共查询到20条相似文献,搜索用时 8 毫秒
1.
2.
3.
4.
Scheres SH 《Journal of molecular biology》2012,415(2):406-418
Three-dimensional (3D) structure determination by single-particle analysis of cryo-electron microscopy (cryo-EM) images requires many parameters to be determined from extremely noisy data. This makes the method prone to overfitting, that is, when structures describe noise rather than signal, in particular near their resolution limit where noise levels are highest. Cryo-EM structures are typically filtered using ad hoc procedures to prevent overfitting, but the tuning of arbitrary parameters may lead to subjectivity in the results. I describe a Bayesian interpretation of cryo-EM structure determination, where smoothness in the reconstructed density is imposed through a Gaussian prior in the Fourier domain. The statistical framework dictates how data and prior knowledge should be combined, so that the optimal 3D linear filter is obtained without the need for arbitrariness and objective resolution estimates may be obtained. Application to experimental data indicates that the statistical approach yields more reliable structures than existing methods and is capable of detecting smaller classes in data sets that contain multiple different structures. 相似文献
5.
Bachtrog D 《Current opinion in genetics & development》2006,16(6):578-585
Sex chromosomes are derived from ordinary autosomes. The X chromosome is thought to maintain most of its ancestral genes over evolutionary time, whereas its Y counterpart degenerates, owing to its lack of recombination. Genomic analyses of young sex chromosome pairs support this view and have shed light on the evolutionary processes underlying loss of gene function on the Y. Studies of ancestral sex chromosomes, however, have also revealed that the process of sex chromosome evolution can be more dynamic than traditionally appreciated. In particular, ancient Y-chromosomes are characterized not only by a loss of genes relative to the X but also by recurrent gains of individual genes or genomic regions, and they often accumulate genes beneficial to males. Furthermore, X chromosomes are not passive players in this evolutionary process but respond both to their sex-biased transmission and to Y-chromosome degeneration, through feminization and the evolution of dosage compensation. 相似文献
6.
Nanda I Zend-Ajusch E Shan Z Grützner F Schartl M Burt DW Koehler M Fowler VM Goodwin G Schneider WJ Mizuno S Dechant G Haaf T Schmid M 《Cytogenetics and cell genetics》2000,89(1-2):67-78
Sex-determination mechanisms in birds and mammals evolved independently for more than 300 million years. Unlike mammals, sex determination in birds operates through a ZZ/ZW sex chromosome system, in which the female is the heterogametic sex. However, the molecular mechanism remains to be elucidated. Comparative gene mapping revealed that several genes on human chromosome 9 (HSA 9) have homologs on the chicken Z chromosome (GGA Z), indicating the common ancestry of large parts of GGA Z and HSA 9. Based on chromosome homology maps, we isolated a Z-linked chicken ortholog of DMRT1, which has been implicated in XY sex reversal in humans. Its location on the avian Z and within the sex-reversal region on HSA 9p suggests that DMRT1 represents an ancestral dosage-sensitive gene for vertebrate sex-determination. Z dosage may be crucial for male sexual differentiation/determination in birds. 相似文献
7.
Marshall Graves JA 《Biology of reproduction》2000,63(3):667-676
In this review I want to argue that, far from being a macho entity with an all-powerful role in male development, the human Y chromosome is a "wimp." It is merely a relic of the X chromosome, and most or all of the genes it bears-including the genes that determine sex and control spermatogenesis-are relics of genes on the X chromosome that have other functions altogether. 相似文献
8.
The recent origin of sex chromosomes in plant species provides an opportunity to study the early stages of sex chromosome evolution. This review focuses on the cytogenetic aspects of the analysis of sex chromosome evolution in plants and in particular, on the best-studied case, the sex chromosomes in Silene latifolia. We discuss the emerging picture of sex chromosome evolution in plants and the further work that is required to gain better understanding of the similarities and differences between the trends in animal and plant sex chromosome evolution. Similar to mammals, suppression of recombination between the X and Y in S. latifolia species has occurred in several steps, however there is little evidence that inversions on the S. latifolia Y chromosome have played a role in cessation of X/Y recombination. Secondly, in S. latifolia there is a lack of evidence for genetic degeneration of the Y chromosome, unlike the events documented in mammalian sex chromosomes. The insufficient number of genes isolated from this and other plant sex chromosomes does not allow us to generalize whether the trends revealed on S. latifolia Y chromosome are general for other dioecious plants. Isolation of more plant sex-linked genes and their cytogenetic mapping with fluorescent in situ hybridisation (FISH) will ultimately lead to a much better understanding of the processes driving sex chromosome evolution in plants. 相似文献
9.
