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General Genetics

by: Clarissa Hermiston DVM

General Genetics BIO 184

Clarissa Hermiston DVM

GPA 3.58


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This 23 page Class Notes was uploaded by Clarissa Hermiston DVM on Monday October 5, 2015. The Class Notes belongs to BIO 184 at California State University - Sacramento taught by Staff in Fall. Since its upload, it has received 15 views. For similar materials see /class/218819/bio-184-california-state-university-sacramento in Biological Sciences at California State University - Sacramento.

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Date Created: 10/05/15
Modified from hHpwwwmhhecombrooker Fall 2006 IO 184 LECTURE 9 Lecture 9 Mendelian Inheritance 4174 4754 my by Mam gummy Drawing of Pim SUMllquot the common ga den pea This plant was wltivated by Ere or Mendel and used to deta mine the rules governing the inheritance of biological traits lavM DIMMb orgFM ng EIF Page 1 Modified from hTTpwwwmhhecombrooker Fall 2006 O 184 LECTURE 9 I Early Theories of InheriTance Many Theories of inheriTance have been proposed To explain The Transmission of herediTary TraiTs a Theory of pangenesis 0 Proposed by HippocraTes ca 400 BC 0 Seeds are produced by all parTs of The body 0 CollecTed in reproducTive organs 0 Then TransmiTTed To offspring aT momenT of concepTion b Theory of preformafionism organism is conTained in one of The sex cells as a fully formed hamLncuLs or miniaTure human being 0 WiTh proper nourishmenT The hamuncuLs unfolds inTo iTs adulT proporTions 0 Drawing from ln ngcn mki ia nk714ml c Blending Theory of Inheritance o FacTors ThaT conTrol herediTary TraiTs are meleable 0 They can blend TogeTher generaTion afTer generaTion 0 Much like mixing food coloring red yellow blends To make orange a compleTely differenT color and The red and yellow colors are forever losT II Gregor Mendel Gregor Mendel39s pioneering experimenTs wiTh garden peas refuTed all of The above Mendel lived from 18221884 and is considered To be The faTher of geneTics Mendel39s success can be aTTribuTed in parT To 0 His boyhood experience in grafTing Trees 0 This TaughT him The imporTance of precision and aTTenTion To deTail 0 His universiTy experience in physics and naTural hisTor o This TaughT him To view The world as an orderly place governed by naTural laws ThaT can be sTaTed maThemaTically Page 2 Modified from hTTpwwwmhhecombrooker Fall 2006 BIO 184 LECTURE 9 Mendel was an AusTrian monk and conducTed his landmark sTudies in a small 115 by 23 fooT ploT in The garden of his monasTery From 18561864 he performed Thousands of crosses He also kepT meTiculously accuraTe records ThaT included quanTiTaTive analysis His work enTiTIed ExperimenTs on PlanT Hybridsquot was published in 1866 However H was ignored for 34 years probably because iT was published in an obscure Journal and There was no undersTanding yeT of chromosome Transmission The behavior of chromosomes in cell nuclei was firsT observed in The 1880s and in 1900 Mendel39s work was rediscovered by Three boTanisTs working independenle Mendel39s work helped explain chromosome behavior during meiosis Mendel chose pea planTs as his experimenTal organism To sTudy The naTural laws governing The Transmission of heriTable TraiTs He performed hybridizaTion experimenTs in which he maTed Two individuals wiTh differenT characTerisTics eg whiTe flowers x purple flowers Such crosses produce offspring called hybrids The garden pea was advanTageous To Mendel because 0 IT exisTs in several varieTies wiTh easily disTinguishable characTerisTics o ITs flower sTrucTure allows for easy crosses 0 IT is possible To selfferTilize The planTs as well as To crossferTilize Them Mendel sTudied seven TraiTs in The garden pea ThaT bred Truequot Such planTs produce The same TraiT over and over again when They are selfed or bred To planTs like Themselves NoTe ThaT each TraiT has Two eaSIy dLs ngw39shabe alTernaTive forms