MOLECULAR BIOLOGY BIO 344
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This 9 page Class Notes was uploaded by Ezequiel Orn on Sunday September 6, 2015. The Class Notes belongs to BIO 344 at University of Texas at Austin taught by David Herrin in Fall. Since its upload, it has received 28 views. For similar materials see /class/181726/bio-344-university-of-texas-at-austin in Biology at University of Texas at Austin.
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Date Created: 09/06/15
Termination The accept 5th mm W Dr nbusume s NEW Erma stup udun mmmsw WWW i I 5 mRNA I MGVLTWMSKY I RF GDP on MGVLTWMSKYF New amnesyzedpowepwe ammu tevmmus s my me wen camuw tevmmus s my me Ham 51cm munquot nu Insert Overview of Translation here 2 pages Between sections 5 and 6 Translation continued 3 TAC CCC CAG GAC TGC ACC TAC TCA TTT ATG AAA ATG 5 5 ATG GGG GTC CTG ACG TGG ATG AGT AAA TAC TTT TAG 3 Transcription of this DNA produces the following mRNA 5 AUG GGG GUC CUG ACG UGG AUG AGU AAA UAC U39U39U UAG 3 Translation of this mRNA produces the following protein NH3 M G V L T W M S K Y F COOH The ribosomes begin translating at the start codon move down the mRNA in the 539 t0 339 direction and stop translating when they encounter a stop codon The protein is being synthesized in the amino t0 carboxy direction lFor simplicity39s sake we will discuss in detail only prokaryotic translation Before we proceed let39s define two regions of the ribosome Psite stands for Peptidylsite The Psite is the area of the ribosome that holds a tRNA that is attached to the growing polypeptide chain Asite stands for Acceptorsite The Asite is an area of the ribosome that accepts the next charged tRNA A charged tRNA is attached to the aInino acid that it encodes P site A site The complete P and A sites are present only in the 70 ribosome By itself the 305 subunit has what we call a partial Psite and a partial Asite 539 page 61 Initiation Initiation is not a function of the complete ribosome but is undertaken by the separate subunits which assemble on the mRNA Examine the figure called Overview 0 f Initiation and Elongation during the following discussion Onejob of the gos bosomal start codon 1n the part1a1 P s1te subunit is to recognize the mRNA start codon and to attach to the mRNA so that the start codon is in the partial Psite 539 w 308 ribosomal subunit How does the 303 subunit recognize the start codon ln prokaryotes this is fairly well understood Prokaryotic mRNAs contain a ribosome binding site that is located 539 to in front of the start codon This sequence is 539 AGGAGG 339 It is called a ShineDalgarno sequence and it is found about 10 bases 539 to the start codon The 165 rRNA in turn contains the sequence 539 CCUCCU 339 These two sequences are antiparallel and complementary to one another Example of a mRNA ShineDalgarno sequence and its relationship to a start codon 539 NNNNNAGGAGGNNNNNNNNAUGNNNNNNN 339 N any nucleotide The 165 rRNA binds the ShineDalgarno sequence which places the start codon in the partial Psite of the 305 subunit page 62 16S rRNA Hbonding with a ShineDalgarno sequence The bigger molecule is the 165 3 y rRNA and the smaller molecule is 5 a mRNA AGGAGGNNNNNNNNAUGNNB But wait In order to translate the ribosome can39t be bound to the mRNA in this fashion It is believed that once the 705 ribosome has been assembled on the mRNA that intramoleculm base pairing between different portions of the 165 rRNA displaces the mRNA39s ShineDalgarno sequence This is shown below Complementary lntramolecular base 10 ShineDalgarno pairing between two sequence regions of the 165 rRNA masks the Shine Dalgarno complementary sequence AACCGUAGG 30 preinitiation complex Once the 305 subunit has bound the mRNA it can accept a tRNA Only one type of tRNA can enter the half Psite It is called the initiator methionyl tRNA and is abbreviated as tRNAinet This particular charged tRNA is the only one that can enter the partial Psite It cannot enter a complete Psite It is reserved for use in the pre initiation complex and is not used during elongation page 63 Methionine The 30 s preinitiation part1 a1 P Slte complex consists of a tRNAplet Charged with initiation factors methionine GTP initiation factors the 39 mRNA and the 305 5 ribosomal subunit 308 pre initiation complex To complete the initiation process the 505 subunit binds the initiation factors are released and energy in