds research in this field, the NAI is currently composed of 15 Lead Research Teams1, which together represent over 700 investigators across the United States. Their research spans the questions outlined in the Astrobiology Roadmap Report, which defines the direction and major objectives for this new field. Lead Team research is supported by NASA through Cooperative Agreements with Ames Research Center, the NASA Center selected to focus on astrobiology, for 5 year periods with the opportunity to recompete for renewal. The next Cooperative Agreement Notice is expected to be announced in early 2002. The NAI also has partnerships with, at present, three international astrobiology organizations; the Centro de Astrobiologia (Spain), the United Kingdom Astrobiology Forum and Network, and the Australian Centre for Astrobiology. The overarching shared goal of the NAI and its members is the conduct of excellent and innovative research in astrobiology, with an emphasis on collaborative work, both within and among its Teams. Focus Groups are an example of NAI’s collaborative infrastructure. Through proposals approved by the NAI, Focus Groups are formed to stimulate new areas of research, promote longdistance collaborations, and contribute to astrobiology space missions. They promote the cooperation of scientists both from several Teams and from scientists not in the NAI to join together for a project of mutual interest. At present, six Focus Groups are chartered. The NAI Mars and NAI Europa Focus Groups concentrate on specific planetary exploration objectives. The NAI Mission to Early Earth, NAI Mixed Microbial Ecogenomics, NAI EvoGenomics, and NAI Astromaterials Focus Groups each address innovative and interdisciplinary efforts formed around a specific topic judged to be of particular significance for advancing the field of astrobiology. All individual members of the NAI are invited, every other year, to attend a science conference to present and discuss new research. The following abstracts represent the papers presented and displayed at the Year 2001 Meeting of the Institute, which was held April 10–12 at the Carnegie Institution of Washington, one of the Lead Team Institutions of the NAI.

Soluble Adenylyl Cyclase as an Evolutionarily Conserved Bicarbonate Sensor

Spermatozoa undergo a poorly understood activation process induced by bicarbonate and mediated by cyclic adenosine 3',5'-monophosphate (cAMP). It has been assumed that bicarbonate mediates its effects through changes in intracellular pH or membrane potential; however, we demonstrate here that bicarbonate directly stimulates mammalian soluble adenylyl cyclase (sAC) activity in vivo and in vitro in a pH-independent manner. sAC is most similar to adenylyl cyclases from cyanobacteria, and bicarbonate regulation of cyclase activity is conserved in these early forms of life. sAC is also expressed in other bicarbonate-responsive tissues, which suggests that bicarbonate regulation of cAMP signaling plays a fundamental role in many biological systems.

An Overview Of The Origin Of Life: The Case For Biological Prospecting On Mars

Studies of the Earth's earliest biosphere have suggested a close coupling between the evolution of early life forms and the physical and chemical evolution of the planetary surface. From a biological perspective there were many similarities between early Earth and early Mars. This has led to the idea that an origin of life event may have occurred on Mars, leading to the development of microbial life. Various theories have been advanced to explain the origin of life on Earth, and these are reviewed with relevance to Mars. If traces of past or present biogenic activity are to be found on Mars, then the most likely place to prospect is several kilometers below the surface where liquid water might be stable. Such prospecting may best lend itself to human exploration.

Spinach CSP41, an mRNA-Binding Protein and Ribonuclease, Is Homologous to Nucleotide-Sugar Epimerases and Hydroxysteroid Dehydrogenases

Spinach CSP41 is part of a protein complex that binds to the 3′ untranslated region (UTR) ofpetDprecursor-mRNA, a chloroplast gene encoding subunit IV of the cytochrome b6/f complex. CSP41 cleaves the 3′-UTR ofpetDmRNA within the stem-loop structure, suggesting a key role in the control of chloroplast mRNA stability. We discovered that CSP41 is homologous to nucleotide-sugar epimerases and hydroxysteroid dehydrogenases while seeking distant homologs of these enzymes with a hidden Markov model-based search of Genpept. This analysis identifiedSynechocystisORF, Accession 1652543 as a homolog. Subsequent analyses show that spinach CSP41 andArabidopsis thaliana2765081 are homologous to theSynechocystisORF. Information from the solved 3D structures of epimerases and dehydrogenases and our motif analysis of these enzymes is used to predict domains on CSP41 that are important in binding and metabolism of mRNA. Cyanobacteria are among the earliest life forms, indicating that the divergence from a common ancestor of nucleotide-sugar epimerases and an mRNA binding protein with ribonuclease activity was ancient.

Permeation of membranes by the neutral form of amino acids and peptides: relevance to the origin of peptide translocation.

