Chapter 17 Transcription RNA Processing and Translation NTP matches base on DNA template RNA polymerase cleaves off two phosphates to catalyze the formation of a phosphodiester bond Initiation begins when a detachable protein subunit sigma binds to the polymerase Bacterial polymerase and sigma form a holoenzyme consisting of a core enzyme RNA polymerase and other required proteins The holoenzyme only binds to specific sections of DNA called promoters which start transcription Bacterial Promoters 40 50 base pairs long and have a particular series of bases in common that are similar or identical to TATAAT which is known as the 10 box 17 1 An Overview of Transcription RNA polymerases are responsible for synthesizing RNA RNA monomers are ribonucleoside triphosphate NTP Have hydroxyl on 2 carbon unlike DNA Strand read by RNA polymerase is the template strand The other strand is the coding strand RNA polymerase does not require a primer to begin transcription Initiation How does Transcription Begin in Bacteria 10 box is 10 bases upstream from from the transcription start site DNA located in the direction RNA polymerase moves is downstream DNA located in the opposite direction is upstream The place where transcription begins is called the 1 site TTGACA also occurs in promoters 35 bases upstream 35 box Events inside the Holoenzyme Sigma binds to 10 and 35 boxes and makes the initial contact with DNA not RNA polymerase Template strand is threaded through a channel that leads to the active site inside RNA polymerase RNA monomeres enter a channel in the enzyme and diffuse tot he active site Reaction catalyzed by RNA polymerase is exergonic and spontaneous because the three Phosphate groups on NTP contain a lot of energy Elongation and Termination Elongation occurs when all the prominent channels and grooves of the enzyme are filled Double stranded DNA goes into and out of one groove ribonucleoside triphosphate enters another and the growing RNA strand exits in the rear Termination ends transcription In bacteria RNA polymerase transcribes a DNA sequence that functions as a transcription terminal signal As soon as the strand of RNA is synthesized it folds to create a short double helix held together by complementary base pairing hairpin This disrupts interaction between the strand and RNA polymerasse Transcription in Eukaryotes Eukaryotes have 3 polymerases RNA polymerase I II and III Each polymerase only transcribes certain types of RNA RNA pol II is the only one that transcribes protein coding genes Promoters re more diverse and include a sequence called the TATA box about 30 bases upstream from the transcription site Instead of using sigma eukaryotic RNA polymerases use basal transcription factors which assemble at the promoter Termination involves the use of the poly A signal The signal is transcribed the RNA is cut by an enzyme downstream of poly A as the polymerase continues to transcribe the DNA template RNA polymerase eventually falls off transcription ends a variable distance from the poly A signal 17 2 RNA Processing in Eukaryotes In bacteria when transcription ends the mRNA is mature and ready to be translated into a protein Eukaryotic genes produce a primary transcript when transcribed and have to undergo a multi step processing before it is functional For protein encoding genes the primary transcript is called a pre mRNA The Startling Discovery of Split Eukaryotic Genes Regions in eukaryotic genes that code for proteins are intermittently interrupted by stretches of hundreds or thousands of intervening bases cid 127 Genes in pieces hypothesis Regions of genes that are part of the final mRNA are called exons because they are expressed Regions not part of the final mRNA are introns because they intervene RNA Splicing As transcription proceeds introns are removed from the RNA strand by splicing Splicing occurs in the nucleus while transcription is still under way and results in an RNA that contains uninterrupted genetic message Splicing is catalyzed by RNAs caled small nuclear RNAs snRNAs These proteins and small RNA macromolecular machines are known as small nuclear ribonucleoproteins snRNPs snRNPs bin to 5 exon intron boundary other snRNPs arrive to form a multipart complex calles a spliceosome the intron forms a loop plus a single stranded stem and then the stem is cut and a phosphodiester linkage links the exons on the other side cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 Strong correlation exists between the number of ribosomes in a given type of cell and the rate at which that cell synthesizes proteins indicating ribosomes as Adding Caps and Tails to Transcripts In pre mRNAs splicing is accompanied by other processing steps 5 cap is added which is a modified guanine with three phosphate groups Poly A tail is added and is about 100 250 adenine nucleotides not present in the DNA strand pre mRNA is complete after these steps and it is a mature mRNA 5 and 3 untranslated regions help stabilize mature mRNA and regulate its translation Protect mRNA from degradation by ribonucleases and enhance efficiency of translation RNA processing is the general term for an of the modifications needed to turn a primary transcript into a mature mRNA Last longer and produce more proteins An Introduction to Translation Ribosomes are the Site of Protein Synthesis the site of protein synthesis Translation in Bacteria and Eukaryotes In bacteria ribosomes attach to mRNAs and begin synthesizing proteins before transcription is complete cid 127 multiple ribosomes attach to each forming a polyribosome This lets many copies of a protein be produced from a single mRNA Transcription and translation can occur concurrently in bacteria because they do not have a nuclear envelope 17 4 The Structure and Function of Transfer RNA tRNA transfer amino acids to a growing polypeptide they act as interpreters during translation cid 127 One end contains an anticodon and the other end contains a codon for an amino acid What do tRNAs Look Like Portions form a stem and loop structure Hairpin tRNAs are folded into an upside down L shape How Are Amino Acids Attached to tRNAs ATP is required to attach an amino acid
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