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Molecular Biology Exam 3 Review - Molecular Biology Exam 3...

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Molecular Biology Exam 3 Review19.1 - IntroductionTranscription is the synthesis of RNA 5’ to 3’ on a template that is 3’ to 5’.Coding strandis whenthe DNA strand that has the same sequence as the mRNA and is related by the genetic code tothe protein sequence that it represents.RNA polymeraseis an enzyme that synthesizes RNAusing a DNA template. RNA polymerase is formally described as a DNA-dependent RNApolymerase. Thepromoterregion of DNA is where RNA polymerase binds to initiatetranscription. Theterminatoris a sequence of DNA that causes RNA polymerase to terminatetranscription. Thetranscription unitis the sequence between sites of initiation and terminationby RNA polymerase and it may include more than one gene. Thestartpointis the position onDNA corresponding to the first base incorporated into RNA. Sequences in the opposite directionfrom the expression areupstream.Sequences before the startpoint are upstream of it.Sequences proceeding farther in the direction of expression with the transcription unit aredownstream.Sequences after the startpoint are downstream.Primary transcriptis the originalunmodified RNA product corresponding to a transcription unit. RNA modification follows theprimary transcript. Gene expression may be regulated at transcription. The regulatory evenduring initiation of transcription is controlled by regulatory proteins.19.2 – Transcription Occurs by Base Pairing in a Bubble of Unpaired DNARNA polymerase separated the two strands of DNA in a transient bubble and uses one strandthat runs 3’ to 5’ as a template to direct synthesis of a complementary sequence of RNA running5’ to 3’. The length of the bubble is approximately 12 to 14 bp. The length of RNA-DNA hybridwithin it is ~8 to 9 bp.19.3 – The Transcription Reaction Has Three StagesInitiationis the recognition of the promoter with melting of DNA and then RNA synthesis begins.Elongationis RNA synthesis within the transcription bubble.Terminationis when transcript isreleased along with RNA polymerase then the transcription bubble closes. Initiation happenswhen RNA polymerase binds to the promoter on the DNA from aclosed complex. RNApolymerase initiates transcription after opening the DNA duplex to form a transcription bubblewhich is theopen complex. RNA polymerase remains bound to the promoter for synthesis of thefirst ~ 9 nucleotide bonds. Initiation is complete once RNA polymerase leaves the promoter.Duringelongationthe transcription bubble moves along DNA and the RNA chain is extended inthe 5’ to 3’ direction by adding nucleotides to the 3’ end of the growing chain. RNA polymerasemoves in the 3’ to 5’ direction along the template. The nascent RNA is paired with the transientlyunwound DNA. When transcription stops bytermination, the DNA duplex reforms and RNApolymerase dissociates at aterminator site. The transcription bubble collapses when the RNA-DNA hybrid is disrupted. The DNA duplex reforms, RNA polymerase dissociates.

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