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Lesson3GenesChromosomesandCellDivisionGeneticsisabranchofbiologythatdealswithgenesandtheirtransmissionfromonegenerationtothenextandtheireffectsonexternaltraitsandcharacteristics.遗传学是生物学的一个分支，它研究基因和基因在上下代间传递以及基因对(生物)外在性状和特征的影响。Colorinheritanceisusedextensivelyintheexplanationofhowgenesinteractandwhycertaingeneticratiosareimportant.However,wewillfindthatgeneticsismuchmorethandominanceandrecessivenessand3:1ratios.体色遗传被广泛地用来解释基因之间如何相互作用以及为什么特定的遗传比率是重要的。然而，我们将发现遗传学研究的内容远不止说明显性和隐性以及3:1的比率。Theycannotbeseenasseparatestructureswiththeaidofamicroscopeuntilthecellisinthepreparatorystagesofdivision.Chromosomesareimportanttothestudyofgeneticsbecausetheycontainthegenes.直到细胞处于分裂的预备期，借助显微镜，才能看到作为独立结构的染色体。由于染色体上携带有基因，对于遗传学的研究来说，染色体是重要的。Anotherdistinguishingfeatureisthecentromerewhichservesasapointofattachmentforthespindlefiberduringcelldivision.ThelocationoftheCentromerealsovariesamongchromosomes.染色体另一个显著的特征是细胞分裂期间与纺锤丝相连接的着丝粒。着丝粒的位置在染色体中也有变化。EachgeneisthoughttobecomposedofonemoleculeofDNA.Therefore,ifaparticularchromosomecontains200genes,itwouldinrealitycontain200moleculesofDNA.每个基因被认为是由DNA的一个分子组成的。因此，如果一个特定的染色体有200个基因，它实际上包含有200个DNA分子。Genescarryouttheirfunctionsbycontrollingthesynthesisofsubstancescalledenzymes.Enzymesaremadeofproteinandusuallyfunctionascatalystsinchemicalreactions.Theactivitiesofacellarecontrolledbymanyhundredsofchemicalreactions.基因通过控制酶的合成来行使其功能。酶由蛋白质组成，通常在化学反应中起催化作用。Thismeansthatthegenethatoccupiesalocusononechromosomecouldalsooccupythesamelocusontheotherhomologouschromosome.However,thegenesthatoccupythesetwohomologousloci(pluralforthelocus)mayormaynotbethesame.这就意味着在染色体上占据一个位点的基因，在另一条同源染色体上占据着相同的位点。但占据这两个同源位点的基因可能相同，也可能不同。Keepinmindthekeypartstothedefinitionofalleles:thegenesmustbeonthesamelocusoftwohomologs,theymustbedifferent,andtheaffectthesametrait.等位基因指：这些基因必须是在两条同源染色体的相同位点上，它们必须是不同的，并且影响相同性状。Whendescribinggenesandsolvinggeneticproblems,itisnecessarytousesymbolstoreprentthegenes.Themostcommonwayistouselettersforgenesymbols.当描述基因和解决遗传问题时，有必要用符号来代表基因。最普遍的方法是用字母作为基因符号。Beforeweconsiderhowgenesaretransmittedfromonegenerationtothenext,thesubjectofcelldivisionshouldbediscussed.Anunderstandingofhowcellsdivide,andhowchromosomesduplicateandaretransmittedtodaughtercellsisvitaltofullerunderstandingofgenesegregationandrecombination.在考虑基因是如何从一代传到下一代之前，首先要讨论细胞分裂问题。了解细胞分裂，对于了解染色体如何复制，如何传递给子细胞，以及更充分地理解基因的分离和重组是至关重要的。TheDNA,andthusthegenes,ineachchromatidareidenticaltooneother.Fromageneticstandpoint,chromosomeduplicationcouldverywellbeconsideredthemostimportantaspectofcelldivision.两条染色单体上的DNA以及基因是相同的。从遗传学的观点看，染色体复制被普遍认为是细胞分解最重要的方面。Theproductionoftwodiploidcellsfromonediploidcellismadepossiblebythefactthatthechromosomesduplicatepriortotheactualdivisionofthecell.在细胞真正分裂以前，染色体复制，因此才能够由一个二倍体细胞产生两个二倍体细胞。Lesson11TheAnimalandIt'sFoodFoodosmaterialwhich,afteringestionbyanimals,iscapableofbeingdigested,absorbedandutilized.Inamoregeneralsenseweusethetermfbod'todescribeediblematerial.饲料是被动物摄取之后能够被消化、吸收和利用的物质。一般来说，我们用“饲料”这一术语来描述可食用的物质。Animalsdependuponplantsfortheirexistenceandconsequentlyastudyofanimalnutritionmustnecessarilybeginwiththeplantitself.Waterfunctionsinthebodyasasolventinwhichnutrientsaretransportedaboutthebodyandinwhichwasteproductsareexcreted.Manyofchemicalreactionsbroughtaboutbyenzymestakeplaceinsolutionandinvolvehydrolysis.