首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1736篇
  免费   55篇
  国内免费   40篇
  1831篇
  2024年   20篇
  2023年   28篇
  2022年   42篇
  2021年   52篇
  2020年   41篇
  2019年   65篇
  2018年   44篇
  2017年   63篇
  2016年   62篇
  2015年   73篇
  2014年   84篇
  2013年   117篇
  2012年   72篇
  2011年   49篇
  2010年   55篇
  2009年   63篇
  2008年   67篇
  2007年   73篇
  2006年   72篇
  2005年   76篇
  2004年   68篇
  2003年   61篇
  2002年   48篇
  2001年   42篇
  2000年   51篇
  1999年   45篇
  1998年   25篇
  1997年   19篇
  1996年   24篇
  1995年   22篇
  1994年   17篇
  1993年   22篇
  1992年   21篇
  1991年   19篇
  1990年   21篇
  1989年   19篇
  1988年   15篇
  1987年   11篇
  1986年   4篇
  1985年   16篇
  1984年   11篇
  1983年   6篇
  1982年   6篇
  1981年   4篇
  1980年   3篇
  1979年   3篇
  1978年   5篇
  1976年   2篇
  1975年   1篇
  1973年   1篇
排序方式: 共有1831条查询结果,搜索用时 15 毫秒
1.
2.
A series of large chromosomal deletions in Streptomyces hygroscopicus 10-22 were aligned on the physical map of the wild-type strain and the mutants were assessed for their ability to produce the aminocyclitol antibiotic 5102-I (jinggangmycin). Twenty-eight mutants were blocked for jinggangmycin production and all of them were found to lack a 300 kb AseI-F fragment of the wild-type chromosome. An ordered cosmid library of the 300 kb AseI-F fragment was made and one of the cosmids conferred jinggangmycin productivity to Streptomyces lividans ZX1. Three of the overlapping cosmids (18G7, 5H3 and 9A2) also hybridized to the valA gene of the validamycin pathway from S. hygroscopicus 5008 as a probe. This gene resembles acbC from Actinoplanes sp. 50/110, which encodes a C7-cyclitol synthase that catalyses the transformation of sedoheptulose 7-phosphate into 2-5-epi-valiolone for acarbose biosynthesis. The valA/acbC-homolog (orf1) of S. hygroscopicus 10-22 was shown to be essential for jinggangmycin biosynthesis as an engineered mutant with a specific in-frame deletion removing a 609 bp sequence internal to orf1 completely abolished jinggangmycin production and the corresponding knock-out mutant (JXH4) could be complemented for jinggangmycin production by the introduction of an orf1-containing construct. Concurrently, the identities of the genes common to S. hygroscopicus strains 10-22 and 5008 prompted a comparison of the chemical structures of jinggangmycin and validamycin, which led to a clear demonstration that they are identical.The first two authors contributed equally to this study.  相似文献   
3.
A population of Chironomus riparius from a Po river station near Moncalieri (a trace-metal polluted station) was studied. In this population was established a great variability of band structure of polytene chromosomes as well as paracentric heterozygous inversions, deletions, deficiencies, partial breaks, diploid chromosome fragments, and changes in functional activity and appearance of heterochromatin. In arms A through F, some bands had an increased size compared to the standard chromosomic map. Some bands appeared in a heterozygous or normal homozygous state or were amplified. In all arms, many condensed stable bands appeared in the decondensed state when compared to the standard map. Asynaptic zones in arms E and G as well as heterozygous Balbiani rings and NORs were established. Very often the 4th chromosome was almost completely heteropycnotic and looded like a pompon chromosome. For the first time in this species, a high frequency of ectopic pairings of different arms was observed. Telomeric regions involved in ectopic pairings had a granular appearance, as did some centromeres. The hypothesis is advanced that such a high frequency of structural rearrangements could be correlated with genomic distribution of specific mobile elements.  相似文献   
4.
In this study a simple electrophoresis approach has been proposed for assessing DNA damage per chromosome in vitro. Novel procedures of gel casting, sample loading, electrophoresis and quantification of damage have been suggested. Sets of Saccharomyces cerevisiae chromosomes subjected to DNA damage by Bleomycin, Co60--radiation alone and in combination with Hoechst were studied in detail. Statistical analyses showed that damage induced by Bleomycin bore linear positive correlation with %GA (r=0.97) and %GT (r=0.61) contents of chromosomes. Samples pre-treated with Hoechst showed much less damage by Co60--irradiation as compared to samples not treated with Hoechst but exposed to Co60--irradiation. The `protective effect of Hoechst' bore linear positive correlation (r=0.8) with %TAT content of chromosomes.  相似文献   
5.
Morphological variation for the NOR chromosome was studied for four half-siblings of a sexual outbreedingTaraxacum, for three siblings of the obligate agamospermT. pseudohamatum, and for two individuals of the agamospermT. brachyglossum. No rearrangement was detected for the 113 chromosomes of sexuals, or for 41 chromosomes of two agamospermous individuals. In the other three agamospermous individuals, 3/16, 5/50, and 5/20 chromosomes showed evidence of chromosomal rearrangement. The majority of rearrangement events (10/13) occurred to the satellite rather than to the body of the NOR-chromosome. It is considered that such high levels of somatic chromosomal rearrangement in agamospermousTaraxacum may be the result of activity by transposable genetic elements. This recombination may be of selective advantage to asexual plants which cannot generate genetic variability through the sexual process.  相似文献   
6.