The flux of amino acids and other nutrient solutes such as phosphate across lipid bilayers (liposomes) is 10(5) slower than facilitated inward transport across biological membranes. This suggest that primitive cells lacking highly evolved transport systems would have difficulty transporting sufficient nutrients for cell growth to occur. There are two possible ways by which early life may have overcome this difficulty: (1) The membranes of the earliest cellular life-forms may have been intrinsically more permeable to solutes; or (2) some transport mechanism may have been available to facilitate transbilayer movement of solutes essential for cell survival and growth prior to the evolution of membrane transport proteins. Translocation of neutral species represents one such mechanism. The neutral forms of amino acids modified by methylation (creating protonated weak bases) permeate membranes up to 10(10) times faster than charged forms. This increased permeability when coupled to a transmembrane pH gradient can result in significantly increased rates of net unidirectional transport. Such pH gradients can be generated in vesicles used to model protocells that preceded and were presumably ancestral to early forms of life. This transport mechanism may still play a role in some protein translocation processes (e.g. for certain signal sequences, toxins and thylakoid proteins) in vivo.

A tooth, a toe, and a vertebra: The genetic dimensions of complex morphological traits

AbstractMost readers probably share with me the profoundly affecting experience of wandering through a natural history museum and being surrounded by the skeletal remains of our vertebrate predecessors. Who does not stand in awe before these scaffolds of the great dinosaurs or ponder the groping stages through which our primate ancestors explored the skeletal and dental opportunities of forest life (Fig. 1)? How did this proliferation of limbs, teeth, and vertebrae, not to mention the complexity we can infer about the overlying anatomy and physiology, arise from the simple early forms of animal life? How did DNA evolve to contain the program for such complexity?

Divergence and speciation as viewed from an insect hybrid zone

Hybridism is examined both from Darwin's point of view and in light of current views on the species concept. It seems that many of the relevant concepts of population genetics and speciation are inherent in primitive molecular evolution and may be useful in explaining divergent evolution from the earliest life forms. When divergent genomes meet they may hybridize and, in many cases, form a hybrid zone. Work on insect hybrid zones is used to exemplify the factors and processes involved. Many such zones are the result of postglacial secondary contact and involve some form of hybrid unfitness. They usually comprise multigenic differences and each gene may have different properties. Two grasshoppers, Podisma pedestris and Chorthippus parallelus, are used as case studies. In the case of P. pedestris, as a result of vegetation analysis, it is possible to be fairly sure that the zone is not environmentally determined but rather the result of heterozygous disadvantage at many loci. In the other species, in which two divergent mate recognition systems meet, there is no evidence of their reinforcement toward speciation, despite the F1 hybrids showing severe testis dysfunction and providing another example of Haldane's rule. A scenario for the postglacial contact and hybridization is proferred in an attempt to explain differences between disjunct sections of the zone. It is argued that gene flow occurs over only very small distances in hybrid zones and that the zones themselves will not move far because they fall in low density traps. Compound hybrid zones may be formed by cycles of range contraction and expansion and the substructure of a species will be determined by its life style. Hybrid zones provide a demonstration that divergence occurs in various characters at different rates and divergence brings with it hybrid disadvantage and frequency dependence. Speciation is seen as some way down this path, but occasional transfers of genetic material may still occur.

Light-dependent pH gradients are generated in liposomes containing ferrocyanide

Ferrocyanide solutions absorb light energy in a photochemical reaction that causes cyanide radicals to be lost from the iron complex. Because cyanide is a weak acid (p K a = 9.2), at pH ranges between 6 and 9 it associates with protons, thereby causing the pH of its solutions to increase. The pH change is proportional to the photon flux and concentration of ferrocyanide. For instance, the pH of an unbuffered 1.0 mM potassium ferrocyanide solution increases from 6 to 9 within seconds after illumination. We have encapsulated ferrocyanide in liposomes, and demonstrated that light-dependent pH gradients were readily produced, alkaline inside. This preparation offers a convenient system for generating pH gradients across lipid bilayer membranes. It also provides a useful model for simulating primitive bioenergetic systems relevant to light energy transduction by early forms of life.