eg purple vs whiTe flowers See Figure 24 in Brooker TexT Mendel did noT sTarT ouT wiTh a hypoThesis To explain The inheriTance of These TraiTs Through The generaTions RaTher he hoped ThaT a quanTiTaTive analysis of crosses mighT provide a hypoThesis ThaT could be rigorously TesTed III Mendel39s FirsT Law Law of SegregaTion In his firsT seT of experimenTs Mendel crossed Two varianTs ThaT differed in only one TraiT eg flower color This is Termed a monohybrid cross 0 Mono 2 one TraiT is followed in The cross Page 3 Modified from hTTpwwwmhhecombrooker Fall 2006 BIO 184 LECTURE 9 0 Hybrid 2 The offspring of The cross are hybrids See Figure 25 in Brooker TexT When The purebreeding parenT planTs were crossed They produced a generaTion of offspring which Mendel called The firsT filial or F1 generaTion However for The TraiTs Mendel chose only one form of The TraiT was expressed in These F1 planTs For example when Mendel crossed a purebreeding purpleflowered pea planT wiTh a purebreeding whiTeflowered planT all of The offspring had purple flowers This clearly showed ThaT quotblending inheriTancequot was noT Taking place buT iT also confused Mendel aT firsT Had The oTher TraiT disappeared alTogeTher To furTher explore whaT was happening Mendel Then selfed The F1 generaTion To produce an F2 generaTion of planTs To his surprise boTh TraiTs reappeared m Ther orgna forms among The F2s Moreover The TraiTs appeared in a predicTable raTio of 3 To 1 These resulTs suggesTed a parTiculaTe inheriTance and Mendel posTulaTed The following Law of SegregaTion To explain whaT he had seen 1 A pea planT conTains Two discreTe herediTary facTors one from each parenT 2 The Two facTors may be idenTical or differenT 3 When differenT facTors of a single TraiT are presenT in The same individual 0 One is quotdominanTquot and iTs effecT can be seen 0 One is quotrecessivequot and is noT expressed 4 During gameTe formaTion in a planT wiTh boTh facTors The paired facTors segregaTe randomly so ThaT half of The gameTes receive one facTor and half receive The oTher Today Mendel39s quotfacTorsquot are called quotgenesquot and The alTernaTive forms of genes eg purple vs whiTe are called quotallelesquot An individual wiTh Two idenTical alleles is Termed homozygous eg AA or aa and an individual wiTh Two differenT alleles is Termed heTerozygous eg Aa The genoType of an individual refers To The specific allelic combinaTion ThaT iT carries while iTs phenoType refers To The TraiTs ThaT The individual acTually expresses Page 4 Modified from hTTpwwwmhhecombrooker Fall 2006 BIO 184 LECTURE 9 See Figure 26 in Brooker TexT Mendel performed The monohybrid crosses for all seven of The TraiTs he chose wiTh The resuITs shown on p 23 of your TexT DaTa from Monohybrid Crossesquot IV PunneTT Squares One excellenT way of visualizing Mendel39s crosses is To draw PunneTT Squares A PunneTT Square is a grid ThaT enables one To predicT The ouTcome of simple geneTic crosses As an example suppose ThaT we were following a cross beTween a purebreeding Tall pea planT TT and a purebreeding dwarf pea planT TT To seT up The PunneTT Square we musT firsT deTermine whaT Types of gameTes each parenT can produce In This case each parenT can only conTribuTe one Type of gameTe To The cross and each gamTe will conTain only one facTor so The PunneTT Square predicTs ThaT only one Type of offspring TT wi resuIT The gameTes from one parenT are placed across The Top of The square and The gameTes of The oTher parenT are placed along The lefT side of The square The gameTes are Then combined ferTilized To produce The offspring unshaded square In a sligthy more complicaTed cross leT39s see whaT happens if Two TT planTs are crossed To one anoTher In This case Mendel39s Law of SegregaTion predicTs ThaT each planT will produce Two gameTe Types T and T in equal proporTions Thus The PunneTT Square would be drawn as follows T T TT TT T TT TT The PunneTT Square now predicTs a 121 genoTypic raTio among The offspring 1 TT 