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During development and metastasis, cells migrate large distances through complex environments. Migration is often guided by chemotaxis, but simple chemoattractant gradients between a source and sink cannot direct cells over such ranges. We describe how self-generated gradients, created by cells locally degrading attractant, allow single cells to navigate long, tortuous paths and make accurate choices between live channels and dead ends. This allows cells to solve complex mazes efficiently. Cells' accuracy at finding live channels was determined by attractant diffusivity, cell speed, and path complexity. Manipulating these parameters directed cells in mathematically predictable ways; specific combinations can even actively misdirect them. We propose that the length and complexity of many long-range migratory processes, including inflammation and germ cell migration, means that self-generated gradients are needed for successful navigation., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

Existence of cellular structures with specific size raises a fundamental question in biology: How do cells measure length? One conceptual answer to this question is by a molecular ruler, but examples of such rulers in eukaryotes are lacking. In this work, we identified a molecular ruler in eukaryotic cilia and flagella. Using cryo-electron tomography, we found that FAP59 and FAP172 form a 96-nanometer (nm)-long complex in Chlamydomonas flagella and that the absence of the complex disrupted 96-nm repeats of axonemes. Furthermore, lengthening of the FAP59/172 complex by domain duplication resulted in extension of the repeats up to 128 nm, as well as duplication of specific axonemal components. Thus, the FAP59/172 complex is the molecular ruler that determines the 96-nm repeat length and arrangements of components in cilia and flagella., (Copyright © 2014, American Association for the Advancement of Science.)

Cadherin-mediated cell adhesion and signaling is essential for metazoan development and yet is absent from all other multicellular organisms. We found cadherin genes at numbers similar to those observed in complex metazoans in one of the closest single-celled relatives of metazoans, the choanoflagellate Monosiga brevicollis. Because the evolution of metazoans from a single-celled ancestor required novel cell adhesion and signaling mechanisms, the discovery of diverse cadherins in choanoflagellates suggests that cadherins may have contributed to metazoan origins.

Large-scale genome sequencing is providing a comprehensive view of the complex evolutionary forces that have shaped the structure of eukaryotic chromosomes. Comparative sequence analyses reveal patterns of apparently random rearrangement interspersed with regions of extraordinarily rapid, localized genome evolution. Numerous subtle rearrangements near centromeres, telomeres, duplications, and interspersed repeats suggest hotspots for eukaryotic chromosome evolution. This localized chromosomal instability may play a role in rapidly evolving lineage-specific gene families and in fostering large-scale changes in gene order. Computational algorithms that take into account these dynamic forces along with traditional models of chromosomal rearrangement show promise for reconstructing the natural history of eukaryotic chromosomes.

Molecular fossils of biological lipids are preserved in 2700-million-year-old shales from the Pilbara Craton, Australia. Sequential extraction of adjacent samples shows that these hydrocarbon biomarkers are indigenous and syngenetic to the Archean shales, greatly extending the known geological range of such molecules. The presence of abundant 2alpha-methylhopanes, which are characteristic of cyanobacteria, indicates that oxygenic photosynthesis evolved well before the atmosphere became oxidizing. The presence of steranes, particularly cholestane and its 28- to 30-carbon analogs, provides persuasive evidence for the existence of eukaryotes 500 million to 1 billion years before the extant fossil record indicates that the lineage arose.

The serial endosymbiosis theory is a favored model for explaining the origin of mitochondria, a defining event in the evolution of eukaryotic cells. As usually described, this theory posits that mitochondria are the direct descendants of a bacterial endosymbiont that became established at an early stage in a nucleus-containing (but amitochondriate) host cell. Gene sequence data strongly support a monophyletic origin of the mitochondrion from a eubacterial ancestor shared with a subgroup of the alpha-Proteobacteria. However, recent studies of unicellular eukaryotes (protists), some of them little known, have provided insights that challenge the traditional serial endosymbiosis-based view of how the eukaryotic cell and its mitochondrion came to be. These data indicate that the mitochondrion arose in a common ancestor of all extant eukaryotes and raise the possibility that this organelle originated at essentially the same time as the nuclear component of the eukaryotic cell rather than in a separate, subsequent event.

Gene expression in eukaryotes is influenced by a wide variety of mechanisms including the loss, amplification, and rearrangement of genes. Genes are differentially transcribed, and the RNA transcripts are variably utilized. Multigene families regulate the amount, the diversity, and the timing of gene expression. The present level of understanding of gene expression in eukaryotes is attributable mainly to biochemical methods rather than to traditional genetics. The new techniques that permit analysis and modification of purified genes of known function will identify both the control regions in eukaryotic genes as well as the molecules within cell that influence gene expression.

