Biology - An Illustrated Guide to Science (794127), страница 8
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mRNA movesbetween the nucleus and therest of the cell.●RRtranscriptiontransfer RNADNA moleculeREnzymes and ATP break hydrogenbonds, and DNA chains separate.Copying the messageThe genes containingessential information forbuilding proteins are kept inthe cell nucleus. These genesare coded lengths of DNA.● The information is copiedonto mRNA in a processcalled transcription.
Thiscopying is essential to get theinformation from the store(the DNA) to the ribosomes,where the manufacture ofproteins occurs.●Free RNA nucleotides findtheir complementarybases on one of the DNAchains.RRRRRRRRRRBuilding mRNAEnzymes break open the DNAmolecule (top right) at thecorrect point to reveal thebase sequence in the middleof the molecule.● Individual RNA nucleotidescan then line up with theDNA using base-matching toensure they are in the correctorder. Enzymes build themRNA molecule and themRNA leaves the nucleus.The enzymes are left behindand can be re-used to buildmore mRNA molecules.●© Diagram Visual Information Ltd.As the newly formed RNA chainis extended, it dissociates fromthe DNA strand to becomemessenger RNA (mRNA).44UNITYKey wordspolypeptide chainribosomeroughendoplasmicreticulumsmoothendoplasmicreticulumRough endoplasmicreticulum: structureRough endoplasmicreticulumRibosomeEndoplasmic reticulumThe endoplasmic reticulum (ER) is anetwork of flat open spaces within acell.
The membrane bounding it iscontinuous with the plasmamembrane surrounding the cell. Thismeans materials can pass along theendoplasmic reticulum until they aredeep within the cell without having tocross over the plasma membrane.● There are two types of ER: smoothendoplasmic reticulum (SER) andrough endoplasmic reticulum (RER).ribosomal subunitscontaining ribosomalRNA (rRNA)●Animal cellsmallsubunitrough endoplasmicreticulumRough endoplasmicreticulumBoth SER and RER are made up ofplasma membrane, but RER has smallbodies called ribosomes attached.These ribosomes made the ER look“studded” or “rough.”● Ribosomes are giant enzyme–RNAcomplexes concerned with proteinmanufacture. They consist of twosubunits that fit together (top rightdiagram) and work as one, usinginformation from mRNA to createpolypeptide chains during proteinsynthesis.● These peptide chains pass into thespace in the ER to fold and assemble,creating more complex proteins.largesubunit●nucleusmitochondrionplasma membraneschematic structureof rough endoplasmicreticulumribosomeSchematic structure of roughendoplasmic reticulum© Diagram Visual Information Ltd.lamellae (layers)—made up of twomembranescavities45Transfer RNATransfer RNA modelsUNITYKey wordsACCloverleaf model oftyrosine transferRNA (tRNA)amino acidanticodoncodonnucleotidepolypeptide chainamino acidbinding siteCtransfer RNATransfer RNAnucleotide chain●hydrogen bondTransfer RNA (tRNA) is an essentialpart of the protein manufacturingprocess.
During translation (see page46), it transfers a specific amino acidto a polypeptide chain at theribosome, where protein issynthesized. In order for this to occur,a specific tRNA molecule must bondwith a specific amino acid.StructureThe twisted, clovershaped tRNA moleculehas two functional sites.At one end of the moleculeis a site for amino-acidattachment and codon recognition.Condons specify the amino acid to belinked into the polypeptide chainbeing synthesized. At the other is theanticodon, three nucleotide basesthat are specific for that amnio acid.● The distance between the amino acidbinding site and the anticodon isconstant no matter how long the tRNAchain is or how many folds it has.●Three-dimensionalrepresentationof tRNAamino acidbinding siteAnticodonsnucleotide chainThere are many different types oftRNA.
