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This 17 page Class Notes was uploaded by Mrs. Lester Zboncak on Friday October 23, 2015. The Class Notes belongs to 002 021 at University of Iowa taught by Staff in Fall. Since its upload, it has received 28 views. For similar materials see /class/228032/002-021-university-of-iowa in Biology at University of Iowa.
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Date Created: 10/23/15
LECTURE 10 TRANSCRIPTION AND TRANSLATION A General Considerations Transcription Formation ofa message using DNA as the source or code Translation Synthesis of a protein by translating the message B Protein Structure String of20 amino acids primary structure 20 amino acids are used There is no restriction on the sequence of usage of particular amino acids The actual sequence of the protein is determined by DNA nucleotide sequence of a gene C Transcription Messenger RNA mRNA is synthesized DNA is used as a template to produce mRNA The part of the DNA to be used will depend on the stretch of DNA gene which is to be transcribed mRNA is complimentary to the DNA of gene That is the nucleotides of mRNA pair with the DNA of the gene to be transcribed This means that the mRNA is not an identical sequence to the gene but is a complementary sequence to that gene D Nature ofRNA AH typeS of RNA are made the same Way and have the same charactensucs RNA Single stranded Ribose as sugar instead of deoxyribose Ribose has 5 carbons just as does deoxyribose of DNA HOC HZ O O H Hoc ii2 0 TH c c c l 3 it i l t H C H H C H AH l lquot 2Deoxyribose Ribose A C G are used in RNA but ThymineT is not used in RNA Instead a base called Uracil U is inserted into RNA every place that would normally be a T if it were DNA Uracil takes place ofT E The genetic code The genetic code is based ofa triplet nucleotide code Each triplet code is called a codon Codons however are triplet nucleotide codes found in RNA not DNA Even though DNA is responsible for coding for a protein IT IS THE TRIPLET NUCLEOTIDE CODES in mRNA that are read or translated That is why it is called messenger RNA it carries the DNA message scour Lenin h by l SO 3 RNA bases in mRNA code for an amino acid Thus each amino acid must have at least one 3 RNA nucleotide code codon But most amino acids are have more than one triplet code ie has more than one codon Look at the chart of codons and find the codons for GLY glycine I has four codons Some codons do not code for an amino acid and these do not add an amino acid to the growing chain These are called STOP codons and they stop the addition of amino acids They stop the formation of the protein it is completed Also one codon is the start codon at which point protein synthesis starts The same start codon is used for all protein synthesis This codon is the triplet code AUG This codon is actually the codon forthe amino acid methionine However when a protein is being produced by translating the RNA the FIRST AUG in the sequence of the mRNA will code for methionine AS WELL AS IT WILL SERVE AS THE START CODON This means that all proteins begin synthesis with the amino acid methionine F The message Message is a molecule of messenger RNA mRNA contains all codons necessary for a protein sequence The mRNA sequence is determined by the sequence of one strand of DNA at a gene DNA r a an mRNA Transcription Nucleus Transom to cytoplasm 0 protein synthesis translationl Cell membrane mRNA is therefore complementary to 1 ofthe DNA strands mRNA therefore has the same sequence as the other DNA strand except that it has U where the DNA has a T Synthesis of the message DNA separates at a gene a Ttwo strands come apart in at the gene RNA nucleotides pair up with exposed DNA bases Same pairing rules as in DNA replication U pairs with A instead of T pairing with A Only one DNA strand is used as template for mRNA RNA nucleotides covalent bond one at a time RNA separates from DNA DNA rebinds into a 2 stranded structure Elongahvn Mcnrxemnlah mm or DNA RNA nucieoildei nwnslmamquot um 39 mmm My mm m RNA leaves nucleus gt 539 a Chan 0 transcvimhm 139 n 7 goes through nuclear pores goes to cytoplasm Contains code for sequence of amino acids to make a protein G mRNA editing mRNA is processed before it is readyto be used as message Some portions are removed introns called intervening sequences these are removed and not used gene WW Chtomosomal DNA Wmquot 1 Wmnz Trans cription RNA synthesis Nuclear RNA K RNA Splicing 8X0 I 830 Z l EXOII 3 I Messenger RNA RNA synthesis and processing Remaining portions are saved exons These are expressed sequences Exons are reconnecterd to each other Thus exons become the mRNA that is expressed ie from which a protein is produced Transfer RNA tRNAL a type of RNA used m protem synthesrs RNA has two mnetrons Tt cames an ammo 5ch There T5 5 specmctRNA foreach ammo 5ch n eontams as part ems RNA nuc eotwde sequence 5 arm sequence wmeh T5 comphmentaryto a codon armoodon An armcodon T5 reeogmzes codon bycomphmentary base Pamng The antrcodon T5 speeme forthe ammo 5ch thattRNAcames Tms codonanticodon pairing aHoWs ammo aerds to be pTaeed mvamulucular basePman Amman mRNA sisic Codons and anticodons Remember that codons are found in the sequence of mRNA Codons are triplet codes In mRNA one codon follows another in the RNA strand So if one starts from the first AUG every 3 triplet following that start codon is another codon These codons which code for the amino acid sequence of a protein can be followed 3 nucleotides at a time clear to the end ofthe mRNA 80 if you know the genetic code you can easily predict the amino acid sequence of the protein which is being coded for The place to start synthesis start codon using an mRNA is at the first AUG codon in the strand It does not usually come at the end but several codons from the end of the mRNA AUG codes for methionine so every protein being synthesized starts with methionine met Also AUG may occur in other places but it only the first one that is t5 lstart codon For Example the following sequence will code for 9 amino acids Remember that producing a protein starts at the first AUG and this codes for methionine as well as being the start codon UUA AUG GCA GUG GGG UUA AAC AAA AAG AUG start 2 3 4 5 6 7 8 9 PROTEIN SYNTH ESIS RIBOSOM ES D39 5 39 quot tie the wuen a new Ribosomes have two subunits which clamp over mRNA Ribosomes have three sites or cavities into which the tRNA will t TRANSLATION Ribosome clamps over mRNA mm mm gt Tymmne Threamne The Start codon AUG is the first AUG in the tRNA and goes into the P site This is the only time a tRNA enters the P site The tRNA attaches with anticodon region UAC to the start codon AUG Methionine amino acid starts protein AUG is codon for methionine Triplet code for next amino acid to be added is already in A site NothNA of the amino acid of the next codon here the codon is AGC the anticodon ofthe tRNA of this codon will be UCG enters the A site and sticks to the codon AGC There are now two tRNA and two amino acids in the ribosome The amino acid from the P site AUG will be covalently bonded to the amino acid in the A site AGC Milli Now the bond to the tRNA in the P site is broken The two amino acids bound together are now attached to the tRNA in the A site THEN The ribosome moves down the mRNA by 3 nucleotide bases 1 codon The tRNA in the P site moves to the E site exit site tRNA in the A site moves to P site still with the short two amino acid chain attached to it The tRNA for next amino acid enters A site and attaches via anticodon The two amino acid chair is covalently bonded to this new amino acid This make a three amino acid chain This three amino acid chain is released from the tRNA in the P site and remains connected to the tRNA in the A site Break this bond This empty tRNA will enter the E site exit site when the ribosome moves 1 codon Ribosome ready for next aminoacyl tRNA 2 GDP e Peptide 9 Translocalion bond formation Cavynulnvnpemsoumunallon in Dubllslvlvmas e lamlncu mg Reactions repeat until stop codon enters A site At stop polypeptide is released from tRNA to become free Finished protein
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