2 TT 1 TT and a 31 phenoTypic raTio 3 Tall 1 dwarf Page 5 Modified from httpwwwmhhecombrooker Fall 2006 BIO 184 LECTURE 9 V Mendel39s Second Law Law of Independent Assortment After completing his monohybrid crosses Mendel extended his experiments by performing a series of dihybrid crosses In a dihybrid cross the hereditary behavior of two dfferent traits eg seed shape and seed color is followed Slmutaneousyin the same cross 0 For example trait 1 2 seed texture round vs wrinkled and trait 2 is seed coor yellow vs green Again Mendel started with purebreeding plants produced a generation of F1 plants and then crossed the Fls to one another to produce an F2 generation Mendel postulated that there were two possible patterns he might observe In the first the two dominant traits would always enter the same gametes and the two recessive traits would enter the other gametes Thus a plant that was Yer would only produce gametes that were YR or yr This is called linked assortment because the traits are linked together through the cross The other alternative is called independent assortment and was what Mendel actually observed In this pattern the dominant forms of each trait have no particular affinity for each other and the plant produces equal numbers of four gamete types YR yr Yr and yR See Figure 27 in Brooker text For example Mendel mated plants that were purebreeding for round yellow seeds with plants that were purebreeding for wrinkled green seeds As expected all of the F1 generation plants were round and yellow Then he crossed or selfed the F1 plants and examined the F2 generation When he did so he discovered that the F2s appeared in phenotypic ratios of 9331 as follows round yellow round green wrinkled yellow wrinkled green H0000 Page 6 Modified from hTTpwwwmhhecombrooker Fall 2006 BIO 184 LECTURE 9 The round yellow and wrinkled green offspring are called parenTals or nonrecombinanTs because They have The same phenoTypic combinaTions as The Two parenTs The oThers are called nonparenTals or recombinanTs because They look differenT from eiTher parenT Figure 28 in Brooker illusTraTes Mendel39s dihybrid crosses and how he inTerpreTed his resulTs Mendel performed dihybrid crosses for all possible combinaTions of The 7 TraiTs he had chosen For all combinaTions buT one he goT close To a 9331 raTio in The F2 generaTion supporTing his model we will discuss The one excepTion laTer VI The Rules of ProbabiliTy SomeTimes iT is easier To use The producT rule Than PunneTT Squares To deTermine The resulTs of a cross This is especially True when more Than Two TraiTs are being followed in The cross or when The relaTive frequency of a parfcuar genoType or phenoType is being soughT The use of This rule is possible because of Mendel39s Law of IndependenT AssorTmenT ThaT is The segregaTion of The facTors for each TraiT in a cross is an independenT evenT from The segregaTion of The facTors of all oTher TraiTs in The cross Consider The following example A cross is made beTween The Two planTs wiTh The genoTypes Aa Yy rr and Au Yy Rr where A purple flowers a whiTe flowers Y 2 yellow seeds y 2 green seeds and R 2 round seeds and r wrinkled seeds WhaT fracTion of The offspring would you expecT To have The genoType Aayyrr To use The producT rule To answer This quesTion you would firsT analyze each TraiT separafey Then you would simply mulTiply The individual probabiliTies TogeTher To arrive 0T 0 final answer No x Aa Aa Ynyy gt y x x 116 rr x Rr rr Page 7 Modified from hTTpwwwmhhecombr39ooker39 Fall 2006 BIO 184 LECTURE 9 The pr oducT rule can also be used To pr edicT The phenoTypes of The offspring of a geneTic cr oss For example using The same cross as above whaT fr acTion of The offspring would you expecT To have purple flowers and yellow wr inkled seeds The easiesT way To solve This Type of problem is To express The phenoType as a genoType and Then proceed as in par T a purple flowers 2 A yellow seeds 2 Y wr inkled seeds 2 rr Aaan A i x i x l 932 VYXYY