在体内，水作为溶剂，运输营养物质到达全身，产生的废物也是在水中被排泄掉。许多有酶参与的化学反应也必须在溶液中发生，包括水解。Inbothplantsandanimals,proteinsarethemajornitrogen-containingcompounds.Inplants,wheremostoftheproteinispresentasenzymes,theconcentrationishighintheyounggrowingplantandfallsastheplantmatures.Inanimals,muscle,skin,hair,feathers,woolandnailscontainprotein.蛋白质是职务和动物体内的主要含氮化合物。在植物体内，大部分含氮化合物是以酶的形式存在，其浓度随年龄增长而降低。对动物而言，其肌肉、皮肤、毛发、羽毛、羊毛和指甲都含蛋白质。Likeproteins,nucleicacidsarealsonitrogen-containingcompoundsandtheyplayabasicroleinthesynthesisofproteinsinalllivingorganisms.Theyalsocarrythegeneticinformationofthelivingcell.在活的有机体中，核酸与蛋白质一样也是含氮化合物，在蛋白质合成中起重要作用，核酸还携带活细胞的遗传信息。Animportantdifferencebetweenplantsandanimalsisthat,whereastheformercansynthesiseallthevitaminstheyrequireformetabolism,animalscannot,orhaveverylimitedpowersofsynthesis,andaredependentuponanexternalsupply.Lesson12EnergyUtilizationKilocalorie(kcal).Theamountofenergyasheatrequiredtoraise1kilogramofwater1C(from14.5Cto15.5C).Equivalentto1000calories.FormerlyreferredtoasalargeCalorieandwassodesignatedbybeingspelledwithacapitalC.千卡(kcal):使1kg睡的温度升高1C(即从14.5C升至15.5C)时所需要的热量，相当于1000卡，也叫大卡，用大写字母C来代替。Bombcalorimeter.Aninstrumentusedfordeterminingthegrossenergycontentofamaterial.Itconsistsofaninsulatedwatercontainerequippedwithacombustionbomb,athermometer,andcertainotheraccessories.爆炸式热量计：一种用于测定某种物质总能的仪器，它由一个绝缘的水容器、一个氧弹、一个温度计和其他的一些附件组成。Grossenergy(GE).Thisisthetotalheatofcombustionofamaterialasdeterminedwithabombcalorimeter-ordinarilyexpressedaskilocaloriesperkilogramoffeedorkcal/kg.总能(GE):物质在氧弹内完全燃烧所释放出的热量，用“千卡/千克饲料”表示。Certainproductssuchascoal,mineraloil,andligninhavehighgrossenergyvaluesbut,becauseoftheirindigestibility,areofnoenergyvaluetotheanimal.Roughageshavegrossenergyvaluescomparabletothoseconcentrates,butthetwodiffergreatlyindigestible,metabolizable,andnetenergyvalues.某些物质如煤炭、矿物油和木质素都有很高的总能值，但由于它们不可消化，对动物来说就没有能量价值。粗饲料在总能上与精料相当，但在消化能、代谢能和净能方面却相距甚远。Fats,becauseoftheirgreaterproportionofcarbonandhydrogen,yield2.25timesmoregrossenergyperkgthandocarbohydrates.因脂肪含碳和氢的比例较高，每千克脂肪的总能都是同等重量碳水化合物的2.25倍。Digestibleenergy(DE).Thisisthatportionofthegrossenergyofafeedwhichdoesnotappearinthefeces.Itincludesmetabolizableenergyaswellastheenergyoftheurineandmethane.消化能(DE):即饲料总能中扣除粪能后剩余的部分，包括代谢能、尿能和甲烷能。Netenergy(NE).Thisisthatportionofmetabolizableenergywhichmaybeusedasneededbytheanimalforwork,growth,fattening,fetaldevelopment,milkproduction,and/orheatproduction.净能(NE):是代谢能中的一部分，动物可以用于做功、生长、育肥、胎儿发育、泌乳和/或产热。Heatincrement(HI).ThisisthedifferencebetweenMEandNE.Itrepresentstheheatunavoidablyproducedbyananimalincidentalwithnutrientdigestionandmetabolism.Ithasbeenreferredtoalsoasworkofdigestion,specificdynamiceffect,andthermogeniceddect.食后体增热(HI):是代谢能和净能的差值，是动物由于营养物质的消化和代谢而不可避免产生的热量。通常也只消化做功、特殊动力效应和热基因效应。MEfigures,ontheotherhand,representamoreprecisemeasureofafeed'senergyvaluetoananimalthandoeitherDEorTDNfigures.IndeterminingtheMEofafeed,allowancesaremadenotonlyfortheenergylossesintheurinebutalsoforthoseofthefermentationgases.另一方面，对动物来说，代谢能值要比消化能和TDN都精确，在确定饲料代谢能时，不仅考虑了尿能，而且也考虑了发酵气体能。