In vitro genetic techniques were used to study the sequence requirements for the initiation of specific transcription. Deletion mutants were constructed around the putative promoter of the adenovirus-2 major late and chicken conalbumin genes. Specific transcription in vitro by RNA polymerase B together with a HeLa cell cytoplasmic extract was used as the test for promoter function. With this approach sequences which are essential for the initiation of specific transcription in vitro, were shown to be located between 12 and 32 base pairs upstream from the 5' end of these genes.

Transfer RNA (tRNA) suppression of nonsense mutations in prokaryotic systems has been widely used to study the structure and function of different prokaryotic genes. Through genetic engineering techniques, it is now possible to introduce suppressor (Su+) tRNA molecules into mammalian cells. A quantitative assay of the suppressor tRNA activity in these mammalian cells is described; it is based on the amount of tRNA-mediated readthrough of a terminating codon in the influenza virus NS1 gene after the cells are infected with virus. Suppressor activity in L cells continuously expressing Su+ (tRNAtyr) was 3.5 percent and that in CV-1 cells infected with an SV40- Su+ (tRNAtyr) recombinant was 22.5 percent.

Class III genes require multiple cellular factors for transcription by RNA polymerase III; these genes form stable transcription complexes, which in the case of Xenopus 5S genes are correlated with differential expression in vivo. The minimal number and identity of the factors required to form both stable and metastable complexes on three class III genes (encoding, respectively, 5S RNA, transfer RNA, and adenovirus VA RNA species) were determined. Stable complex formation requires one common factor, whose recognition site was analyzed, and either no additional factors (the VA gene), a second common factor (the transfer RNA gene), or a third gene-specific factor (the 5S gene). The mechanism of stable complex formation and its relevance to transcriptional regulation were examined in light of the various factors and the promoter sequences recognized by these factors.

Six structural homologs of spermidine and five of its precursor, putrescine, were studied for their ability to prevent cytostasis of cultured L1210 leukemia cells induced by alpha-difluoromethylornithine (DFMO), a specific inhibitor of putrescine biosynthesis. High-performance liquid chromatography and competition studies with spermidine indicated that the homologs, which vary in the length of the carbon chain separating the amines, penetrated the cells. The structural specificity of the spermidine carrier was defined. Three of the six spermidine homologs supported cell growth during a 48-hour incubation in the presence of DFMO, indicating that a two-carbon extension of spermidine structure was tolerated for biological function. Two of the five putrescine homologs supported growth after being converted by the cells to their respective spermidine homologs. The central nitrogen of spermidine appears to be essential for function since diamines of chain length comparable to that of spermidine did not prevent DFMO cytostasis. No more than 15 percent of the spermidine normally present in L1210 cells was required for cell proliferation in the presence of DFMO.

Clathrin-coated membranes are intimately associated with a variety of protein transport processes in eukaryotic cells, yet no direct test of clathrin function has been possible. The data presented demonstrate that Saccharomyces cerevisiae does not require clathrin for either cell growth or protein secretion. Antiserum to the yeast clathrin heavy chain has been used to isolate a molecular clone of the heavy chain gene (CHC1) from a library of yeast DNA in lambda gt11. Clathrin-deficient mutant yeast have been obtained by replacing the single chromosomal CHC1 gene with a disrupted version of the cloned DNA. Cells harboring a nonfunctional chc1 allele produce no immunoreactive heavy chain polypeptide, and vesicles prepared from mutant cells are devoid of clathrin heavy and light chains. Although clathrin-deficient cells grow two to three times more slowly than normal, secretion of invertase occurs at a nearly normal rate. Therefore protein transport through the secretory pathway is not obligately coupled to the formation of clathrin-coated vesicles.

A number of genes in higher organisms and in their viruses appear to be split. That is, they have "nonsense" stretches of DNA interspersed within the sense DNA. The cell produces a full RNA transcript of this DNA, nonsense and all, and then appears to splice out the nonsense sequences before sending the RNA to the cytoplasm. In this article what is known about these intervening sequences and about the processing of the RNA is outlined. Also discussed is their possible use and how they might have arisen in evolution.

The differences in the biochemistry of messenger RNA formation in eukaryotes compared to prokaryotes are so profound as to suggest that sequential prokaryotic to eukaryotic cell evolution seems unlikely. The recently discovered noncontiguous sequences in eukaryotic DNA that encode messenger RNA may reflect an ancient, rather than a new, distribution of information in DNA and that eukaryotes evolved independently of prokaryotes.