gt Y 4 4 2 r r er gt r r Page 8 Mudified frum Nip wwwmhh2 cumbruuker F9 2006 O IE4 LECTURE 3 Lecfur e 3 Translafion of mRNA shincmignna smwcnrc I mkllynliim NA a Juimiwmlrma 339 insriwit 339 mkalymic mm 539 din cosiihasuni i suuunn R bnmn gtL mn mg 9 sh inc i mRNA by Ribosomes 39 In pmka e is mcognin and basepaired by the 165 ribosomal RNA in the mRNA r actua nie iated by cap bind39ng pretens not shown The ribesenie then scans down the mRNA and finds the eoer start codon for that gene ht www in v books by on e r 177 I Ovaview of Translafian Th2 fruitsafle of the mkNA codons info amino acid sequences leads to the synthesis of poypeptides which then fold andor aggregate to form funchbnamolecules caiied proteins The word translation is wenchosen because the chemical language of nucleic acids in rnizm is being changed into the chemical ianguage orpoiypeptides during the process Page Modified from hTTpwwwmhhecombrooker BIO 184 Fall 2006 LECTURE 3 ProTeins are The acTive parTicipanTs in cell sTrucTure and funcTion II They are The work horsesquot of The cell The main funcTion of The geneTic maTerial is Therefore To encode The producTion of cellular proTeins m 16 correcf ce af 16 proper fme and m sufabe amoun 7 s Early ExperimenTs A Archibald Garrod early 20fh CenTury Was The firsT To propose a relaTionship beTween genes and proTein producTion Garrod sTudied paTienTs who had defecTs in Their abiliTy To meTabolize cerTain compounds He was parTicularly inTeresTed in alkapTonuria o PaTienTs bodies accumulaTe abnormal levels of homogenTisic acid alkapTon 0 Disease characTerized by black urine and bluishblack discoloraTion of carTilage and skin He proposed ThaT alkapTonuria was due 7 0 a mssng enzyme namely homogenTisic acid oxidase Garrod also knew ThaT alkapTonuria follows a recessive paTTern of inheriTance He Thus pro posed ThaT a r39eafbnshp exsfs befween fhe inherfance of le fraif and Me miterfame of a defec ve enzyme 0 He described The disease as an inborn error of meTabolismquot B Beedle and TaTum early 19405 George Beadle and Edward TaTum were also inTeresTed in The relaTionship among genes enzymes and TraiTs 0 They specifically wanTed To know wheTher each gene coded for a single enzyme or wheTher each gene coded for many enzymes Their geneTic model was Neurospora crassa a common bread mold 0 Their sTudies involved The analysis of simple nuTriTional requiremenTs Page 2 Mudified fmm hiip vvvwvmhhe cumbruuker Fall 2000 o 184 LECTURE 3 They analyzed more than 2000 strains that had been irradiated to produce mutations y una a w unaa However in each case growth was restored if only a single vitamin is added to the minimal medium 1st strain gt Pyridoxinfz 2nd strain gt Thiamine 3rd strain gt p aminobenzloic acid Iquot L I I o In the mutant strains agszlc defect In aruger prevented the synthesis of one protein required to produce that vitamin o Beadle and Tatum39s concluded that a rinyh m annulled flu syn uu39s 0quot 139 This was referred to as the one geneone enzyme theoryquot In later decades this theory had to be modified in two ways 1 Enzymes are only one category of proteins 2 Some proteins are com posed of two or more different polypeptides The term polypeptide denotes amenquot The term protein denotes famevim For exam Elfz Adult hemoglobin is coded by two genes proper y and then bind together to make functional hemoglobin Pagea Modified from hTTpwwwmhhecombrooker Fall 2006 BIO 184 LECTURE 3 B Khorana early 19605 The Gene l ic Code which links The sequence of nucleoTides in an mRNA To The sequence of amino acids in a polypepTide was deciphered in The early 1960s 0 More Than one seT of experimenTs were performed 0 Only The research performed by a Team led by Gobind Khorana will be described here Gobind Khorana and his collaboraTors developed a novel meThod To synThesize RNA 0 They firsT creaTed shorT RNAs 2 To 4 nucleoTide long ThaT had a defined sequence 0 These were Then linked TogeTher enzymaTically To creaTe long copolymers 