BIOL 118 1st Edition Lecture 16 Outline of Last Lecture I Introduction How Cells Build Protein II Overview of Transcription III Characteristics of RNA Polymerase IV Initiation How Does Transcription Begin V What Role Does Sigma Play in Initiation VI Bacterial Promoters Outline of Current Lecture I In Bacteria Sigma Subunits Initiate Transcription II Most Bacteria Have Several Types of Sigma Proteins III What Occurs Inside the Holoenzyme IV Elongation Termination V Eukaryotic Promoters VI Transcription Initiation in Eukaryotes VII RNA Processing in Eukaryotes VIII The Discovery of Eukaryotic Genes in Pieces a Exons b Introns IX RNA Splicing X Adding Caps Tails to RNA Transcripts XI An Introduction to Translation XII Transcription Translation in Bacteria XIII Transcription Translation in Eukaryotes XIV How Does an mRNA Triplet Specify an Amino Acid XV What Happens to the Amino Acids Attached to tRNA XVI The Characteristics of Transfer RNA XVII What Do tRNAs Look Like XVIII How Many tRNAs Are There XIX The Structure Function of Ribosomes XX Ribosomes and the Mechanism of Translation XXI The Phases of Translation a Phase 1 Translation Initiation i Translation Initiation in Bacteria b Phase 2 Translation Elongation i Is the Ribosome an Enzyme or a Ribozyme ii Moving Down the mRNA These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute c Phase 3 Translation Termination i Release Factors ii Post translational Modifications Current Lecture In Bacteria Sigma Subunits Initiate Transcription Transcription begins when o Sigma as part of the holoenzyme complex binds to the 35 10 boxes o Sigma not RNA polymerase makes the initial contact with DNA that starts transcription supports hypothesis that sigma is a regulatory protein Most Bacteria Have Several Types of Sigma Proteins Each type allows RNA polymerase to bind to a different type of promoter therefore a different kind of gene What Occurs Inside the Holoenzyme Sigma opens the DNA double helix o The template strand is threaded through the RNA An incoming ribonucleoside triphosphate pairs with a complementary base on the DNA template strand o RNA polymerization begins forming an mRNA transcript o Once initiation phase of transcription is complete sigma dissociates from the core enzyme Elongation Termination During the elongation phase of transcription o RNA polymerase moves along the DNA template o Synthesizes RNA in the 5 3 direction Transcription ends with a termination phase o RNA polymerase transcribes a transcription termination signal coded for in the DNA template into RNA In bacteria the transcription termination signal o Codes for RNA that forms a hairpin structure o Causes the RNA polymerase to separate from the RNA transcript ending transcription Eukaryotic Promoters Eukaryotes have a much more diverse complex series of promoters than do prokaryotes Many of the eukaryotic promoters include o A unique sequence called the TATA box o Centered about 30 base pairs upstream of the 1 Transcription Initiation in Eukaryotes As with bacteria the RNA polymerase does not bind directly to the promoter In eukaryotes a group of proteins called basal transcription factors o Bind to DNA promoter o Initiate transcription Basal transcription factors perform a similar function to bacterial sigma proteins o Basal transcription factors include many proteins o Not part of holoenzyme RNA Processing in Eukaryotes In bacteria o The information in DNA is converted to an mRNA transcript directly In eukaryotes o The product of transcription is an immature primary transcript or pre mRNA Before eukaryotic primary transcripts can be translated o They have to be processed in a complex series of steps Eukaryotic genes are much larger than their corresponding mature mRNA The Discovery of Eukaryotic Genes in Pieces The protein coding regions of eukaryotic genes are interrupted by non coding regions o To make functional mRNA the noncoding regions must be removed Exons The coding regions of eukaryotic genes Part of the final mRNA product Introns The intervening noncoding sequences Not in final mRNA RNA Splicing The transcription of eukaryotic genes by RNA polymerase o Generates a primary RNA transcript that contains exons introns Introns are removed by splicing Small nuclear ribonucleoproteins snRNPs o Form a complex called a spliceosome Spliceosome catalyzes the splicing reaction Adding Caps Tails to RNA Transcripts Primary RNA transcripts are also processed by the addition of a 5 cap a poly A tail RNA processing is complete upon o Addition of the cap o Addition of the poly A tail o Completion of splicing The product is a mature mRNA The 5 cap serves as a recognition signal for the translation machinery The poly A tail extends the life of an mRNA by protecting it from degradation An Introduction to Translation In translation o The sequence of bases in the mRNA is converted to an amino acid sequence in a protein Ribosomes o Catalyze translation of the mRNA sequence into protein Transcription Translation in Bacteria In bacteria transcription translation occur simultaneously Bacterial ribosomes o Begin translating an mRNA before RNA polymerase has finished transcribing o Multiple ribosomes attached to an mRNA form a polyribosome Transcription Translation in Eukaryotes In eukaryotes transcription translation are separated mRNAs are synthesized processed o In the nucleus o Then are transported to the cytoplasm for translation by ribosomes How Does an mRNA Triplet Specify an Amino Acid There were two hypotheses regarding the specification of amino acid sequence by a sequence of nucleotide bases o mRNA codons amino acids interact directly o Francis Crick proposed That an adapter molecule holds amino acids in place While interacting directly specifically with a codon in mRNA The adapter molecule was later found to be a small RNA called transfer RNA tRNA What Happens to the Amino Acids Attached to tRNA Experiments with radioactive amino acids revealed o They are lost from tRNAs o Incorporated into polypeptides synthesized in ribosomes Inspired the use of transfer in tRNA s name o Because amino acids are transferred from the RNA to the growing end of a new polypeptide The experiment also confirmed o Aminoacyl tRNAs act as the interpreter in the translation process question asked in class o tRNAs are Crick s adapter molecules Characteristics of tRNA ATP is required to attach tRNA
View Full Document
Unlocking...