ActualNEvalues,however,havebeendeterminedforonlyalimitednumberoffeeds.MostavailableNEvaluesareestimateswhichhavebeencalculatedforvariousfeedsbasedontheircompositionanddigestibilityinrelationtothecompositionanddigestibilityofthefeedsonwhichactualNEvalueshavebeendetermined.在最后的分析中，某种程度上净能是最有利用价值的，因为它最精确地反映了动物的能量需求以及不同饲料满足这种需要的能力。净能值不仅考虑了尿能和发酵气体能量损失，而且还考虑了营养物质代谢产热损失或食后体增热损失。但是实际测得的净能值是有限的几种饲料，大部分可利用的净能值是根据各种饲料的组成与其消化性来估算的。Lesson15WhatareViruses.Virusesaresubmicroscopic,obligateintracellularparasites.Theyaretoosmalltobeseenbyopticalmicroscopes,andtheyhavenochoicebuttoreplicateinsidehostcells.病毒是亚显微的、专性细胞内寄生生物，病毒的体积非常小以至于在光学显微镜下不能被观测到，另外他们只能在宿主细胞内进行复制。.However,thisshortdefinitionisnotcompletelyadequate.Itisnotaproblemtodifferentiatevirusesfrommulticellularorganismssuchasplantsandanimals.然而这个简短的定义是不充分的。区分病毒与其他多细胞生物很容易，例如动物和植物.However,afewgroupofprokaryoticorganismsalsohavespecializedintracellularparasiticlifecyclesandoverlapwiththisdescription.然而，一些原核生物也具有专性细胞内寄生史与我们的定义相重叠。.Acommonmistakeistosaythatvirusesaresmallerthanbacteria.Thoughthisistrueinmostcases,sizealonedoesnotdistinguishthem.一种常见的错误的观点认为病毒比细菌小，虽然大多数情况下这种观点是正确的。但是仅通过大小是不能区分它们的。.Researchhasrevealedthattheevolutionaryrelationshipbetweenvirusesandotherlivingorganismsisperhapsmorecomplexthanwaspreviouslythought.研究发现，病毒与其他生物的进化关系或许比之前想象的要复杂的多Lesson161.Asubstancethatcanenterthebodyandstimulatetheproductionofantibodiesiscalledanantigen.能够进入机体并且刺激机体产生抗体的物质称作抗原nthecontextofdiseasepreventionthiscouldbeadisease-productionorganism.Theantibodyinturnhasadetrimentaleffectupontheorganismthatstimulateditsproduction.在提到疾病预防时，抗原通常指致病微生物。微生物能刺激抗体产生，反过来，抗体对微生物具有不利影响。.Animalscanacquireimmunitynaturallyorartificially;theanimalmayplayanactiveroleorapassiveroleinthedevelopmentoftheimmunity;andimmunitymaylastforonlyafewweeksoritmaylastforseveralyears.动物可以自然地或人工地获得免疫力。在免疫力形成的过程中，动物扮演着主动或被动的角色。免疫力可持续几周或数年。.Activeimmunityisacquiredbytheintroductionofantigensintothesystem.Theantigensinturnstimulatetheproductionofantibodieswhichprovideacertainlevelofimmunitytofutureintroductionsofthatparticularantigen.主动免疫是由抗原进入机体而获得的。抗原能刺激抗体生成，抗体可以针对再次进入机体的特定抗原提供某一水平的免疫力。.Thebetterwaytodevelopactiveimmunityinananimalisthroughtheintroductionofavaccine.Avaccineisanantigenpreparationthecontainseithertheinfectiousagentitselforitsmetabolicproducts.动物形成主动免疫的更好方式是注射疫苗。疫苗是一种含有病原体或其代谢产物的抗原制剂。.Anothertypeofvaccinecontainsliveorganismsthathavebeenmodifiedsotheyarenotasharmfulastheuntreatedorganisms.Theyarecalledmodifiedlivevaccinesorattenuatedvaccines.另一类疫苗含有经过改良的活微生物，其危害性远低于未经处理的微生物，称为减毒活疫苗或弱毒疫苗。.Sometypesofrabiesandbrucellosisvaccinesareproducedinthisway.Stillanothertypeofvaccinecontainsonlytoxinsproducedbytheinfectiousorganismsratherthantheactualpathogensthemselves.某些狂犬病疫苗和布式杆菌疫苗就属此类。还有一类疫苗仅含有感染性微生物产生的毒素，而非病原体本身.Asecondmajortypeofacquiredimmunityiscalledpassiveimmunity.Withthiskindofimmunity,theanimaldoesnotplayanactiveroleinthedevelopmentofitsimmunity.第二种主要的获得性免疫类型称为被动免疫。在被动免疫中，动物在免疫形成的过程中没有扮演主动角色。.Theserumcanbeseparatedandpreparedforinjectionasanantiserum.Swineerysipelasandscarletfeverinhumansaretwodiseasesforwhichantiserumshavebeenprepared.分离的血清可被制成注射用抗血清。现已制备了人的猪丹毒和猩红热两种疾病的抗血清。.Forthisreasonititimportantthatnewbornanimalsobtainthefirstmilkfromtheirmothers.ColostrumalsocontainsahighpercentageofproteinandvitaminAwhicharebeneficialtothenewbornanimals.因为新生动物迟到妈妈的初乳是很重要的。初乳中还含有很高比例的蛋白质和维生素A,这些对新生动物十分有