Each type transfers oneparticular amino acid to a growingpolypeptide chain.● When messenger RNA (mRNA) entersthe ribosome, tRNA anticodons on thetRNA molecule recognize and bind tothe appropriate codon on an mRNAmolecule, bringing the correct aminoacid into sequence for the formationof the polypeptide chain.anticodon (mRNAbinding site)anticodon (mRNAbinding site)© Diagram Visual Information Ltd.●46UNITYKey wordsanticodoncodonmessenger RNAribosometransfer RNACodonsProteins are molecules madeof amino acids joined in aparticular sequence. If theseamino acids are arranged inthe wrong order, the proteinwill not function.● The sequence of amino acidsis coded by a sequence oforganic bases in DNAmolecules in the nucleus.Each amino acid is coded by asequence of three bases calleda codon (see top diagram).So MET codes for methionine,ACC codes for tryptophan.● Messenger RNA (mRNA) is acopy of the codons on theDNA.
The mRNA molecule canpass out of the nucleus to theribosomes on the endoplasmicreticulum.●Messenger RNAtranslationmRNA translation in the cytoplasmPTRMETtRNA withanticodon UACand carryingmethionine(MET) binds tocorrect codonAUG on mRNA.ribosomecodonUACAUGUGGMETAt the ribosome the mRNAacts as a template for otherRNA molecules to attach to.These molecules are thetransfer RNA (tRNA) moleculesthat carry amino acids neededfor protein synthesis.● A tRNA molecule with ananticodon that fits the nextavailable space on the mRNAmolecule slots into position.Its amino acid is held in thecorrect position for enzymesto join it to a growing chain ofamino acids formed at theother end of the tRNAmolecule.● Once the amino acid is joinedon, it is released from thetRNA, which detaches fromthe mRNA.
The tRNA can bereused when it has had thecorrect amino acid reattachedfrom the pool in the cell.© Diagram Visual Information Ltd.UCGCCILETRPUACACCAUGUGGanticodonAUCGCCpeptide bondA peptide bondforms betweenmethionine andtryptophan.UThe first tRNAmolecule returnsto the cytoplasmto pick up anothermethioninemolecule. Theribosome shifts,and a third tRNAmolecule bindsto mRNA.METACAUGACUGILEACCUAGUGGAUCTRPCAAALTRPMETThe process isrepeated.UGAUTranslating themessage●ACCAmRNAtRNA moleculewith the correctanticodon bindsto the codon atthe second site.It carriestryptophan(TRP).tRNAGGCGCCALAILEUAGCGGAUCGCCGG47Gene controlUNITYKey wordsGene inductionDNAenzymeoperonß-galactosidase in Escherichia colioperonRegulatorgenePromoterOperatorOnly when neededrepressor bound tooperator blockingRNA polymerasebinding siteRNApolymeraserepressorOperon derepressed(structural gene switched on)RegulatorgenePromoterOperator● ß-galactosidaseis an enzymeinvolved in the breakdown oflactose.
The gene that producesthis enzyme is usually switchedoff, and yet when lactose isfound, the gene switches onquickly, and the enzyme isproduced.StructuralgeneStructuralgeneGene typesThe gene that produces ß-galactosideconsists of two lengths of DNA: theregulator gene, and the operoncontaining structural operator andpromoter genes.● The structural gene produces theenzyme when the operator switches iton. The operator, in turn, is controlledby the promoter. The promoter workswith an enzyme called RNApolymerase to switch on the operatorand the structural gene.● However, RNA polymerase must beable to link with the promoter forthis to happen, and this isnormally blocked by a chemical,called a repressor, that binds tothe operator.●Blocking the repressorRNA polymerasebound to promoter●repressorinactivatedby inducerinducer (lactose)lactoseThe repressor is a molecule producedby the regulator gene.
If no lactose ispresent, the repressor binds to theoperator. When lactose is present, itbinds with the repressor and preventsit from binding with the operator. Thisallows RNA polymerase to bind withthe promoter and so switch on thestructural gene.© Diagram Visual Information Ltd.Operon repressed(structural gene switched off)48UNITYKey wordstransformationTransformationGenetic transformation of pneumonia bacteriaExperiments of Griffiths, 1928The dead mice had living S-typePneumococcus in the bloodstream.PneumococcusPneumococci are a group of bacteriathat can cause illness in animals andhumans.