    
The goals of this work were to create germ-cell-stage-specific cDNA libraries from mouse spermatogenic cells and to employ a novel two-step genetic screen to identify gene sequences present during the critical meiotic stage of spermatogenesis. Highly enriched germ-cell fractions were prepared from adult and juvenile mouse testes, and purity of these fractions was extensively analyzed by light and electron microscopy. Standard techniques were used to prepare cDNA libraries from populations of mixed leptotene and zygotene (L/Z) spermatocytes, pachytene (P) spermatocytes, and round spermatids. These libraries were analyzed with respect to representation of sequences from ubiquitously expressed genes, and from genes expressed at specific germ-cell stages as well as from genes expressed in testicular somatic cells. For the first step of the screening procedure, testicular cDNA was prepared from mutant mice carrying the T(X;11)38H chromosomal translocation that causes spermatogenic arrest at early meiotic prophase. This mixed cDNA probe was used to screen the libraries from L/Z and P spermatocytes to detect sequences that failed to hybridize. The clones identified were characterized for ability to hybridize to various germ-cell-specific cDNAs to verify that they represented sequences present in normal spermatogenic meiotic cells. These clones were then subjected to a second screening with another mutant probe; this time the cDNA probe was from testes of sterile mice bearing the T(X;16)16H chromosomal translocation that causes spermatogenic arrest at late meiotic prophase. This screen identified 27 clones that were not represented in testicular cDNA from T38-bearing mice or from T16-bearing mice. These clones may represent sequences essential for normal completion of the genetic events of meiosis during spermatogenesis. Likewise, the secondary screen identified 19 clones that were not represented in testicular cDNA from T38-bearing mice but were represented in testicular cDNA of T16-bearing mice. These clones are thus gene sequences present in spermatogenic cells during the time from early meiotic prophase to mid-to-late prophase. This strategy represents the first use of genetic aberrations in differential screening to identify genes expressed at specific times during mammalian spermatogenesis. © 1996 Wiley-Liss, Inc.  相似文献   
7.
The ability of a genotype to change its phenotype was once considered rather a nuisance -- making it difficult to define a genotype. This led to the idea that there was a problem called 'instability'. But quite early it was recognized that stability was under genetic control, and was a character like other attributes of an individual. From this realization came the idea that there were two sides to the character of 'instability', and that the ability to change could be important. This ability was thus given the title of 'plasticity'. Once recognized, it became clear from surveys of different species and populations that plasticity can (i) be a complex character, and (ii) be selected to fit species to the particular demands of different environments. For plants, which cannot meet variations in environment like animals by behavioural responses, phenotypic plasticity can be very important. Plants should therefore be valuable tools for unravelling the mechanisms of plasticity whilst also demonstrating its contribution to fitness experimentally. We ought also to be able to demonstrate that appropriate genetic variability is available through which complex responses can be built up by selection. Genes must exist not only to determine character means, but also to determine character response, which adds interesting complexity to our ideas about evolution.  相似文献   
8.
Chromosomal aberrations induced by cobaltous chloride in mice in vivo   总被引:1,自引:0,他引:1  
The effects of cobaltous chloride in inducing chromosomal aberrations were observed on laboratory bred mice in vivo after single oral administration of different fractions (1/10, 1/20, 1/40) of the lethal toxic dose of the salt. Bone marrow cells were flushed out and processed for chromosome studies following colchicine, hypotonic, giemsa, air drying procedure. The parameters screened were chromosomal aberrations, with and without gaps and break per cell. Slides were screened after the expiry of 6, 12, 18, and 24 h. Statistical analysis indicated the clastogenic effects of the salt. The degree of chromosome damage was directly related to the concentration, and also to the period after administration. The different stages of the cell cycle were affected.  相似文献   
9.
    
The major aim of this study was to test three hypotheses: 1) more complex traits of the hand are less prone to developmental insults and therefore show lower fluctuating asymmetry (FA) as compared with simple traits; 2) the manifestation of FA correlates with the variability of the trait (i.e., CV); and 3) FA is an organ-wide property, and therefore a concordance exists between the FA measures of different traits in hand bones. Seventy-two bilateral measurements of hand bones, were made from plain-film radiographs of 365 cadavers. A complex trait was considered as the total length of the three phalanges of a finger and their contiguous metacarpals. Simple traits were considered to be the lengths of individual bone that made up the complex trait. The following results were obtained: 1) on the average simple traits, composing the complex trait, show much higher FA than the corresponding complex trait, but this result is expected if there is no correlation (or low correlation) between FA of simple traits within the complex trait, due to random direction of right-left differences; 2) strong and highly significant correlation was observed between FA and CV of studied traits, regardless of sex and age of individuals; and 3) the majority of FA measurements of hand bones showed no correlation. However, correlations between some sets of FA traits were highly significant. They were interpreted, although not specifically tested, as the result of a tight relationship between traits related not only developmentally but also by active performance of the same function. Am J Phys Anthropol 107:125–136, 1998. © 1998 Wiley-Liss, Inc.  相似文献   
10.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号