CHAPTER 17 Transcription RNA Processing and Translation 3 9 2014 Transcription RNA polymerases enzymes responsible for synthesizing mRNA performs template directed synthesis in 5 3 direction unlike DNA polymerase does not require primer to begin transcription bacteria contain 1 RNA polymerase eukaryotes have 3 types of RNA polymerase I II and III each polymerase transcribes only certain types of RNA RNA pol II only polymerase that transcribes protein encoding genes template strand strand that is read by the enzyme non template coding strand strand whose sequence matches the RNA that is transcribed from template strand codes for a polypeptide does not match RNA exactly bc RNA has uracil and not thymine sigma protein subunit regulatory protein subunit that must bind to RNA polymerase before transcription can begin controls which sigma proteins used are one of ways that bacterial cells regulate gene expression responsible for guiding RNA polymerase to specific locations promoter sequences on DNA strand where transcription begins o most bacteria have many types of sigma proteins that each enables RNA polymerase to bind to a different type of promoter different gene holoenzyme enzyme made of a core enzyme other required proteins formed from sigma and RNA polymerase together o made of core enzyme prokaryotic RNA polymerase that acts as active site for catalysis synthesizes RNA o binds to promoters specific binding sites of DNA that promote the start of transcription bacterial promoters are 40 50 base pairs long 10 box located 10 bases upstream from transcription start site series of bases sequence is TATAAT 35 box located 35 bases upstream from 1 site series of bases sequence is TTGACA o o eukaryotes have a more diverse complex series of promoters than prokaryotes TATA box centered 30 base pairs upstream of the 1 site basal transcription factors in eukaryotes group of proteins that RNA polymerases use to recognize promoters and initiate transcription assemble at the promoter and RNA polymerase follows transcription begins when sigma as part of holoenzyme complex binds to 35 and 10 boxes sigma not RNA polymerase makes initial contact with DNA of promoter after recognizing holoenzyme made by RNA polymerase and sigma determines where direction that RNA polymerase will start synthesizing RNA begins transcription downstream DNA located in direction RNA polymerase moves during transcription upstream DNA located in the opposite direction RNA polymerase moves during transcription initiation first phase of transcription in holoenzyme sigma opens DNA double helix template strand goes through RNA polymerase active site RNA polymerization begins when incoming NTP pairs w complementary base on template strand of DNA initiation phase completes as RNA polymerase extends mRNA from 1 site phase ends when sigma dissociates from core enzyme o o o o o o o o o o o o o o o 3 9 2014 elongation phase of transcription that begins when RNA polymerase begins moving across DNA template synthesizing RNA RNA synthesized in 5 3 direction all prominent channels grooves in enzyme filled double stranded DNA goes in and out of one groove ribonucleoside triphosphates enter another RNA strand exists in rear RNA elongates in 5 3 direction termination ends transcription RNA polymerase transcribes a transcription termination signal in the DNA template into RNA in bacteria transcription termination signal codes for RNA that forms a hairpin structure separating RNA polymerase from RNA transcript to end transcription in eukaryotes polyadenylation poly A signal sequence that terminates protein coding genes RNA cleaved downstream of signal transcription ends variable distances from here after termination transcription of eukaryotic genes by RNA polymerase creates primary RNA transcript that contains exons and introns exons coding regions of eukaryotic genes part of final mRNA product introns intervening noncoding sequences NOT part of final mRNA product removed by splicing splicing is catalyzed by a spliceosome which is made up of small nuclear ribonucleoproteins snRNPs 5 cap and poly A tail added to primary RNA transcripts o 5 cap is a recognition signal for translation machinery poly A tail extends life of mNA by protecting it from degradation addition of cap poly A tail completion of splicing completes RNA processing produces mature mRNA differences in RNA processing btwn bacteria eukaryotes in bacteria in eukaryotes information coded in DNA is converted to mRNA directly product of transcription is immature primary transcript pre mRNA immature primary transcripts must be processed in a complex series of steps before primary transcripts can be translated eukaryotic genes much larger than corresponding mature mRNA o o o o o o o o o o translation sequence of bases in mRNA converted to amino acid sequence in protein o in bacteria o in eukaryotes transcription and translation may occur simultaneously bacterial ribosomes begin translating mRNA before RNA polymerase has finished transcribing it multiple ribosomes attached to an mRNA form a polyribosome transcription and translation processes are separated mRNAs synthesized and process in the nucleus then exported to cytoplasm for translation by ribosomes 1 polypeptides grow from ribosomes translating mRNA Translation ribosomes catalyze translation of mRNA sequence into proteins o contain protein and ribosomal RNA rRNA composes the active site of the ribosome catalyzes peptide bond formation 3 9 2014 o o o o o o o o o o o o o o ribosome is a ribozyme RNA with catalytic activity can be split into two subunits small subunit holds mRNA in place during translation large subunit is where peptide bonds form during translation 3 distinct tRNAS line up within ribosome A site site of ribosome that accepts aminoacyl tRNA P site site of ribosome where peptide bond forms that adds amino acid to growing polypeptide chain E site site of ribosome where tRNAs no longer bound to amino acid exit ribosome synthesizes protein in a 3 step sequence aminoacyltRNA carrying correct anticodon for mRNA codon enters A site 1 2 peptide bond forms btwn amino acid on aminoacyl tRNA in A site growing polypeptide on tRNA in P site 3 ribosome moves ahead three bases all 3 tRNAs move down one position the tRNA in E site exits hypotheses regarding specification of amino acid sequence by a sequence of nucleotide bases o mRNA codons and amino acids interact directly o an adapter molecule transfer RNA or tRNA holds amino
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