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1
macromolecules
proteins
composed of monomers
DNA
RNA
Unit xx, Lesson xxUnit xx, Lesson xx
Essential Questions• Whatisamonomer,andwhatrelationdoesithavetothestructureand
functionofpolymers?
• HowdonucleicacidmonomersinfluencethefunctionofDNAandRNA?
• Howdoesthesequenceofanaminoaciddeterminethethree-dimensionalstructureoftheprotein?
• Whatdoesdirectionalitymeanwhenreferringtonucleicacidsandproteins?
Unit 7, Lesson 1
Nucleic Acids and Proteins
Keywordsamino acid
dehydration synthesis
fatty acids
monomer
monosaccharides
non-polar molecule
nucleotide
nucleic acid
peptide bond
polar molecule
polymer
protein
rRNA
Copyright © 2011, K12 Inc. All rights reserved. This material may not be reproduced in whole or in part, including illustrations, without the express prior written consent of K12 Inc.
Unit 7, Lesson 1 Nucleic Acids and Proteins 2
Set the StageXXX
Set the StageAlthough one missing amino acid in a polypeptide or the wrong nucleotide in a nucleic acid sequence are small differences, they can have serious consequences for an organism. The wrong nucleotide in DNA or RNA can result in the wrong codon being used during the translation process and then the wrong amino acid being inserted into the final sequence. When the primary sequence of a protein is wrong, the protein will not fold in the same way it would have, and ultimately the function of the protein will be different or nonexistent. This can cause serious problems in a biological system that is dependent on thousands of pieces working harmoniously together for the wellbeing of the organism. If one part does not work, others might not either, the way a car would not work without a steering wheel.
An example of a problem that missing amino acids or wrong nucleotides can cause in humans is the disease of sickle-cell anemia, a genetic blood disorder that produces abnormal red blood cells with a tendency to form unwanted clots in the blood stream due to their sickle shape. This disease is the result of one monomer, glutamic acid, being substituted for the amino acid valine in the primary sequence of hemoglobin. It is only one example of the thousands of genetic disorders that can occur when one monomer is out of place.
Macromolecules are polymers made of connected monomers.Alllivingthingsareconstructedprimarilyfromfourmajormacromolecules:nucleicacids,proteins,lipids,andcarbohydrates.Nucleic acidisimportantinstoring,transmitting,andmakingusefultheinformationnecessaryfortheprocessesoflife.Proteiniscomposedofaminoacidsthatareimportantforlifefunctions.Lipidsarecomposedoffats,oils,phospholipids,steroids,andwaxes.Carbohydratesarelargeorganicmoleculesmadefromcarbon,oxygen,andhydrogen.
Figure 1. Sickle-shaped red blood cells are the result of a single base-change mutation.
nucleic acid a macro-molecule important in storing, transmitting, and making useful the information necessary for the processes of life
protein a macromole-cule composed of amino acids that are important for life functions
Copyright © 2011, K12 Inc. All rights reserved. This material may not be reproduced in whole or in part, including illustrations, without the express prior written consent of K12 Inc.
Unit 7, Lesson 1 Nucleic Acids and Proteins 3
Themoleculesnucleicacids,proteins,lipids,andcarbohydratesarereferredtoas“macro”becauseofthelargenumberofatomsusedintheirconstructionandbecauseoftheirlargemolecularweight(oftenover100,000daltons).Themajor-ityofmacromoleculesarepolymers(PAH-luh-muhr),largemoleculesofrepeat-inggroupsofmolecularlyidentical,nearlyidentical,orcloselyrelatedsubunits.Thesepolymersubunitsareknownasmonomers(MAH-nuh-muhr).Withinanyparticularcell,thousandsofdifferentmacromoleculescanbefoundcarryingoutmanyimportantfunctions—allcontributingtotheprocessoflife.
Monomers are important to polymer structure and function. Eachclassofmacromoleculeismadeupofdifferenttypesofmonomers.Nucleicacidmonomersarecallednucleotides;proteinmonomersarecalledamino acids;lipidmonomersarecalledfatty acids;andcarbohydratemonomersarecalledmonosaccharides(MAH-nuh-SA-kuh-riyd).Alltypesofmonomersareorganicmolecules,meaningthattheycontaincarbon-to-carbonbonds.Com-monlythemonomerswillhavemanycarbonatomsbondedtogetherrepeatedlytoformchains,resemblingthestructureoffattyacids.Thesechainsarereferredtoascarbon“backbones”becauseoftheirrepetitiveappearance.Othertimesthecarbonatomsmaytaketheformofrings,astheydoinmonosaccharidesandnucleotides.