0 They used These copolymers in a cellfree TranslaTion sysTem To see whaT polypepTides were made 0 For example when a copolymer of UC was made UCUCUCUCUCUCUCUCUCUC was TranslaTed as eiTher serine or leucine This meanT ThaT UCU and CUC musT code for These Two amino acids TABLE 1 3 5 Examples of Copolymers That Were Analyzed by Khorana and Colleagues Synthetic Amino Acids RNA Codon Possibilities Incorporated into Peptides UC UCU CUC Serine leucine AG AGA GAG Arginine glutamic acid UG UGU GUG Cysteine valine AC ACA CAC Threonine histidine UUC UUC UCU CUU Phenylalanine serine leucine AAG AAG AGA GAA Lysine arglnine glutamic acid UUG UUG UGU GUU Leucine cysteine valine CAA CAA AAC ACA Glutamine asparagine threonine UAUC UAU AUC UCU CUA Tyrosine isoleucine serine leucine UUAC UUA UAC ACU CUU Leucine tyrosine threonine The synthetic RNAs were linked together to make copolymers Page 4 Modi BIO III Fall 2006 fied from hTTpwwwmhhecombrooker 184 LECTURE 3 Characteristics of The GeneTic Code 1 IT is a TripleT Code Since life uses 20 amino acids biologisTs reasoned ThaT The geneTic code musT conTain aT leasT 3 leTTers o A oneleTTer code could only encode 4 amino acids 0 A TwoleTTer code could only encode 16 amino acids 0 A ThreeleTTer code could encode 64 amino acids more Than enough Khorana39s Team provided experimenTal supporT for a 3leTTer code Each 3leTTer combinaTion is called a codon The code is arbiTrary buT musT have been adopTed very early in The evoluTion of life since all organisms use almosT The same code TABLE 1 3 2 The Genetic Code Second position U A G UUU UCU UAU UGU U Phe Tyr Cys U UUC UCC s UAC UGC C er UUA L UCA UAA Stop UGA Stop A eu UUG UCG UAG Slop UGG Trp G CUU CCU CAU Hg CGU U IS CUC CCC CAC CGC C C Leu Pro Arg CUA CCA CAA CGA A 5 GIquot 2 E CUG CCG CAG EGG G 3 3 3 n a E AUU ACU AAU AGU U E E Asn Ser E AUC Ile ACC AAC AGC C A Thr AUA ACA AAA AGA A Lys Arg AUG MelStan ACG AAG AGG G GUU GCU GAU GGU U Asp GUC GCC GAC GGC C 6 Val Ala Gly GUA GCA GAA GI GGA A u GUG GCG GAG GGG G 2 IT has START and STOP codons Page 5 Modified from hTTpwwwmhhecombrooker Fall 2006 184 LECTURE 3 o AU6 which specifies meThionine sTarT codon o AU6 also specifies addiTional meThionines wiThin The coding sequence 0 UAA UA6 and U6A TerminaTion or sTop codons 2 If is degenerate o More Than one codon can specify The same amino acid 0 For example 66U 66C 66A and 666 all code for lysine o In mosT insTances The Third base is The degeneraTe base 0 IT is someTime referred To as The wobble base 3 The code is nearly universal o All living organisms use The same code 0 Only a few rare excepTions have been noTed and These are ThoughT To have evolved offer The code was firsT esTablished 4 All nucleofides are read only once 0 Once TranslaTion has begun nucleoTides in The mRNA are read in successive TripleTs codons o NucleoTides are never skipped or read more Than once The figure below illusTraTes The process of TranslaTion of an mRNA using The 6eneTic Code Co ding strand 539 r r 339 DNA H lGGGGCTCGGGGAATGG a V V CGGGAGCCC39CTTACC 5 Transcription 5 339 mRNA I Unlranslaled Siart region codon An codons Translation l Il il l U CAUCGCUGGUGA CCCCGAGC CCUUACC o lRNA Paiypepiide Page 6 Modified from hTTp39wwwmhhecombrooker BIO 184 NoTe ThaT The mRNA begins wiTh a 539 unTranslaTed region Fall 2006 LECTURE 3 o In oTher words The START codon is noT aT The 5 end of The mRNA buT somewhaT downsTream 3 0 Although iT is noT shown here The STOP codon is likewise noT aT The 3 end of The mRNA There is a 339 unTranslaTed region afTer iT IV ProTein STrucTure There are four levels of sTrucTures in proTeins 1 Primary Is iT s amino acid sequence WiThin The cell The proTein will noT be found in This linear sTaTe RaTher iT will adapT a compac r 3D sTrucTure Indeed This folding can begin during TranslaTion The progression from The primary To The 3D STrucTure is dicTaTed by The amino acid sequence wiThin The polypepTide Page 7 Modified from thwwwmhhecombrooker BIO 184 0 There are 