One particular type can killmice and has two distinct types: rough(R-type), which does not kill, andsmooth (S-type), which is always fatal.● Rough and smooth refer to the outercoat of the Pneumococcus organisms.rough (R-type)non-virulentPneumococcus●smooth(S-type) virulentPneumococcusDead bacteriaDead bacteria cannot cause illness.Experiments with heat-killed bacteriain mice showed this.● When heat killed S-type bacteria wereinjected into mice with live R-typebacteria, the mice died. Live R-type donot kill mice, so the S-type must haveinfluenced them in some way. The Rtype were said to be transformed bythe dead S-type.●The active componentFurther work looked at whatcomponent in the S-type bacteria wasproducing the transformation.● S-type bacteria were killed, and thevarious components separated.Different mice were injected withdifferent extracts from the S-typebacteria along with live R-types.● The only mice that died had beeninjected with DNA from the S-type.This showed that it was the DNA thathad the power to transform the R-typebacteria.●heat-killed R-typePneumococcusheat-killed S-typePneumococcusheat-killedS-typePneumococcus+Experiments of Avery, et al, 1944protein fromS-typePneumococcusrough (R-type)non-virulentPneumococcus+DNA fromS-typePneumococcus© Diagram Visual Information Ltd.rough (R-type)non-virulentPneumococcusrough (R-type)non-virulentPneumococcus+49Genetic engineeringUNITYKey wordsTransferring genesplasmidrestrictionenzymeBacteriumnucleoidBacterial chromosomes●plasmidPlasmidForeign DNAstickyendcomplementarystrands of DNAcomplementarystrands of DNAAnnealingPlasmid and foreign DNAjoin at their sticky ends.CleavagePlasmid is cleaved byrestriction endonuclease.sticky endBacteria do not have a nucleus.
TheirDNA is found in a compacted masscalled the nucleoid. Bacteria alsocontain a plasmid, a small DNAmolecule that can be transferred fromone cell to another. Plasmids arecommonly used in geneticengineering to transfer genes.Sticky endsThe two strands in the DNA helix ofthe plasmid are “mirror images” ofeach other. Where one side hasadenine the other has thymine; whereone has cytosine the other hasguanine.● Restriction enzymes, which recognizespecific, short nucleotide sequences,can cut the plasmid to produce a gapwith “sticky ends.” These enzymes donot cut straight across the DNAstrand—they split the two strandsapart so that one end sticks outbeyond the other.
Because the singlestrands of DNA have complementarybases, they can bind to a portion ofDNA with appropriate basesprotruding from their “sticky ends.”●PlasmidsCareful use of restriction enzymesallows genetic engineers to cut outlengths of DNA with sticky ends thatcorrespond to the gaps in a brokenbacterial DNA. The foreign genes canthen be added and the DNA rejoinedto make a circular plasmid.● The plasmid can be inserted intoanother bacterium where it can beexpressed. In this way, for example,the gene responsible for producingthe hormone insulin can be splicedinto a bacterium.nucleoidTransformationBacterium picks upmodified plasmid.reconstituted plasmidacting as vector forforeign DNA© Diagram Visual Information Ltd.●50CONTINUITYKey wordsanaphasecentromerechromatidchromosomegameteinterphasemetaphasemitosisprophasetelophaseMitosis in an animal cellMitosisInterphaseProphaseplasma membraneMetaphasesister chromatidscytoplasmcentriolesnucleolusCell division●All living cells divide.
There are twomethods of division: m itosis, whichproduces copies of the original cell,and meiosis which is only used toproduce gam etes, the reproductivecells in plants and animals.Interphase●Cells do not divide all of the time—they are present in a state calledin terphase. In interphase, the DNA inthe chromatin threads is dividing andmultiplying to produce copies of allthe genes in the cell—but this processis invisible.nuclearmembranecentriolescentromerecentromereattached tospindle equatorchromatin threadEarly anaphaseLate anaphasespindlefiberEarly telophaseMitosisMitosis is a continuous process thatinvolves four main stages.● In prophase, chromatin is condensedinto short, thick chrom osom es.
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