In addition to the numerous carbon atoms and bonds in thesemolecules, monomers may contain arrangements of atoms and bonds,called functional groups, which influence the function of the polymersinuniqueways.A common functional group found in amino acids is theamino group −NH2 (Figure 2). Other commonly occurring functionalgroups include carboxyl groups (−COOH) in amino acids, methyl groups(−CH3),andphosphategroups(–PO3)innucleicacids.Thevaryingchemicalpropertiesofthesefunctionalgroups,accordingtohowtheyareattachedtothevariouscarbonringsandchains,arethereasonthatdifferentmonomershaveuniquecharacteristics.
nucleotide a subunit of a nucleic acid that consists of a phosphate group, a five-carbon sugar, and a nitrog-enous base
amino acid the organ-ic molecules from which proteins are made
fatty acids the organic molecules that serve as the units from which lipids are made
monosaccharides the organic molecules from which carbohydrates are made
Figure 2. Functional groups are groups of atoms that attach to carbon backbones. Each group has different chemical properties that influence chemical interactions. These six functional groups are commonly found in biological chemicals.
O H hydroxyl group
O
Ccarbonyl group
O
C O Hcarboxyl group
H N
Hamino group
S H sulfhydryl group
O
O P OH
OHphosphate group
polymer large molecules of repeating groups of molecularly identical subunits
monomer building block molecules for polymers
Copyright © 2011, K12 Inc. All rights reserved. This material may not be reproduced in whole or in part, including illustrations, without the express prior written consent of K12 Inc.
Unit 7, Lesson 1 Nucleic Acids and Proteins 4
Ultimately,thestructuresofthemonomersdeterminethecharacteristicsandfunctionofpolymersandtheorganismasawhole.Thestructureaffectstheorder,orientation,andenvironmentalinteractionofthelargermacromolecule.Themac-romoleculepropertiesthatariseduetothesemonomercharacteristicsarecalledemergentproperties.
Sincemonomerbuildingblocksconstitutethestructuresofalllifeonearth,andsincestructuredirectlyinfluencesthefunctionoforganisms,thislessonwilldevotesignificanttimetotheseimportantmoleculesandthemacromoleculestheyconstruct.
Connecting and disconnecting monomers occurs through condensation and hydrolysis reactions.Animportantpartofbiologyisknowingwhatpiecesmakeupmacromolecules.Howtheyareputtogetherandtakenapartisalsoimportantbecausemacromol-eculesarefrequentlyconstructedanddeconstructedwithmajorimplicationsforthefunctionoforganisms.Whenyoudigestfood,forexample,thecellsliningyourdigestivetractabsorbmonomersofthelipids,carbohydrates,andproteinsyoujustateandusethemasbuildingblockstoconstructtheirownmacromolecules.Thisutilizationispossiblebecauseoftwoopposingchemicalreactionsthatutilizemonomers,water,andenzymes.
Areactioncalleddehydration synthesis,orcondensation,connectstwomonomers.Thisprocessoccurswhenanenzymeremovesan−OHgroupfromtheendofonemonomerandan−Hatomfromanothermonomer.The−OHand−Hjointogether,creatingwater,whilethetwomonomersbecomeconnectedthroughacovalentbond(Figure 3).Thisprocessrequiresaninputofenergyfromthecelltobeaccomplished.Hydrolysis(watersplitting)breaksmonomersapartbydoingtheexactopposite.H2Oisattachedtoanenzymeandisbrokenwhenan−Hatomisconnectedtoonemonomerwhilethe−OHbecomesconnectedtotheother,breakingthecovalentbondthatholdsthemonomerstogether.
dehydration synthesis an enzymatic reaction that connects two monomers while creating a water molecule
Figure 3. A dehydration synthesis reaction occurs when an –OH group is removed from one chemical and an –H is removed from another. The two chemicals are bound together with a covalent bond, and a molecule of H2O is produced.
H N
H
C C OH
H2O
R
H O
H N
H
C C OH
R
H O
H N
H
C C
R
H O
N
H
C C OH
R
H O
+
+
Copyright © 2011, K12 Inc. All rights reserved. This material may not be reproduced in whole or in part, including illustrations, without the express prior written consent of K12 Inc.