20 amino acids fhaf may be found in polypepfides 0 Each confuins a different side chain or R group Vaiine Val v Glycine Gly G Alanine Ala A Leudne Leu L lsnleucine lle a Nonpoiar aiipnaiic amino acids Proline Pro F Fall 2006 LECTURE 3 Nonpolar amino acids hate span 9h plasma membrane of various organelles or le call iisal Phonyiaianinn Phc F Tyrosine Tyr Y Tiypiopnan Trpi w 4 h Nonpoiar aromallc amino acids sea Serine Sens Cysteine0sC TlireuniiieiThIT MemiunineMeiM AsparagilieAsn N Gimmineieinm ci Puian neuiral amiriu acius 000 i Asuanlc adthst Gimamlc aameiu E Pmlinn Pm P Hisli 5 His H Lysine Lys K id i oian acimc aminn Buds 4c Poian basic aminu acids Page 8 Polar amino acids like water prof ins 4 Modified from hTTpwwwmhhecombrooker Fall 2006 BIO 184 LECTURE 3 2 Secondary o The primary sTrucTure of a proTein folds To form regular repeaTing shapes known as secondary sTrucTures 0 There are Two Types of secondary sTrucTures o a helix 0 b sheeT 0 These are sTabilized by The formaTion of hydrogen bonds beTween aToms along The quotbackbonequot of The polypepTide 3 TerTiary o The shorT regions of secondary sTrucTure in a proTein fold inTo a Three dimensional TerTiary sTrucTure o This is The final conformaTion of proTeins ThaT are composed of a single polypepTide o The TerTiary sTrucTure is sTabilized via inTeracTions beTween R groupd of The amino acids I Ionic bonds I HydrophicinTeracTions I Disulfide bonds I Hydrogen bonds I Van der Waals forces 4 QuaTernary o ProTeins made up of Two or more polypepTides have a quaTernary sTrucTure o This is formed when The various polypepTides associaTe TogeTher To make a funcTional proTein o OfTen sTabilized by disulfide bonds See Figure 136 Brooker V TRNA STrucTure and FuncTion A The AdapTor HyoThesis Page 9 Modified from hTTpwwwmhhecombrooker Fall 2006 BIO 184 LECTURE 3 In The 1950s Francis Crick and Mahon Hoagland proposed The adapTor hypoThesis which hypoThesized ThaT fRNAs play a direcf role in le recagni an of cadans in le mRNA 0 In parTicular The hypoThesis proposed ThaT TRNA has Two funcTions 1 Recognizing a 3base codon in mRNA 2 Carrying an amino acid ThaT is specific for ThaT codon To The TranslaTion machinery During mRNATRNA recogniTion The anTicodon in TRNA binds To a complemenTary codon in mRNA Phenylalanine Praline I Phenylaiine a g Proline anticodon anticodon A A G G G c U U C C C G 9amp33 mHNA Phenylaline Proline codon codon B TRNA Secondary STrucTure The secondary sTrucTure of TRNAs exhibiTs a Cloverleaf paTTern conTaining 0 Three sTemloop sTrucTures 0 Variable region 0 An accepTor sTem amino acid binding siTe o 339 single sTrand region anTicodon o The acTual Threedimensional or TerTiary sTrucTure involves addiTional folding See Figure 1310 Brooker Page 10 Modified from hTTpwwwmhhecombrooker Fall 2006 BIO 184 LECTURE 3 C Modified Nucleofides and The Wobble Hyofhesis In addiTion To The normal A U G and C nucleoTides TRNAs commonly conTain modified nucleofides o More Than 60 of These can occur As menTioned earlier The geneTic code is degenerafe o WiTh The excepTion of serine arginine and leucine This degeneracy always occurs aT The codon39s posiTion 0 To explain This paTTern of degeneracy Francis Crick proposed in 1966 The wobble hypofllesis o In The codon anTicodon recogniTion process The firsT Two posiTions pair sTrichy according To The A U G C rule 0 However The Third posiTion can acTually quotwobblequot or move a biT Thus ToleraTing cerTain Types of mismaTches The modified nucleoTides in The TRNA anTicodon allow This wobbling To occur VI The Ribosome TranslaTion occurs on The surface of a large macromolecular complex Termed The ribosome A ribosome is composed of sTrucTures called The large and small subunifs 0 Each subuniT is formed from The assembly of I ProTeins I Ribosomal RNA rRNA Polypeptide Ribosomes conTain 3 discreTe siTes iRNA o PepTidyl siTe P siTe o Aminoacyl siTe A siTe o ExiT siTe E siTe mRNA Page 11 Modified from httpwww nith combrook Faii 2006 BIO 134 LECTURE 