【正文】 Frederick Griffith39。The Search for the Chemistry and Molecular Structure of nucleic AcidsNuclei acid, originally isolated by Johann Miescher in 1871, was identified as a prime constituent of chromosomes through the use of the redstaining method developed by Feulgen in the early 1900s.他們的工作為其他工作者鋪平了道路，即精確地闡明了酶影響了復(fù)雜的新陳代謝途徑。Beadle 和Ephrussi在三十年后對(duì)果蠅眼睛顏色的研究發(fā)現(xiàn)特殊基因與相關(guān)反應(yīng)的生物合成有關(guān)。伴性遺傳又導(dǎo)致了不分離現(xiàn)象的發(fā)現(xiàn)，即在減數(shù)分裂中，染色體對(duì)不分離。s work was rediscovered, Walter Sutton and Theodor Boveri independently proposed that the hereditary units might be located on chromosomes. Experiments to prove this hypothesis were carried out by Thomas Hunt Morgan and his students at Columbia University, in research on the sex chromosomes of fruit flies. Morgan39。在1900年，他的著作再被發(fā)現(xiàn)利用。Mendel presented his ideas in 1866 in a scientific paper published by the Brunn Society for Natural History. Unfortunately, the meaning of his research was not understood by other scientists of the day. His work was rediscovered in 1900 by Carl Correns and Hugo de Vries. 1866年，孟德?tīng)栐谧匀皇飞习l(fā)表了他的科學(xué)論文，陳訴了他的觀點(diǎn)。不完全顯性說(shuō)明了兩個(gè)等位基因?qū)Ρ硇投加杏绊?。特例是，不完全顯性。s laws is inplete dominance, a phenomenon in which offspring of a cross exhibit a phenotype that is intermediate between those of the parents. However, inplete dominance reflects the fact that both alleles for the trait in question exert an effect on the phenotype. The alleles themselves remain separate. 雙因子雜合試驗(yàn)，兩個(gè)特征是如何相互影響遺傳的。Mendel39。為驗(yàn)證此理論，他做了測(cè)交實(shí)驗(yàn)，即基因型未知的植物與純合的隱性基因植物雜交。s first law: the law of law states that for a given trait an organism inherits one allele from each parent. Together these alleles form the allele pair. When gametes are formed during meiosis, the two alleles bee separated (halving of chromosome number).To gain evidence for his theory Mendel performed test crosses, mating plants of unknown genotype to plants that were homozygous recessive for the trait of interest. The ratio of dominant phenotypes (if any) in the progeny makes clear whether the unknown genotype is heterozygous, homozygous dominant, or homozygous recessive. 分離定律，生物只遺傳父母本等位基因?qū)Φ囊粋€(gè)等位基因。The results of Mendel39。因此，生物的表型與基因型是不同的。相反，稱雜合。此遺傳單元就是今天共識(shí)的等位基因。在子2代中顯形與隱性比為3∶1。For each of the characters he studied, Mendel found that one trait was dominant while the other was recessive. In the second filial (F2) generation, the ratio of dominant to recessive was 3:1. Mendel deduced that this result was possible only if each individual possesses only two hereditary units, one from each parent. The units Mendel hypothesized are today known as alleles, alternative forms of genes. Genes are the basic units of heredity. An organism that inherits identical alleles for a trait from each parent is said to be homozygous for that trait。例如，種子顏色，植株高度，這些特征只有兩個(gè)明確的可能性。s Classic ExperimentsMendel studied genetics through plantbreeding experiments with the garden pea, a plant species that is selffertilizing and breeds true (each offspring is identical to the parent in the trait of interest). To test the blending theory, he focused his research on seven distinct characters. Each of these characters, such as seed color and plant height, present only two, clearcut possibilities. He also recorded the type and number of all progeny produced from each pair of parent pea plants, and followed the results of each cross for two generations.孟德?tīng)柾ㄟ^(guò)豌豆實(shí)驗(yàn)研究遺傳學(xué)，豌豆是自花授粉植物和純品系。直到他去世后，他的理論才被理解和接受。當(dāng)他還是大學(xué)生時(shí)就提出了物質(zhì)的粒子屬性。這兩個(gè)早期觀點(diǎn)合起來(lái)形成融合理論：子代擁有父母本混合的遺傳特征，而不完全象親代。 theory of pangenesis and August Weismands germ plasm theory. Based on experiments with mice, Weismann proposed that hereditary information in gametes transmitted traits to progeny. Both of these early views incorporated the blending theory: they held that heritable traits of the two parents blend, so that the distinct characteristics of each are lost in offspring. 遺傳學(xué)的早期理論包括泛生說(shuō)和種質(zhì)理論。也可以產(chǎn)生新的基因并在種群中蔓延。但一個(gè)簡(jiǎn)單災(zāi)難性事件或疾病都可能摧毀一個(gè)細(xì)胞群體。It also allows new gene forms to arise and spread through populations.有絲分裂和減數(shù)分裂在傳遞遺傳信息過(guò)程中各有優(yōu)勢(shì)。A prime benefit of sexual reproduction is that it provides genetic variability and a ready mechanism for the elimination of deleterious mutations.A major disadvantage of the asexual mode is that a single catastrophic event or disease may destroy an entire population of genetically identical organisms.s successful genetic plement, requires little or no specialization of reproductive organs, and is more rapid than sexual reproduction.產(chǎn)生4個(gè)單倍體，父母染色體隨機(jī)分配。著絲粒最終分離，每個(gè)姊妹染色單體分向兩極。由此而導(dǎo)致4個(gè)子細(xì)胞染色體數(shù)減半。It is this event that results in the halving of the chromosome number in the four daughter cells that result from meiosis.正如在有絲分裂中一樣，兩個(gè)同源染色單體通過(guò)蛋白質(zhì)和RNA橋配對(duì)形成聯(lián)會(huì)復(fù)合體。Unlike the anaphase of mitosis, however, during anaphase I the two chromatids of each chromosome stay joined at the centromere and move together to one of the two poles of the cell.The homologous pairs stay together when they align on the metaphase plate.As in mitosis two chromatids exist for each chromosome at the beginning of prophase 1. During this phase the homologous chromosomes undergo synapsis, or pairing, which is brought about by a bridging structure of proteins and RNA called the synaptonemal plex.如有絲分裂，它也是發(fā)生在DNA復(fù)制后并有連續(xù)的兩個(gè)核分裂。在植物細(xì)胞中，在赤道板形成新的細(xì)胞板。動(dòng)物細(xì)胞與中心粒相連，而在植物和真菌細(xì)胞中，紡錘體與微管組織中心的離子相連。在分裂后期，纖維開(kāi)始變短，染色單體分離。紡錘體微管由兩極向赤道板延伸。在分裂末期，在每套染色體周圍形成核膜，細(xì)胞質(zhì)發(fā)生分裂。在分裂前期后期和分裂中期前期，濃縮的染色體與紡錘體相連，最后以正確的角度排列在赤道板上。During telophase nuclear envelopes begin to form around each set of chromosomes, and division of the cytoplasm takes place.生物學(xué)家將有絲分裂劃分為4個(gè)階段。Next, during anaphase, the two sister chromatids of each chromosome split, and one from each pair is drawn toward each pole of the cell.Eventually the chromosomes bee arranged in a plane (called the metaphase plate) at a right angle to the spindle fibers.As prophase ends and metaphase begins, the condensed chromosomes bee associated with the spindle.At the beginning of prophase the chromosomes each consist of two highly condensed chromatids attached to each other at a cen