TranscriptionLesson4

Transcription(DNAàRNA)

• DNAcodewrittenasanRNAcode

• Occursinthenucleus

• Threeparts:1. Initiation

2. Elongation

3. Termination

1.Initiation

• RNApolymerasebindstoDNAandunwindsitnearthebeginningofageneà

bindingoccursatapromoter

• PromoterhasaTATAboxthatisrecognizedbyRNApolymerase(specialized

sequenceofDNAhighinAandT)

2.Elongation

• Noprimerisneeded

• mRNAismadeina5’à3’ directionusing3’à5’DNA(templatestrand)

• OppositestrandtoDNA(notused)à codingstrand(“U”replaces“T”à pairswith“A”)

3.Termination

• RNApolymerase

recognizesa

termination

sequenceand

releasesthe

RNAtranscript

Post-TranscriptionalModifications

• RNAstrandiscalledprecursor

messengermRNA(pre-mRNA)

• Beforepre-mRNAcanleavethe

nucleusà additional

modifications

• Vulnerabletoenzymesand

conditionsoutsidethenucleus

Poly(A)Tail

• Poly-ApolymeraseaddsaPoly(A)Tail(50-250adeninenucleotides)tothe3’end

• Protectsitfromenzymesinthecytosol(degradation)

5’Cap

• 5’Cap(7Guaninenucleotides)addedto5’end byacomplexofcappingenzymes(CE)

• InitialattachmentsiteformRNAstrandtotheribosome

• AllowsmRNAtoattachtoribosomestobegintranslation

• Protectionfromenzymes

DNAà RNAà pre-mRNAàmRNA

• “Capping”and

“tailing”

•mRNAactsas

intermediarybetween

DNAandribosomes

IntronsandExons

• Genescontaincodingregions

(exons)andnon-coding

regions(introns)

• Introns(non-codingregions)

areremoved• Donotcodeforproteinsà willnot

allowproteintofoldproperly

mRNAsplicing

• Occursinaspliceosomeà complex

formedbypre-mRNAandseveralsmall

nuclearribonucleoproteins (snRNPs)

• snRNPbindstointronandpre-mRNA

• Recognizeandformcomplementary

basepairswithmRNAsequence

• Causeintrontoloopoutandbringexonstogether

• Activespliceosomeformed,releasing

intronandjoiningexons

AlternativeSplicing

• DifferentmRNAsfrompre-mRNA(exonsandintrons)

• Alternativesplicingàmorethanonetypeofpolypeptidesencodedbyasinglegene

• Humansà 20,000genes=100,000proteins

Homework

•Role-playduetomorrow

• Reminder:Presentation,loose-script,individualanalysis

•VocabularyQuiz#1tomorrow(Lesson1-3)

•PracticeQuestions• (Nelson2003):pp.249#1,2,3,4,5a,7*,8