3 Tnensieiion can be viewed as occurring in three sieges 1 Initiation In bacteria I The binding of mRNA to the 305 subunit is facilitated by u ribosomul binding site or ShineDulgurno sequence 0 This is complementary to a sequence in the 165 rRNA within the small bacterial ribosomul subunit ShineDalgamo Stan sequence codon 16 rRNA 39 I I x 305 subunl i sequence is comziemsniaw 1C a Puiiion ui a 5 ms RNA iomtm m Ihs use VHNA 5 3 IFE puri es ihe bindng o he iniiiazun Rm Page 12 Mudmed from hiip qu mhhe cumbruuker BIO 184 Fall 2005 LECTURE 3 Mel ased The 503 Subunit associates l le and lFa are rele In salaryNI Quite dirrenent than in baetenia o o o kzcognifion of the mkNA by the nibosoine is thnough pnoteins cap bainding pnoteins attached to the 539 cap of the mkNA These ane joined by a com plex consisting of the 405 subunit tizNAinet and other initiation factors The entine assembly moves along the mkNA scanning for the night start codon Once it finds this we usually but not always the inst one it eneountens the 405 subunit binds to it The 605 subunit then joins forming the Bus initiation complex P991213 Modified from hTTpwwwmhhecombrooker Fall 2006 BIO 184 LECTURE 3 2 ElongaTion 0 During This sTage The amino acids are added To The polypepTide chain one aT a Time See Figure 1318 Brooker 3 Termination o The final sTage occurs when a sTop codon is reached in The mRNA 0 In mosT species There are Three sTop or nonsense codons I UAG I UAA I UGA 0 These codons are noT recognized by TRNAs buT by proTeins called release facTors I Indeed The 3D sTrucTure of release facTors mimics ThaT of TRNAs See Figure 1319 Brooker VII The NewlyTranslaTed PolypepTide PolypepTide synThesis has a dl39r39ec anaI39fyThaT parallels The 539 To 339 orienTaTion of mRNA 0 During each cycle of elongaTion a pepTide bond is formed beTween The lasT amino acid in The polypepTide chain and The amino acid being added CH3 OH i CH2 fit I CH2 CH2 CH2 H i Amino H3N 39 C C N C C N C C O Carboxyl terminal 1 II I ll terminal end H 0 o H 0 end I Methionine Tyrosine Cysteine 5 Sequence in m RNA in Directionaiity in a polypeptide and mRNA Page 14 Modified from h pwwwmhhecombrooker Fall 2006 O 184 LECTURE 3 VIII Polypeptide Localization The amino acid sequences of newlysyn l39hesized polypep l ides con l39ain sor i ing signalsquot l39ha l39 l39ell The cell where l39hey belong These are especially imporl l39an l39 in eukar yo l es where each sor i39ing signal is recognized by a specific cellular componen l These cellular componen l s facili l39a l39e l39he sor i39ing of l he pr o l ein l39o i l39s cor r ec l compar i men l Many polypep l ides will no l39 fold properly and become funci39ional pr o l eins un l39il l39hey are properly localized wi l39hin The cell Some l in39les mu l39a l39ions lead l39o changes in The amino acid sequence of l he polypep l ide l39ha l39 pr even l i l39s localiza l39ion 0 Such polypep l ides ar e even l39ually degr aded o Localiza l39ion mu l39a l39ions are usually null mu l39a l39ions l39he pr o l ein coded by The gene has no residual func l ion in The cell TA 3 LE 1 3 8 Sorting Signals in Eukarymic Proteins Type ul signal nasciinlian Miladiunlliial largeling signal Nuclear lgcalizauun Slgnal hargad lesiuuss and usually one ol mole pmllnes Pemxisrzmal 39 l H largeling signal ZGamlnoasld sequence al llla aminu lennnlus SRP signal a 39 39 39 ER lelenliun signal a 1 ul nllllamir Arirl Gnlgl iaianliuii slgnal Lysosurllal largeling signal Desllllailan ul a cellular Plolein Type nl signal llle Pmlein conlainswnliin lls Amino Acid Sequente Cylusal No slgnal ienuiiau Mitochondriaquot Milorhnndriallalgelinl signal Nuclear Nunlanilunalizaliun signal Pemximllla l Eluxisomal laigaliiig signal ER SRP signal and an ER mention signal Golgi SW signal and a Gulgl Ielemlull Slgnall lymsnme SRP signal and a lysusuinaliaigeung slgnalv Plasma membrane 39 Senell on SW signal lln adrllllnrlal signal ieuuueii Page 15


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