RNA evolution and the origins of life

The evolution of RNA is likely to have played an important role in the very early history of life on Earth but it is doubtful that life began with RNA. Consideration of what came before RNA must take into account relevant information from geochemistry, prebiotic chemistry and nucleic acid biochemistry.     

Evidence for the evolution of multiple genomes in arbuscular mycorrhizal fungi.

Ancient asexuals directly contradict the evolutionary theories that explain why organisms should evolve a sexual life history. The mutualistic, arbuscular mycorrhizal fungi are thought to have been asexual for approximately 400 million years. In the absence of sex, highly divergent descendants of formerly allelic nucleotide sequences are thought to evolve in a genome. In mycorrhizal fungi, where individual offspring receive hundreds of nuclei from the parent, it has been hypothesized that a population of genetically different nuclei should evolve within one individual. Here we use DNA-DNA fluorescent in situ hybridization to show that genetically different nuclei co-exist in individual arbuscular mycorrhizal fungi. We also show that the population genetics techniques used in other organisms are unsuitable for detecting recombination because the assumptions and underlying processes do not fit the fungal genomic structure shown here. Instead we used a phylogenetic approach to show that the within-individual genetic variation that occurs in arbuscular mycorrhizal fungi probably evolved through accumulation of mutations in an essentially clonal genome, with some infrequent recombination events. We conclude that mycorrhizal fungi have evolved to be multi-genomic.     

Grain-size evidence for multiple origins of the reticulate red clay in southern China

The loess-paleosol sequences in Chinese Loess Plateau record the multi-cyclic paleo-climatic changes in the Quaternary period[1]. Presently, what interests people most is that at what degrees and scales the climates in southern China had corresponded to the alternating changes be-tween the glacial and interglacial climates in northern China in the Quaternary period. Many researchers[2―10] hope to find some potential Quaternary sediment materials in southern China, just as the loess-paleoso…     

DNA cleavage catalysed by the ribozyme from Tetrahymena.

An RNA enzyme derived from the self-splicing intervening sequence of Tetrahymena thermophila catalyses sequence-specific cleavage of an oligodeoxyribonucleotide substrate. Compared with RNA, the DNA substrate is bound very weakly and is cleaved very slowly, revealing the importance of the RNA 2'-hydroxyl group in both the binding and chemical steps. The finding that catalysis by RNA can extend to DNA substrates indicates new possibilities for the transposition of intervening sequences and for the design of DNA cleavage agents with novel sequence specificities.     

Novel guanosine requirement for catalysis by the hairpin ribozyme

THERE is much interest in the development of 'designer ribozymes' to target destruction of RNAs in vitro and in vivo. Engineering of ribozymes with novel specificities requires detailed knowledge of the ribozyme-substrate interaction, and a rigorous evaluation of sequence specificity. The hairpin ribozyme catalyses an efficient and reversible site-specific cleavage reaction. We have used mutagenesis and in vitro selection strategies to show that RNA cleavage and ligation has an absolute requirement for guanosine immediately 3' to the cleavage-ligation site. This G is not required for efficient substrate binding, rather, its 2-amino group is an essential component of the active site required for catalysis.     

墨西哥玉米草组培体系的建立及愈伤组织切片观察

墨西哥玉米草是与玉米最近野生近缘种之一,可多茬刈割,产量高,品质好,被誉为青饲料之王,在畜牧业中扮演着重要角色,因此对墨西哥玉米草的深入研究具有重大的科研价值和经济效益．虽然墨西哥玉米草杂交体系已有较多报道,但墨西哥玉米草组培体系却鲜有研究．组培快速繁殖体系是通过生物技术对墨西哥玉米草进行品质改良的基础,因此建立高效的墨西哥玉米草组培快速繁殖体系具有重要意义．文中以墨西哥玉米草 8493 种子为实验材料,在含有2,4-D和6-BA的MS培养基上可以诱导幼苗基部的膨大节间,其在黑暗条件下两周可以诱导出愈伤组织．石蜡切片观察显示愈伤组织具备胚性愈伤特性．     

苞叶杜鹃离体培养及种质试管保存体系的建立

以苞叶杜鹃新生嫩芽为外植体,应用均匀设计法筛选其最适合的培养基,建立了苞叶杜鹃的离体培养和种质试管保存体系。结果表明：适合苞叶杜鹃基部直接再生芽苗的诱导培养基为DR+TDZ(1.80 mg/L)+IBA(005 mg/L),此条件下芽苗分化率为100%;生根培养基为MS(改良)+IBA(0.08 mg/L)+NAA(0.02 mg/L)+KT(0.15 mg/L),这一条件下芽苗生根率达99%以上;试管保存培养基为1/7MS+B9(1.30 mg/L)+根皮苷(2.80 mg/L),常温条件下保存时间超过35个月。以苞叶杜鹃再生植株茎节为材料进行快繁的结果表明,在35 d的培养周期内,平均增殖倍数超过55倍。     

Susceptibility variations of multiple origins of loess from the Ily Basin (NW China)

The magnetic susceptibility of loess from the Ily Basin,northwestern China shows maximum values in S0 paleosols but minimum values in other paleosols,the mechanism of which has been well debated.In this work,systematic magnetic measurements were made on a representative section from Neleke county.The results show that the loess horizons(L1,L2 and L3) have multi-domain magnetite grains of aeolian origin,S0 is characterized by production of pedogenetic ultrafine-grained ferrimagnetic minerals,and the other paleosols(S1,S2,and S3) are characterized by the formation of nonferrimagnetic minerals associated with waterlogging.The correlation between the low concentration of ferrimagnets,high paramagnetic content,high magnetic coercivity remanence,fine ferrimagnetic grain size and intensified pedogenesis suggest two competing processes of pedogenetic enhancement and pedogenetic depletion in the lower paleosols.Pedogenetic depletion dominates and is responsible for the low susceptibility.Changes in magnetic grain size distribution occur during pedogenetic depletion.The susceptibility variations are of multiple origins in the loess of the Ily Basin.Pedogenetic enhancement,pedogenetic depletion,and allochthonous input of magnetic minerals should all be taken into account to explain the variations of magnetic parameters.     

RNA substrate binding site in the catalytic core of the Tetrahymena ribozyme.

In catalysis by group I introns, the helix (P1) containing the RNA cleavage site must be positioned next to the guanosine binding site. We have identified a conserved adenine in the catalytic core that contributes to the stability of this arrangement and propose that it accepts a hydrogen bond from a specific 2'-OH in P1. Such base-backbone tertiary interactions may be generally important to the organization of RNA tertiary structure.     

发酵乳杆菌B44益生性能的体外评价

发酵乳杆菌B44是一株分离自保加利亚家庭手工制作酸奶的乳酸菌株.体外实验发现:发酵乳杆菌B44具有较强的疏水性(79.67％)和低酸高碱的静电作用,可耐受1 mg/mL溶菌酶,在口腔中具有较佳的黏附和存活能力;此外,具有较高的自聚率(13.34％)以及与变形链球菌(Streptococcus mutans)的共聚集作用...     

In vitro fertilization: MRC tests the climate

It is reported that Britain's Medical Research Council has published guidelines for MRC-supported research involving human embryos obtained through in vitro fertilization. In ethics committee-approved projects, researchers may use deliberately fertilized ova or, with parents' consent, fertilized ova left over from therapeutic programs. In vitro culture of human embryos is permitted only up to the implantation stage. Conditions are also set for experiments on interspecies fertilization and on freezing of embryos. Concern over physician and public reception of these guidelines has been expressed by the British Medical Association.