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1、2019-2020年高二生物基因表达的调控 人教版教案示例第三节 基因表达的调控教学模式：小组讨论探究学习教学手段：实物投影仪课时安排：一课时教学过程：引言巴黎巴斯德研究所的Jacquse Monod和Francois Jacob发现大肠杆菌（E.coli）在不含乳糖只含葡萄糖的培养基中不分泌半乳糖苷酶，在只含乳糖的培养基中才能分泌半乳糖苷酶，能把乳糖水解为半乳糖和葡萄糖。经多次反复实验发现，大肠杆菌在没有乳糖的环境中不产生编码半乳糖苷酶的mRNA，因而不合成半乳糖苷酶，这一调节机制是发生在转录这一环节上的。他们终于在19601961年提出了乳糖操纵子模型（lacoperon model），开
2、创了基因表达机制研究的新领域。这一模型很好地说明了原核生物基因表达的调节机制，是生物学领域中一个划时代的突破。为此，他们于1965年获得了诺贝尔生理学和医学奖。新课教师指导学生活动提供以下英文材料。学生可以翻阅课本，以英文阅读理解的方式学习乳糖操纵子的工作原理，可以结合成四人学习小组，进行讨论；教师将教材中乳糖操纵子的结构图用实物投影仪投射在大屏幕上，由学生翻译各结构名称，可以用英文和中文阐述乳糖操纵子是如何调空半乳糖苷酶的表达的。材料一(material one)：Organization of bacterial genesAn important feature which determ
3、ines how gens transcription is regulated in bacteria is the organization of the genes as operonsThese are transcriptional units in which several genes,usually encoding proteins with related functions,are regulated togetherOther genes also occur which encode regulatory proteins that control gene expr
4、ession in operonsMany different operons have been identified in E.coliMost contain genes that encode proteins involved in the biosynthesis of amino acids or the metabolism(新陈代谢) of nutrients(营养物质)Operons are classified as inducible(诱导的)or repressible(抑制的)Inducible operons contain genes that encode e
5、nzymes involved in metabolic pathwaysExpression of the genes is controlled by a substrate(底物) of the pathway An example of an inducible operon is the lacoperon which encodes enzymes required for the metabolism of lactose Repressible operons contain genes that encode enzymes involved in biosynthetic
6、pathways and gene expression is controlled by the end product of the pathway which may repress expression of the operon or control it by an alternative mechanism called attenuationAn example of a repressible operon is the trp operon which encodes enzymes involved in the biosynthesis of tryptophan(色氨
7、酸)材料二(material two)： Regulation of gene expressionFor a bacterium to function it is not necessary that all of its genes are transcribed at all timesTo conserve(保存)energy and resources(资源)bacteria regulate the activity of their genes so that only Those gene products necessary for the cells functions
8、are producedFor example,it would be wasteful for a bacterium to produce enzyrnes required to synthesize an amino acid that was already available(可获得的)to it from its environmentRegulation of gene expression allows bacteria to respond to changes in their environment,typically(典型地)to the presence or ab
9、sence of Nutrients(营养物质)材料三(material three)：The lac operonthis operon contains three genes encoding enzymes required by the Ecoli bacterium for the utilization(利用)of the disaccharide(二糖)sugar lactose These are lactose permease(半乳糖苷透性酶),which transports lactose into the cell,-galactosidase(-半乳糖苷酶)whi
10、ch hydrolyzes lactose into its ponent sugars (glucose and galactose)and -galactosidasetransacetylase(-半乳糖苷乙酰)which is also involved in the hydrolysis of lactosedThese enzymes are normally present in Ecoli at very low levels but in the presence Of lactose their levels rise rapidlyThe three genes in t
11、he lac operon are knownAs lac Z,Y and A,and encode -galactosidase,lactose permease and-galactosideTransacetylase,respectively(分别地)The genes are sequential and are transcribed as a single mRNA from a single pronoterAnother regulatory gene,lac I,which is Expressed separately,lies upstream of the opero
12、n and encodes a protein called the lac repressor which regulates the expression of the lac Z,Y and A genesIn the absence of lactose the lac repressor binds to a DNA sequence called the operator positioned between the lac promoter and the beginning of the lac Z gene When bound to the operator,the lac
13、 repressor blocks the path of the RNA polymerase bound to the lac Promoter upstream of it and prevents transcription of the lac geneswhen the cell encounters(遇到)lactose a few molecules of the lac enzymes present in the cell allow lactose to be taken up and metabolized(新陈代谢)Allolactose,an isomer(异构体)
14、of lactose produced as an intermediate(中间物)during the metabolism of lactose,act As an inducerIt binds to the lastose repressor and changes its conformation ,Such that it can no longer bind to the operator The path of the RNA polymerase is no longer blocked and the operon is transcribedLarge numbers
15、of enzyme molecuules are produced which take up loctose and metabolize itThe presence of lactose thus induces the expression of the enzymes needed to metabolize it When the lactose is used up lacrepressor retures to its original conformation and again,binds thelac opeator preventing transcription an
16、d switching off the operon材料四（material four）：Gene expression in eukaryotic cellThe human genome is estimated to contain as many as 100 000 genes which are subject to plex patterns of regulation that are,as yet,inplete understoodIn eukaryotic cells not all of the genes present in the genome are activeCells express about 15% of their genes;t
