Chapter 16 Transcrip1on How does informa on from DNA produce cellular proteins Learning objec ves By the end of this class you should be able to Understand what strand of DNA is the template strand Describe how the cell uses the DNA template to create mRNA Describe what is happening during inita on elonga on and termina on of transcrip on Describe how prokaryo c transcrip on di ers from eukaryo c transcrip on Reading Frame The cell reads the mRNA in pairs of three These three nucleo des are called a codon Each speci es one amino acid Three nucleo des equal one amino acid One Codon Deciphering the genetic code AUG CCU GGA Met Pro Gly GGG CCU CCU X G CCU X CGU Central Dogma What is Transcrip on The rst step in conver ng gene c informa on into proteins is to synthesize a messenger RNA Transcrip on is the synthesis of from a template The reac on is catalyzed by RNA polymerase RNA Polymerase Transcrip on RNA Polymerase Like DNA polymerase RNA polymerase performs a template directed synthesis in the direc on Unlike DNA polymerase RNA polymerase does not require a primer to begin The enzyme cannot start transcribing without a detachable protein subunit called sigma First Step Ini a on RNA Polymerase cannot ini ate transcrip on on its own Instead a detachable protein subunit called sigma must bind to the polymerase for transcrip on to begin Sigma binds to speci c regions of called promoters Promoter Sequences In prokaryotes a TATAAT box is always 10 nucleo des upstream of the of transcrip on In addi on 35 nucleo des upstream is the conserved sequence TTGACA All prokaryo c promoters have a 10 and 35 box but the sequences outside of the boxes vary Promoter Sequences Con nued Sigma binds to promoters and directs RNA polymerase where to begin transcrip on Most bacteria have several sigma proteins that bind to di erent promoters Thus transcrip on can be regulated for speci c The type of sigma protein in RNA polymerase determines the type of gene to be transcribed Promoters in Eukaryotes Many promoters in Eukaryotes include the TATA box around 30 bp upstream The loca on of the TATA box does not correlate with the site of transcrip on Does not contain a TTGACA box Unlike prokaryotes eukaryotes use a number of proteins called transcrip on factors to direct RNA polymerase to the site of transcrip on First Step Ini a on Di erent bind to di erent promoters and determine which genes will be transcribed Once sigma binds the helix opens up and the template strand is threaded through a channel that leads to the ac ve site of RNA pol Sigma is released and the reac ons is spontaneous because NTPs have so much energy Elonga on Phase In the interior of the enzyme a group of amino acids called a rudder help steer the strands through the channels Another group of amino acids called the zipper help to separate the newly synthesized RNA from the DNA Enzymes ac ve site catalyzes the addi on of nucleo des at 50 nucleo des per second Termina on Phase In most cases transcrip on stops when RNA polymerase reaches a DNA sequences that func ons as a transcrip on termina on signal This codes for RNA that forms a structure The forma on of this hairpin is thought to disrupt the interac on between the RNA transcript and RNA pol Transcrip on in process mRNA DNA Di erences between Prokaryotes and Eukaryotes Start a er Eid Break
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