consulned in the form of one highenergy phosphate bond derived from GTP Completed 705 initiation complex P Site which has a complete P and A site uiuoiqieui l 539 1IC GGG 339 708 initiation complex What purpose do the initiation factors serve The initiation factors help to make translation proceed in one direction The assembly of the 705 initiation complex is very much like an enzymatic reaction It even consulnes energy in the form of GTP Like all reactions it is reversible The reversal of initiation however requires the interaction of all of the molecules described above Once the 505 subunit binds energy is consulned and the initiation factors dissociate from the complex The loss of the initiation factors and the consulnption of energy make it very unlikely that all of the players will be in the correct position to get the reaction to ow backwards Therefore once begun initiation tends to ow only in the forward direction met Furthermore it is the initiation factors that recognize the tRNAi and enable it to enter the partial Psite page 64 Elongation NN N stands for a codon in the mRNA AA 1 stands for aInino acid 1 AA 2 stands for aInino acid P Slte 2 etc A site A charged tRNA enters the A site 39 I Hbonding with the codon 539 i 1 NNNi 339 Peptide Bond Formation AA 1 Catalysis of a peptide bond between AA1 and AA2 Peptide bond formation is catalyzed between AA 1 and AA 2 P site Transl ocation AA 1 The ribosome translocates t0 the next codon the uncharged tRNA leaves and a new charged tRNA enters the A site A site This step requires EFG and c0nsu1nes energy in the form of NNNi 339 GTP Peptide Bond Formation Catalysis of peptide bond formation between AA 2 and AA3 A Site page 65 Transl ocation AA 1 Uncharged tRNA leaves the ribosome translocates to the next codon and a new charged tRNA enters the A site P Slte A site This step requires EFG and consulnes energy in the form of GTP SVENNNiNNN This process ofelongation occurs cyclically until a stop codon is reached Termination The last step in translation Termination occurs when the a stop codon enter the Asite of a ribosome At this thne the ribosome binds a releasing factor and GTP The GTP is hydrolyzed energy is consulned and the last tRNA the mRNA the two ribosomal subunits and the protein dissociate Examine page 2 of the Overview of Initiation and Elongation Do all proteins really begin with methionine Yes and No Translation of all protein does begin with methionine But for some proteins an enzyme comes along and clips off the 15 to 30 amino acids from the N terminus Therefore mature proteins do not all begin with methionine Polycistronic mRNA Translation differs in two fundaInental ways in prokaryotes and eukaryotes We have already discussed the fact that only prokaryotes have concomitant transcription and translation The second way is that prokaryotes produce polycistronic mRNAs while eukaryotes produce only monocistronic mRNAs The word cistron is synonymous with gene page 66 Translation of a polycistronic mRNA Initiation Elongation Termination Initiation Elongation Termination O 23 gt 0 0 p Q 513344 t U4 0 S mRNA region made from mRNA region made from the rst cistron gene the second cistron gene As a rule each cistron is treated as a 39separate39 mRNA by the ribosomes That is the processes of initiation elongation and termination are completely independent Each cistron has its own ShineDalgarno sequence its own start codon and its own stop codon An occasional exception to this rule exists Ask me about it if you like How are polycistronic mRNAs produced Polycistronic mRNAs gene 1 are transcribed as a single transcription unit that is a single mRNA that contains the information from mRNAs more than one gene eue Don39t forget that translation of these mRNAs is also occurring I Gene 1 Cene 2 i 4 DNA sunultaneously With mmlotion Transcription transcription starts here stops here What types of genes are expressed in the same polycistronic mRNA Polycistronic mRNAs are usually produced from genes that are involved in the same biochemical pathway For instance the five enzymes needed by E coli to synthesize tryptophan are produced from a single polycistronic mRNA All of these genes are organized 39head to tail39 in the bacterial genome and are transcribed as one long mRNA This is a very efficient way to control gene expression Can you figure out why page 67
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