Unit 7, Lesson 1 Nucleic Acids and Proteins 5
DNA and RNA are important macromolecules. Thedouble-helixmacromoleculeDNAcontainsthegeneticinformationnecessarytoguideconstructionofalltheproteinsinacellandthusallowthecontinuityoflife.Asdiscussedpreviously,DNA’ssingle-strandedcounterpart,RNA,comesinthreeforms,eachwithaspecificfunctionthataidsproteinsynthesis.MessengerRNA(mRNA)transportstheinformationfromDNAtotheribosomes.Attheribosomes,tRNAtransfersthegeneticinformationintoasequenceofproteins,andrRNAservesasthebuildingblockofribosomes.ThesevariousDNAandRNApolymersaregenerallyreferredtoaspolynucleotides.
The structure of DNA and RNA determines their function. ThenucleicacidsDNAandRNAhavemanychemicalandstructuralsimilaritiesbutcanfunctiondifferentlybasedonthenucleotidemonomersfromwhichtheyareconstructed.DNAmonomersarecalledDNAnucleotides,andRNAmonomersarecalledRNAnucleotides.Bothtypesofnucleotidescontainafive-carbonsugarring,aphosphategroup,andanitrogen-containingbase(Figure 4).Thenitrogen-containingbaseconnectstothefirstcarbonofthesugarring,andthephosphateattachestothefifthcarbonofthesugarring.Thesenucleotidemonomerslinktogetherbyformingcovalentbonds,whichformarepeatingsugar-phosphateback-bonewiththesugarringofonenucleotideattachedtothephosphateofthenext.
TherearesomesubtlebutimportantdifferencesbetweenthestructuresofDNAandRNA(Figure 5).Onedifferenceisinthefive-carbonsugarrings.ThesugarinDNAnucleotides,deoxyribose,lacksanextra−OHfunctionalgroupthatthesugarinRNA,ribose,contains.ThisextrafunctionalgrouppreventsRNAfromformingthestabledouble-helixstructureofDNA.
Figure 4. A DNA nucleotide is made of a five-carbon sugar ring, a phosphate group, and a nitrogen base. Nucleotides are joined together where the phosphate group binds to the fifth carbon of the sugar ring.
Figure 5. RNA and DNA are both nucleic acids made of nucleotides. RNA is a single strand, and DNA is a double strand. RNA contains uracil instead of thymine, which is found in DNA.
S
P
P
SP
TA
S
P
P
S
CG
S
P
P
S
AT
S
P
P
S
TA
S
P
S
GCU
uracil
sugar-phosphate backbone
base pair
nucleotide
Tthymine
Aadenine
Aadenine
Gguanine
Gguanine
Ccytosine
Ccytosine
tRNA transfer RNA: a type of RNA molecule that carries amino acids to the site of protein synthesis in ribosomes
rRNA ribosomal RNA: a type of RNA molecule that makes up the main part of the structure of a ribosome
Copyright © 2011, K12 Inc. All rights reserved. This material may not be reproduced in whole or in part, including illustrations, without the express prior written consent of K12 Inc.
Unit 7, Lesson 1 Nucleic Acids and Proteins 6
Thenitrogen-containingbasesarealsoslightlydifferentinDNAandRNA.DNAcontainscytosine,guanine,adenine,andthymine.InRNA,thebasethy-mineisreplacedwithuracil.Thesebasespairwitheachothertoformtherungsofthestepladderwithinthedoublehelix.Cytosineandguaninearecapableofpairingwitheachotherwhileadenineandthyminepairwitheachother.InRNA,adeninepairswithuracil.ThevariationinthesequenceofthesebasesisthecodeforthegeneticinformationthatDNAcontains.Basedonthedifferentmonomertypesthatareusedtoconstructthem,DNAandRNAarecapableofperformingverydifferentfunctions.
Proteins are macromolecules that have many functions in the cell. Proteinsplayasignificantroleinbiology:proteinmakesupover50%ofthemassofthetypicalcell,notcountingwater.Proteinscarryoutmanyfunctionswithincellsincludingstructuralsupport,moleculartransportation,andcell-to-cellcom-munication.Chemicalprocessesareacceleratedbyspecializedproteinscalledenzymes,whicharefoundabundantlyinlivingsystems.Althoughthousandsofproteinshavealreadybeendiscoveredandanalyzed,manyscientists,aidedbycontinualadvancesintechnology,continuetoinvestigatetheremainingundescribedproteins.