10.
11.
Richard J Bennett 《Genome biology》2009,10(7):230-4
Analysis of the genomes of multiple Candida species reveals shared traits associated with pathogenesis, but also striking differences in sexual reproduction. 相似文献
12.
Identification of two teleost homologs of the Drosophila sex determination factor, transformer-2 in medaka (Oryzias latipes) 总被引:3,自引:0,他引:3
Shiraishi E Imazato H Yamamoto T Yokoi H Abe S Kitano T 《Mechanisms of development》2004,121(7-8):991-996
Transformer-2 (Tra2), an RNA-binding protein, is an important regulator in Drosophila sex determination. In vertebrates, however, the role of Tra2 homologues is not known. We identified two teleost homologues of Tra2, which we named Tra2a and Tra2b, in medaka (Oryzias latipes). Furthermore, we demonstrated that both Tra2 mRNAs were predominantly expressed in germ cells of both sexes before the onset of sex differentiation, suggesting that both Tra2 homologues might be involved in the sex differentiation in medaka. 相似文献
13.
14.
15.
16.
In order to elucidate a possible role of estrogen receptor in the gonadal sex differentiation and the sex reversal with sex steroids, we examined for the formation of testis or ovary in transgenic medaka fish overexpressing the medaka estrogen receptor under the constitutive medaka beta-actin promoter. The transgenic fish underwent the genetically determined gonadal differentiation and showed the same sex-reversal rates as those of wild-type non-transgenic fish after treatments with estrogen and androgen. These results present invaluable data to reconsider the role of estrogen receptor in the gonadal sex determination. 相似文献
17.
18.
A. C. Joshi 《Journal of genetics》1939,38(1-2):353-356
19.
Plant sex determination and sex chromosomes 总被引:15,自引:0,他引:15
Charlesworth D 《Heredity》2002,88(2):94-101
Sex determination systems in plants have evolved many times from hermaphroditic ancestors (including monoecious plants with separate male and female flowers on the same individual), and sex chromosome systems have arisen several times in flowering plant evolution. Consistent with theoretical models for the evolutionary transition from hermaphroditism to monoecy, multiple sex determining genes are involved, including male-sterility and female-sterility factors. The requirement that recombination should be rare between these different loci is probably the chief reason for the genetic degeneration of Y chromosomes. Theories for Y chromosome degeneration are reviewed in the light of recent results from genes on plant sex chromosomes. 相似文献
20.
Genetic sex-determining systems in vertebrates include two basic types of heterogamety, which are represented by the XX/XY and ZZ/ZW types. Both types occur among amphibian species. Little is known, however, about the molecular mechanisms underlying amphibian sex determination. Recently, a W-linked gene, DM-W, was isolated as a paralog of DMRT1 in the African clawed frog Xenopus laevis, which has a female heterogametic ZZ/ZW-type sex-determining system. The DNA-binding domain of DM-W shows high sequence identity with that of DMRT1, but DM-W does not contain a domain with homology to DMRT1's transactivation domain. Importantly, phenotypic analysis of transgenic individuals bearing a DM-W-expression or -knockdown vector strongly suggested that DM-W acts as a female sex-determining gene in this species. In this minireview, we briefly describe the sex-determining systems in amphibians, discuss recent findings from the discovery of the DM-W gene in terms of its molecular evolution and its function in sex determination and ovary formation, and introduce a new model for the ZZ/ZW-type sex determination elicited by DM-W and DMRT1 in X. laevis. Finally, we discuss sex-determining systems and germ-cell development during vertebrate evolution, especially in view of a conserved role of DMRT1 in gonadal masculinization. 相似文献