Dualism in the early history of Norwegian associations

The early forms of organizational life in Norway display many similarities with the other Nordic countries, but also special features of their own. These features to a great extent have their roots and parallels in countries outside the Nordic region, but particularly Norwegian conditions are also part of the picture. The conventional model namely, development from the corporations of feudal society, via the associations of liberalism and the Nachtwachterstaat (which were often closely connected with the ruling elite), to the mass organizations of the late nineteenth century does not entirely fit Norway or at least requires substantial modification. Moreover, in Norwegian the term association has been used both in the nineteenth century and by twentieth-century scholars in a fairly broad sense as synonymous with forming and selskap (both of which mean society), and it will also be used in that way in this article. The term estate (stand) was often used both at the time and by later historians, but the terminology of the classic estate-based society is to some extent inappropriate when applied to Norway. Part of the explanation is that Norway had a small nobility and few towns and that in many parts of the country the peasantry had long been engaged in a wide variety of economic activities, including some fishing, forestry, the handicrafts which led to thinking in monetary terms and to the use of money. Liberal currents of thought were strong in leading circles inside the young state of Norway, especially from the 1830s and 40s onwards. The emancipation of the state from its traditional tasks and the dismantling of public regulations were seen as the ideal.. Britain was the great model in this respect. There are certainly examples of the state/public sector using or encouraging associations in order to avoid direct intervention or activity by the state. Measures taken by the state were generally disguised as

Interpreting the behavior of enzymes: purpose or pedigree?

To interpret the growing body of data describing the structural, physical, and chemical behaviors of biological macromolecules, some understanding must be developed to relate these behaviors to the evolutionary processes that created them. Behaviors that are the products of natural selection reflect biological function and offer clues to the underlying chemical principles. Nonselected behaviors reflect historical accident and random drift. This review considers experimental data relevant to distinguishing between nonfunctional and functional behaviors in biological macromolecules. In the first segment, tools are developed for building functional and historical models to explain macromolecular behavior. These tools are then used with recent experimental data to develop a general outline of the relationship between structure, behavior, and natural selection in proteins and nucleic acids. In segments published elsewhere, specific functional and historical models for three properties of enzymes--kinetics, stereospecificity, and specificity for cofactor structures--are examined. Functional models appear most suitable for explaining the kinetic behavior of proteins. A mixture of functional and historical models appears necessary to understand the stereospecificity of enzyme reactions. Specificity for cofactor structures appears best understood in light of purely historical models based on a hypothesis of an early form of life exclusively using RNA catalysis.

Upper Proterozoic evaporites in the Amadeus basin, central Australia, and their role in basin tectonics

The Amadeus basin, an isolated intracratonic basin in the center of the Australian continent, contains an upper Proterozoic to mid-Paleozoic stratigraphic succession of shallow-marine sediments that, in places, exceeds; 14 km in thickness. The Gillen Member of the Bitter Springs Formation, which occur toward the base of the upper Proterozoic succession, includes evaporites which are among the world9s earliest (∼0.8 to 0.7 Ga). Because of their age, the evaporites have been cited in discussions of sea-water chemistry and have been the focus of scrutiny for early life forms. In spite of their importance, the evaporites are poorly known, particularly from the viewpoint of their depositional and tectonic setting. In an attempt to rectify this deficiency, more than 6,000 km of seismic data were analyzed, in conjunction with a field and well-log study of the unit. The Late Proterozoic Amadeus basin appears to have consisted of two major poorly circulated anoxic sub-basins, which perhaps opened to the ocean to the southeast through the Adelaide geosyncline. Data relating to facies are limited but suggest that deposition of the evaporites was cyclic and followed the patterns identified in other major evaporite basins, the carbonates and sulfates being closer to the basin margins and later stage halite and possibly potassium salts being toward the basin center. The evaporites were deposited in a shallow-marine setting at the time of a relative sea-level high stand. The apparent sea-level high, may relate to basin dynamics, whereas the cyclicity of the evaporites may be due to eustatic sea-level controls acting on the barrier to allow intermittent inflow of sea water. Salt tectonism began shortly after the evaporites were deposited and continued throughout basin development. Consequently, most of the major anticlinal structures have salt cores. The geometry of the salt structures suggests that during their growth, the mean strain rate was 10 −16 s −1 , a rate typical of large salt structures. Growth on these salt structures has played an important role in controlling later sedimentation, particularly during the Cambrian.

On the primacy of primordial RNA

The ability of RNA to catalyze biochemical reactions is used to develop a self-consistent picture of how a primordial RNA could have given rise to the necessary factors and processes of early life forms. Essential to this proposal is the impact of RNA structural domains, “selected” by thermodynamic criteria, on the structure of early proteins (exons) and the assembly of functional complexes. Based on this analysis, the chronological appearance of informational molecules follows the order: primordial RNA, proteins whose structures are determined by primordial RNA sequences and finally DNA.

The Test-Tube Baby Debate: An Anthropological View

In the ongoing debate about the fate of 'surplus' human embryos in the test-tube baby programme, technically referred to as blastocysts, arguments continue as to how these early forms of life are to be treated. Some support research on these entities because of the potential benefits that might accrue, while other groups vehemently oppose any research at all because they would see it as a violation of the sanctity of human life. And earlier this year the British Parliament also became embroiled in the debate with the tabling of Enoch Powell's Unborn Children (Protection) Bill. A summary of the major arguments is given by Smith.! However, despite their differences, both sides in the dispute share one important argument namely, each tends to refer to nature, to the natural order of things, as a grounding for its particular position. Each group sees fit to draw upon what it sees as its own objective view of nature, to legitimate its position on the ethics of research.