Amino acid monomers form proteins and determine protein function. Proteinmacromoleculesconsistofassortedaminoacidmonomersthatbecomelinkedtogetherviadehydrationsynthesis(Figure 6)likebeadsonanecklace.Atypicalproteinmightcontainabout100linkedmonomers.Yetallproteinsareconstructedfromonly20differentaminoacids.AswithDNAnucleotides,theorderinwhichthepiecesareputtogetherisnearlylimitless.Formingdifferentproteinsusingonly20aminoacidsiscomparabletoformingmanywordsinalanguageusingonly26lettersinthealphabet.Similartotheorderinnucleotides,theorderinwhichtheseindividualaminoacidmonomersarejoinedultimatelydeterminesthefunctionandpropertiesofeachprotein.
H
H
C C
O
OHR
N
Amino Group Carboxyl Group
H
Figure 6. An amino acid consists of an amino group, a hydrogen atom, a carboxyl group, and the variable R group.
1SeLF-CheCK
How are monomers
capable of interacting in
such a way that allows
dehydration synthesis or
hydrolysis reactions
to occur?
Copyright © 2011, K12 Inc. All rights reserved. This material may not be reproduced in whole or in part, including illustrations, without the express prior written consent of K12 Inc.
Unit 7, Lesson 1 Nucleic Acids and Proteins 7Unit 7, Lesson 1 Nucleic Acids and Proteins
Figure 7. R groups attach to the central atom of an amino acid molecule. The bond location on each R group is represented by a single-sided bond.
AMINO ACIDS AND THEIR R-GROUPS
Amino Acid R-Group Amino Acid R-Group
GlycineH
Threonine CHCH3HO
AlanineCH3
Cysteine H2C
SH
Proline* H
N
H2C C C
H2C CH2 OH
OAsparagine
CH2
NH2
CO
ValineCH
H3C CH3
Glutamine CH2
C
H2C
OH2N
Isoleucine
C H2
CHH3CCH3
SerineH2C
OH
Leucine
CH
H3C CH2
CH3
Aspartic acidCH2
OH
CO
Methionine
H2C
H3C
CH2
S
Glutamic acid CH2
C
H2C
OHO
Tryptophan
HC
C HC
HC
C
C
CH
NH
CH2
CH
Lysine
CH2H2CH2N
CH2CH2
Phenylalanine
HO
HC
C C
HC
CH
CH2
CH
Arginine CH2
HNH2C
C N H
CH2
H2N
Tyrosine
HOC
C HC
HC
CH
CH2
CH
Histidine
HC
C CH2
NH
N
HC
*Because the R group binds to the NH2 as well as to the stem, the entire amino acid is shown instead of just the R group.
Copyright © 2011, K12 Inc. All rights reserved. This material may not be reproduced in whole or in part, including illustrations, without the express prior written consent of K12 Inc.
Unit 7, Lesson 1 Nucleic Acids and Proteins 8
Aminoacidsarecomposedofacentralcarbonatomconnectedonfoursidesbyacarboxylgroup,anaminogroup,ahydrogenatom,andavariablegroupreferredtoastheRgroup.ItistheRgroupthatdiffersfromaminoacidtoaminoacid,makingeachonechemicallyuniqueandprovidingeachonewithspecificchemicalproperties.
Whentwoaminoacidsarejoinedtogetherduringadehydrationreaction,theyformapeptide bondwhereintheaminogroupfromoneaminoacidjoinswiththecarboxylgroupoftheotheraminoacid(Figure 7).Aspecialenzymehelpsformapeptidebondbyfacilitatingthedehydrationsynthesisprocessthatremovesawatermoleculeandconnectsthemonomerstogether.
Protein monomers interact chemically with one another. AminoacidscanbeclassifiedbasedonthetypeofRgrouptheypossess.Non-polarRgroupsarecomposedofachemicalstructurethatrepelspolarmoleculessuchaswater.PolarRgroupsdojusttheopposite,chemicallyattractingotherpolargroups,includingwater.Whenthepolarandnon-polarRgroupsofdifferentaminoacidsinteractwitheachother,theresultmaybeahydrophobicorhydrophilicinteraction.Inhydrophobicinteractions,thenon-polargroupsinteractbyclusteringtogetherontheinsideoftheproteinawayfromthewaterontheoutside.InhydrophilicinteractionsthepolarRgroupsinteractsothattheyfaceoutwardswheretheyareexposedtowater.Ahelpfulwaytorememberthisistoknowthathydrophobicmeans“waterfearing,”sothesemoleculesareinsolubleinwater,andhydrophilicmeans“waterloving,”sothesemoleculesaresolubleinwater.Theseinteractionsoccurprimarilybecausepolarmoleculescanhydrogenbondwitheachotherbuttheycannothydrogenbondwithnon-polarmolecules.
