GEN MICROBIOLOGY BIOL 2051
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Date Created: 10/13/15
Chapter 8 Transcription Translation and Bioinformatics RNA Synthesis Transcription making an RNA copy of part of a DNA strand one short segment of DNA is copied into a section or RNA Differences between DNA amp RNA DNA made of deoyribonucleotides 39 Double stranded 39 Thymine in DNA not found in RNA 39 Sugardeoyribose RNA made of ribonucelotides leave of deoxy 39 Single stranded 39 Nitrogen base uracil so where the isT in DNA there is a U in RNA 39 Sugar ribose Three types of RNA 1 mRNA Contains information used to make proteins Eventually translated into proteins 2 tRNA a Carries amino acids to build the polypeptide during translation 3 ara a Component of ribosomes site of protein synthesis 39 Template strand DNA strand that is being copied into complementary mRNA 39 RNA polymerase enzyme that transcribes DNA into RNA 39 Bacterial RNA polymerase 4 subunits a 2 compies a beta a beta prime and an Process of transcription 3 phases 1 Initiation a Start tanscibition 2 Elongation a Add on to it 3 Termination a End the process 39 Transcription of RNA from DNA involves the enzyme RNA polymerase which adds ribonucleotides onto 3 end of growing RNA chain 39 Does not need a primer can start a RNA chain all by itself 39 Unlike DNA polymerase RNA polymerase needs no primer RNA polymerase recognizes a specific start sequence on the DNA called the promoter and starts transcribing DNA into RNA at that site Transcription lnitiation Sigma factor binds RNA polymerase Forms RNA polymerase holoenzyme RNA polymerase binds promoter Sequence on DNA 10 and 35 bases upstream of start site Polymerase unwinds the DNA at promoter Open complex Once complex is opened the Sigma factor released Promoter Terminator DNA W4 v HF Initiation Elongation Termination Promoters 39 Specific DNA sequences indicating start site of transcription Two important regions in Bacteria 35 sequence 10 sequence Pribnow box means upstream of start site Prokaryotes have a single RNA polymerase with a sigma subunit it only 39ob it to help find the promoter The sigma factor binds to the promoter to initiate transcription Multiple sigma factors 7 kn lo Guides RNA polymerase to most genes kn IS Active when cell is stressed by heat Heat shock response Transcription Elongation Core RNA polymerase moves down the template DNA strand from 3 to 5 39 RNA strand is made 5 to 3 ribonucletides are added to the 3 end complementary to the DNA RNA polymerase RNA Rho r dependent termination Rho r factor binds to the mRna Slides along mRNA to polymerase Breaks polymerase amp mRNA off of DNA Rhoindependent termination Series of U residues downstream of pause site GCrich sequence forms stem loop DNARNA UA base pairs are least stable Even less stable if polymerase is stalled mRNA breaks off of DNA polymerase released 39 Eukaryotes do not have sigma factors 39 Transcription factors help eukaryotic RNA polymerases to bind to promoters o Eukaryotes have 3 RNA polymerases llllll 39 In prokaryotes genes that code for the enzymes in a specific pathway are often lined up in groups called operons 39 Ex There are 3 enzymes involved in lactose utilization so there are 3 genes These are lined up in an operon called the lac open 2 lacZ lacY lacA 3 DNA RNA pol may transcribe the entire operon and produce one long mRNA molecule called polycistronic mRNA lacZ lacY lacA l transcription lacZ lacY lacA mRNA 39 Once the mRNA is made for the lac operon it is used to make 3 proteins 39 Translation process of making a protein using mRNA as template 39 There is 1 protein for each gene in the lac operon lacZ lacY lacA DNA l transcription lacZ lacY lacA mRNA l translation WW MNWVWVW WNWWW proteins Bgalactosidase lac permease transacetylase 39 Multiple genes can be cotranscribed forming a polysictronic mRNA 0 ln eukaryotes a single gene is transcribed at a time Protein Synthesis Translation Process of making a polypeptide chain from mRNA Codons 39 There are 20 amino acids found in proteins 39 How can 4 RNA bases AUGC code for 20 amino acids 39 A group of 3 RNA bases code for each amino acid this is called a codon a group of 3 mRNA bases and each group of there codes for one amino acid 5 39AUGUUUACCAGA TTTT Met PheThrArg The genetic code is degenerate 39 There are 64 possible codons but only 20 amino acids 39 Some amino acids that have several codons 39 Ex There are 3 codons for isoleucine AUU AUC AUA don t have to memorize 39 A single amino acid may be encoded by several different but related codons The quotwobblequot concept Base pairing between tRNA and mRNA is more flexible for the third base than the first two This allows some tRNA to recognize more than one codon For example 7 Arg codons are EU QC EA 6 Transfer RNA tRNA 0 tRNA small RNA molecules that acts as adapters in the process of translation 0 Each tRNA has a binding site for a codon at one end and a binding site for an amino acid 39 There are tRNA molecules for 61 of the 64 codons these are called