An intron in the 23S ribosomal RNA gene of the archaebacterium Desulfurococcus mobilis

The archaebacteria have been defined, at a molecular level, as constituting a third primary kingdom consisting of the methanogens, the extreme halophiles, and the sulphur-dependent extreme thermophiles. In reaching this conclusion, Woese and colleagues used the 16S ribosomal RNA as an approximate chronometer for evolutionary time and demonstrated that, at a nucleotide sequence level, the archaebacteria are as different from the eubacteria and eukaryotes as the latter kingdoms are from one another. Current research on archaebacteria is yielding valuable insights into the evolutionary relationships between archaebacteria, eubacteria and eukaryotes, and into the early forms of cellular life. Here, we extend this knowledge by providing the first evidence for the occurrence of an intron within any prokaryotic ribosomal RNA. The intron was found within the 23S rRNA gene of the sulphur-dependent and anaerobic Thermoproteale Desulfurococcus mobilis, which was isolated from hot acidic springs in Iceland at temperatures up to 97°C. The intron contains 622 base pairs (bp); it is very A+T-rich (65%) compared with the 23S rRNA gene (34%), and it exhibits a large open reading frame. The splicing site occurs in domain IV of the 23S RNA at a position close to that of an intron of the lower eukaryote Physarum polycephalum; the intron does not readily fall into one of the three classes of eukaryotic nuclear introns because it has features in common with those of classes I (rRNA) and III (transfer RNA).

Filamentous microfossils from the 3,500-Myr-old Onverwacht Group, Barberton Mountain Land, South Africa

The Swaziland Supergroup, Barberton Mountain Land, South Africa, has long been regarded as a promising location for the Earth's oldest fossils because it includes some of the most ancient well-preserved sedimentary rocks, many of which contain carbonaceous matter. Although there have been numerous reports of microfossils from Swaziland Group rocks1–7, the biogenicity of most of the structures has been questioned8–10. Although some of the organic spheroids are probably biogenic10,11, the best early Archaean simple spheroids are generally regarded as ‘possible microfossils’10 because organic spheroids may form abiotically in several ways12. The discovery of less-simple biological morphologies is therefore important in establishing the existence of early life forms in the early Archaean. Uniformly-sized curving filaments, especially tubular ones, are difficult to explain as anything other than the fossil remains of filamentous organisms. Here we report the discovery of numerous filaments from two different stratigraphical positions in the 3,500-Myr-old Onverwacht Group of the Swaziland Supergroup. Their morphologies and abundance provide convincing evidence for the existence of bacteria- or cyanobacteria-like organisms on the Earth during the early Archaean. This supports recent reports of similar filamentous microfossils from 3,500-Myr-old rocks from Western Australia13.

Electronmicroscopy and circular dichroism of the dynamics of the formation and dissolution of supramolecular forms of Z-DNA.

Previous electronmicroscopic studies had shown that N-acetylaminofluorene (AFF)-substituted poly(dG-dC)-poly(dG-dC) in the Z conformation, in 10mM Mg++, condensed into periodically banded, branched structures. We now show that similar structures are seen when poly(dG-dC)-poly(dG-dC) is converted to the Z conformation by heating to 60 degrees C in 1mM Mn++ or to 65 degrees in the presence of 0.5mM Mn++. We demonstrate that these banded structures form in solution, i.e. they are not artifacts of the preparative procedures used for electronmicroscopy, by crosslinking the Z conformers in solution with DL-diepoxybutane (DEB), and then restoring the solution to conditions that favor return to the B conformation. Circular dichroism (CD) and immunochemical studies showed that the Z conformation was maintained and the banded supramolecular structures were still seen by electronmicroscopy. Electronmicroscopy and CID were also used to follow the dissolution of the supramolecular structures by controlled scission of the crosslinks with the eventual return to the short double stranded molecules typical of the B conformers. During this process, supercoiled structures, both toroidal and interwound, were observed. The relationship of the toroids to the banded structure is discussed in the context of two possible structures for the condensed polynucleotide. We conclude that DNA, whether in the B or Z conformation, is extremely flexible in the presence of appropriate counter ions, and we present evidence that earlier estimates of their persistence lengths are too high. The inherent tendency to form condensed, highly organized structures is a property of DNA that could play an important role in its "packaging," and in its functions, and might have been critical for the evolution and replication of early life forms.