Inadditiontohydrophobicandhydrophilicinteractions,aminoacidscaninteractinotherways.Ionicbondscanformbetweenchargedatoms(anionsandcations)indifferentmolecules.APolarbondisatypeofcovalentbondthatdoesnotsharetheelectronsequallybetweenthetwoatomsbecauseoneismoreelec-tronegativethantheother.VanderWaalsinteractionsoccurwhenpositiveandnegativechargedareasofmoleculesgetclosetoeachotheruntiltheyweaklysticktooneanother.WhentwoRgroupscontainingsulfhydrylgroups(−SH)approacheachotherinafoldedprotein(thesearefoundoncysteinemonomers),theywillformadisulfidebridgewiththetwosulfuratomsformingabond.
Proteins can obtain four different configurations depending on how the monomers interact chemically with one another. Aproteincontainsnumerousaminoacidsheldtogetherbypeptidebondsinauniquesequenceknownasitsprimarystructure(Figure 8).Theprimarystructurecanbecomefolded,however,ashydrogenbondsformbetweenthecarbonandnitrogenelementsofthepolypeptidebackbone,formingwhatisknownasthesecondarystructureoftheprotein.Thissecondarystructuremaytaketheconformationofeitheranαhelix(alphahelix),acoiledorspiralshape,oraβpleated(betapleated)sheet,resemblingrowsofmonomersfoldedbackoneachother.
2SeLF-CheCK
How do DNA
nucleotides and RNA
nucleotides differ from
one another?
non-polar molecule a molecule that lacks a dipole or permanent positive and negative areas
polar molecule a molecule in which there are positive and negative areas
peptide bond a chemical bond connect-ing two amino acids with the carboxyl group of one and the amino group of the other
Copyright © 2011, K12 Inc. All rights reserved. This material may not be reproduced in whole or in part, including illustrations, without the express prior written consent of K12 Inc.
Unit 7, Lesson 1 Nucleic Acids and Proteins 9
Allofthepreviouslydescribedchemical-bondinginteractionsoccurbetweentheRgroupsofthevariousaminoacidsinaproteinandtheenvironmentwheretheinteractionoccurs.Thus,proteinscanspontaneouslyarrangethemselvesintoauniquethree-dimensionalconfigurationcalledthetertiarystructure.Althoughthebondsthatformtocreatethesethree-dimensionalconfigurationsareweak,theycanstabilizethestructureenoughsothatitremainsfolded.
Morethanonepolypeptidechaininafunctioningproteincauseinteractionsthatformthequaternarystructure.Mostfunctionalproteinsexistinthisform.SimilarchemicalinteractionsinvolvingtheRgroupsandtheenvironmentoccurinboththequaternarystructureandthetertiarystructure.
A protein’s environment interacts chemically with protein monomers and can affect protein structure. Justasproteinmonomerscaninteractwitheachotherchemically,resultinginchangesofconformation,theycanalsointeractwithmoleculesintheirsurroundingenviron-ment.Infact,itispossibletochangetheenvironmentofaproteintochangethestructureofaprotein.AlteringthepH,saltconcentration,ortemperatureoftheenvi-ronmentcanresultindenaturation,achangeofconformationthatcausesfunctionlossthatmayormaynotbereversible.Whenaneggisplacedinahotfryingpan,theheatcausesdenaturationoftheproteins,resultinginacookedegg.Becausethisdenaturationdisturbstheprimarystructureoftheprotein,itisnotpossiblefortheproteinsinacookedeggtorevertbacktotheiruncookedform.However,iftheprimarystructureisundisturbed,itispossibletoreturnaproteintoitsfunctionalshape.Thisiscommonlydoneinthelaboratorybyremovingthedenaturingagentfromatest-tubesolution.Forexample,thepHcanbechangedbacktowhatitwasoriginally,allowingtheproteinmonomerstoreformtheirchemicalbondswitheachotherandtore-fold.