sense codons The other 3 codons are called nonsense codons have no tRNA that will code to them they end the process of translation 3UAC5 One or more transfer RNAs exist for each amino acid found in a protein 39 Enzymes called aminoacyltRNA synthetases attach an amino acid to a tRNA The anticodon 39 3 nucleotides at the bottom of the tRNA that are complementary to a codon on the mRNA n drawing above the anticodon is UAC The anticodon is written from 3 to 5 What codon is this anticodon complementary to AUG usually the start codon 39 AUG is the codon for which amino acid 39 MET A codon will baseipair with a sequence of 3 bases on a tRNA called the anticodon Translation of mRNA occurs from a start codon AUG to a stop codon 3 different stop codons I From start codon to stop codon is called an open reading frame ORF 39 Codon table shows all possible codons ampthe amino acid that each on codes for look at one and make sure you know howto use o e 39 Each ofthe 61 sense codons that all specify amino acids 39 There are tRNA molecules forthe 61 sense codons 0 There are no tRNA39s forthe 3 nonsense codons UAG UGA UAA The ribosome 39 Ribosomer large complex of proteins and RNA that links amino acids togetherto form proteinslocation of protein synthesis 39 There are 2 ribosomal subunits 305 and 505 in prokaryotes 39 5 stands for Svedberg unit a measure of mass and shape 39 The 2 subunits togetherform a 70s ribosome merege together in such a way that you end up with a 70s ribosome mRNA There are three sites on the ribosome acceptor A site where the charged tRNA t RNA which has a charge attached to it first binds peptide P site where the growing polypeptide chain is held egtltt E site where the tRNA exit from Translation initiation so intiation in the first step ShinerDalgarno seduenceribosome binding site mRNA seduence involved in binding ofthe mRNA to the ribosomes 39 The mRNA fits into a groove in the 305 subunit ofthe ribosome 39 The amp has 3 holes called the P peptide site the A acceptor site and the E egtltit site 5 mRNA 5 mRNA 39 Initiation factors bind ribosome to mRNA so there is one codon in the P site AUG and one codon in the A site 39 Aminoacylated tRNA s with correct anticodons fit into these 2 sites amp form hydrogen bond with the mRNA codons 39 Ribosomes forms a peptide bond between the 2 amino acids 39 After peptide bond forms there is a chain of 2 amino acids hooked to the tRNA in the A site 5 mRNA 5 mRNA XXXYYY xxxvvy P A P A Translation elongation 39 Translocation ribosome moves up the mRNa one codon so the tRNA with the chain of amino acids id in the P site so now the A site is open for another tRNA can come in 39 EFTs EFTu EFG bring GTP energy that are necessary for the joining of the amino acids 39 For polymerization amp movement of ribosome along mRNA 39 A new aminoacylated tRNA binds to the vacant A site 39 chain of amino acids is linked to the amino acid in the A site 39 Now there is a chain of 3 amino acids 39 Ribosome moves u the mRNa 1 more codon amp repeats the cycle until the complete chain is made 3 5 mRNA 3 During each step of amino acid addition the ribosome advances three nucleotides one codon along the mRNa and the tRNa moves from the acceptor to the peptide site Translation termination Prote l39l Occurs when a nonsense codon which does not code for an amino acid is in the A site Releasing factors undock ribosome from mRNA protein amp ribosome are released from the mRNA httpwwwwwnortoncomcollegebiologymbioanimationsmainaspchnoch08a01 htt www outubecom watchvNJxob kPEAo Eukaryotic translation occurs in the cytoplasm spatially separated from transcription transcription and translation cannot occur in the same place In prokaryotes several ribosomes can translate a single mRNA molecule simultaneously forming a complex called a polysome ln prokaryotes transcription amp translation can occur at the same time Coupled Transcritoin and translation can noever occur at the same time in eukarytoes Polysome more than lribosome translating an mRNA at once To function correctly proteins must be properly proteins must be folded properly Folding may occur spontaneously or may involve proteins called molecular chaperones which are proteins that help fold Many proteins must be transported into or through cell membranes Such proteins are synthesized with a signal seguence that is recognized bu the cellular export apparatus and is removed wither during or after export Degradation All cells constantly rebuild themselves Cytoskeleton is rebuilt Cell wall grows and is remodeled Proteins are degraded when not needed Amino acids reused where a ling chain in a protein peptide bonds broken and reused Proteins survive for minutes to days Seguence shape function determine halflife Proteases cut proteins Cut at specific amino acid sequences Proteasomes degrade proteins Eukaryotes add signal to proteins Ubiquitin tag causes degradation Added to the end of the protein and that s the signal that that proten can be degraded
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