The unique shape of each protein results in a unique function.Thefunctionofeachproteinisdeterminedbyitsstructure.Itisthroughtheprimarystructureofaproteinthatthefinaltertiaryorquaternarystructuretakesshapeandbecomesafunctionalprotein.Thefinalshapeofaproteinallowsittointeractwithotherproteinsandmoleculesasrequiredincellprocesses,muchasakeyandalockinteract.Oneproteinwillfunctioninacertaincapacityandincurparticularresultsthatanother
Figure 8. Proteins are made of amino acids connected by peptide bonds. These polypeptide chains can fold to form the complex three-dimensional structure of a protein.
peptide bonds amino acids
tertiary structure quaternary structureprimary structure secondary structure
Copyright © 2011, K12 Inc. All rights reserved. This material may not be reproduced in whole or in part, including illustrations, without the express prior written consent of K12 Inc.
Unit 7, Lesson 1 Nucleic Acids and Proteins 10
proteinisnotabletoincur,facilitatedbyitsstructure.Thoughthisconceptmayseemstraightforwardinatextbook,forscientiststryingtomakesenseofthethousandsofproteinsinteractingwithmoleculesincellsthroughoutanorganism,discoveringhowtheseinteractionsoccurinreallifeisanintense,ongoing,andoftenpuzzlingtask.
Macromolecules have directionality. Nucleicacidsandproteinsshareacharacteristic,directionality,whichaffectshowothermoleculesinteractwiththem.Directionalityexistswheneachendofthemoleculeisdifferentfromtheother.Thischaracteristicofsomemacromoleculescanaffecthowtheyfunctionandhowothermoleculesinteractwiththem.
The directionality of nucleic acids affects their function. InDNAandRNA,oneendhasasugarmoleculewhiletheotherhasaphosphatepart.Thesugarendandphosphateendhavespecificnamesderivedfromthecarbonnumbertheyareattachedto.Inthesugar-phosphatebackbone,theendwithaphosphategroupattachedtoitisknownasthe5’(fiveprime)end,whiletheonewiththesugarisknownasthe3’end(Figure 9).Thenitrogenousbasesbranchofffromanendwheretheymaypairwiththeircomplementarybasesonanothernucleicacidthatrunsantiparalleltothefirstnucleicacid—its5’to3’backboneisgoingintheoppositedirection,thesamewaythetrafficoneachsideofatwo-waystreetmovesinoppositedirections.Forexample,ifthesequenceononestrandofDNAwascoded5’AGTACto3’,itsantiparallelstrandthatcomprisesthecomplementaryhalfofthedouble-helixwouldread3’TCATGto5’.
hydrogen bonds
5’ 3’
3’ 5’
S
P
P
S
TA
S
P
P
S
CG
S
P
P
S
AT
S
P
P
S
TA
S
P
S
GC
sugar-phosphate backbone
nucleotide base pairs
sugar-phosphate backbone
P
3SeLF-CheCK
What environmental
factors might affect a
protein’s function?
Figure 9. DNA replication takes place in a 5’ to 3’ direction. The leading strand is constructed continuously while the lagging strand is constructed in short seg-ments known as Okazaki fragments. These fragments are then connected to form a continuous strand of DNA.
Copyright © 2011, K12 Inc. All rights reserved. This material may not be reproduced in whole or in part, including illustrations, without the express prior written consent of K12 Inc.
Unit 7, Lesson 1 Nucleic Acids and Proteins 11
DuringDNAreplication,directionalityimpactshowthenewDNAstrandsareputtogether.Elongationmusttakeplaceina5’to3’direction.Therefore,theDNApolymerasemovingdownthe3’to5’parentalDNAstrandhasnoproblemfollow-ingtheruleforelongationdirectionalitywhenthenewleadingstrandofDNAisconstructed.Forthepolymeraseworkingonthe5’to3’parentalstrand,however,itisnecessarytoconstructthisnewlaggingstrandofDNAinshortsegmentsbetween1,000and2,000nucleotideslong.TheseshortsegmentsofunconnectedDNAareknownasOkazakifragmentsuntiltheyareeffectuallystitchedtogetherbytheenzymeDNAligase.Duringthetranscriptionprocess,RNAiscompiledfromits5’endtoits3’end.TheRNApolymerasemovesdownstreamalongtheDNAgeneitistranscribingina3’to5’direction.DirectionalityalsoaffectsthefunctionofRNAduringthetranslationprocess.Theribosomeattachestothe5’endofmRNAasguidedbythepresenceofa5’capandpoly(A)tailonthe3’end.Becauseofthisattachment,tRNAcanassemblethecorrectproteinsequencesusingthethreenucleotide-widecodons.
The directionality of proteins affects their function.Proteinsalsoutilizedirectionality:oneendisalwaysacarboxylgroupwhiletheotherisanaminogroup.Thereareenzymesinourbodiesthatonlyworkononeendortheotherofapolypeptide.Thisisthecasewithsomediges-tiveenzymes.
4SeLF-CheCK
What are some
implications of
directionality in DNA
and RNA function?
Extensions • Locateandreadafewcurrenteventarticlesorwatchafewvideosabout
geneticengineering.Howmightadvancesinthefieldofgeneticengineeringaffectyounowandinthefuture?
• Findouthowgenetictestingisconducted.Whataresomeethicalprosandconstousingthistechnology?
• Exploregeneticdiseases.Whataresomeexplanationsforwhytherearesomanydifferentgeneticdiseases?
SummaryMacromoleculesarethebuildingblocksofalllivingthings.Similarly,monomersarethebuildingblocksofmacromolecules.Polymersareconstructedanddestructedthroughchemicalreactionsutilizingenzymes,water,andmonomers.DNAandRNAnucleotidesjointoformthe“backbone”and“stepladder”featuresofDNAandRNA.Functionalgroupsareimportantaspectsofmonomerstructurethatal-lowfordifferentchemicalbondstoformbetweenmolecules.Proteinsinparticularareaffectedbychemicalbondingbecausethesebondscausethemtoreshapeintosecondary,tertiary,andquaternarystructures.Environmentalvariablescaninterferewiththesechemicalbondsandcauseproteinstodenature.Somemacromolecules,includingDNA,RNA,andproteins,areaffectedbydirectionality,whichalsoimpactshowtheyareabletofunction.
Copyright © 2011, K12 Inc. All rights reserved. This material may not be reproduced in whole or in part, including illustrations, without the express prior written consent of K12 Inc.
Unit 7, Lesson 1 Nucleic Acids and Proteins 12
1. Monomers can interact in dehydration synthesis and hydration reactions because enzymes facilitate these interactions.
2. Compared to DNA, the sugar in RNA contains an extra –OH functional group, and the base thymine is replaced with uracil in RNA nucleotides.
3. Temperature, pH, and salt concentration are three factors that may interfere with protein function.
4. Because of directionality, a leading strand and a lagging strand are necessary during DNA replication, and special tags are necessary during translation to guide the ribosome to the correct binding location.
SeLF-CheCK ANSweRS
ConnectionsSinceallittakestowreakhavocinanorderlybiologicalsystemistoreplaceonemonomerforanotherinaprotein’sprimarysequence,itseemssystemswouldbedisorderedmoreoften.ButonlyoneintenbillionDNAnucleotidesgetsthroughthesystemtocauseproblems.Howisitpossible,whenthousandsofproteinsareproducedinthehumanbody,forthingsnottogowrongmoreoften?Theanswerliespartiallyinabuilt-insafetycheckthatoccursduringtheDNAreplicationpro-cessitself.DNApolymerase,thesameenzymethatputsnucleotidebasesinplacetobeginwith,double-checksitsworkasitgoes.Anymistakethatgetsthroughthisprocesshasasecondchancetobecorrectedthroughmismatchrepair,aprocessthatinvolvesaspecialenzymetofixwronglyplacednucleotidesinacompletedsequence.Thankstothesetwoprocesses,thechanceofsomethinggoingwrongduringproteinsynthesisisgreatlyreduced,whilethechancesofproducingafullyfunctionalorganismareincreasedsubstantially.
Copyright © 2011, K12 Inc. All rights reserved. This material may not be reproduced in whole or in part, including illustrations, without the express prior written consent of K12 Inc.
Unit 7 Lesson 1All images © K12 Inc. unless otherwise noted. 2 Normal red blood cell and a sickle cell. Dr. Stanley Flegler/Visuals Unlimited, Inc.
Copyright © 2011, K12 Inc. All rights reserved. This material may not be reproduced in whole or in part, including illustrations, without the express prior written consent of K12 Inc.