Genetics Study Guide Exam 3 Chapter 12 Genetic Code Transcription Transcription o AKA Gene Expression o Process by which a base sequence in DNA into RNA Enzyme responsible is RNA Polymerase How the genetic code was deciphered o Original evidence Genetic code was a comma less triplet code o Genetic Code Organization Degenerate and ordered o Protects against point mutations Universal Analysis of Insertions and Deletion Mutations in bacteriophages One base insertion deletion o Loss of infection Two base insertions deletions o Loss of infection Three base insertions deletions o Infectivity restored 1E Coli 2grind 3centrifuge 4supernatant ribosomes tRNA 5add ATP radioactive amino acid 6incubate 37 C 7precipitate protein 8analyze protein o Cell free Translation Repeating Copolymers o Synthetic mRNA s AKA homopolymers UUU Polyphenylalanine AAA PolyLysine CCC Polyproline o To count the different codons in order to separate them to identify the code you must create a three codon sequence from the 1st 2nd and 3rd position o Adding one more nucleotide to the sequence changes the whole sequence of events Example In vitro translation o Repeating copolymer CUCUCUCUCUCUCU Gives leucine and serine CCUCCUCCUCCUCCU gives leucine serine proline o It is already known that CUU codes for leucine What are the codons for serine and proline Serine UCC UCU Proline CCU Solution o CUCUCUCUCU Codons CUC and UCU o CCCUCCUCCUCCU Codons CCU CUC and UCC o CUC UCU leucine and serine o CCU CUC UCC leucine serine proline o CUU leucine o Using the chart if CUU leu CUC leu Anything with CU leu o If CUC leucine UCU and UCC must serine CCU proline Purine Pyrimidine o Adenine and guanine o Cytosine and Thymine Uracil Base substitutions o Misense Changes the amino acid o Nonsense Changes the amino acid codon into a stop codon o Silent No amino acid change o Base Transitions Most mutations affecting the 3rd position do not change the amino acid Mutations affecting the 2nd position often result in replacing one amino acid with a similar one Mutations in which a purine is switched with another purine o A with G Pyrimidine is switched with another pyrimidine o U with C 5 10 times MORE common than base transversions o Base Transversions vice versa Mutations in which a purine is switched with a pyrimidine or o A with C A with U G with U G with C o U with A U with G C with A C with A o Effect of a Base Insertion Frame Shift Insertions and deletions change the meaning of all downstream codons Change the amino acid Start Codon o AUG DNA RNA Met o RNA chain is complementary to DNA chain o RNA Polymerase Enzyme o Movement downstream 5 to 3 synthesis DNA template No primer needed o AUG is where transcription begins RNA polymerase recognizes and binds to promoter sequence o Promoter Where RNA polymerase binds Binding initiates transcription Bacterial promoters 2 sequences o 10 TATA sequence o 35 sequence Transcription in Eukaryotes 3 different RNA polymerases Promoters are large and complex RNA is processed after transcription Transcription and translation are separated spatially o mRNA Processing 1 Capping 2 Addition of PolyA tail 5 cap and polyA tail are necessary for mRNA stability and transport out of the nucleus 3 Removal of introns splicing Introns o Intervening noncoding sequences within the gene o Donor site o Branch site 5 end of intron o Acceptor site 3 end of intron o Origin Near the 3 end of intron Self splicing introns in protozoans Exons o The coding sequence o Exon shuffling The reason why introns evolved Alternative splicing combines different exons to make different proteins from the same gene Splicing o Introns are cut out and exons are joined o Separates the 5 splice site from the ligate and the o Many genes in Eukaryotes are mostly introns Bigger the gene larger amount of introns strand o Splicing Removal of introns and splicing of exons Specific bases are found at the intron exon borders Spliceosome A particle of enzymes that cuts at the intron exon borders 1 Spliceosome Ss recognizes the sequence at the intron exon and join the exons borders and branch points 2 Ss pulls the exons together and promotes rearrangement of the phosphodiester bonds Intron is cut out and exons are joined 3 Lariat formed Introns excised Exons ligated Monocistronic o Structure of a typical Monocistronic Prokaryotic and Eukaryotic One mRNA codes for one protein Start and stop codons for translation are located inside of the ends of the mRNA Promoter 10 and 35 for prokaryotes o Only 10 TATA for eukaroytes Transcription start 1 Intron is between exons PolyA tail added at the other side of the cap site o Splicing gone wrong B Thalassemia Underproduction of the beta chain of hemoglobin Frame shift creates a nonfunctional beta chain which is degraded o IVS1 frame shift Chapter 13 Translation and Proteins Translation o Translation Apparatus o mRNA Message o tRNA bring amino acids to the ribosome o Ribosomes complexes of proteins and RNA s rRNA o Steps in Protein Synthesis 1 tRNA charging o Adaptor molecules that associate codons with specific amino acids o 45 different tRNA s are used Wobble position Complementary base pairing between the mRNA codon and the tRNA anticodon o There are 20 aminoacyl tRNA synthetases One for each amino acid Amino Acid binding site 3 end Recognize and join the correct tRNA with the correct amino acid 2 Chain initiation Small subunit binds to mRNA at position of start codon AUG Places the start codon in the p site Initiation complex mRNA binds to small subunit along with initiation factors IF1 2 3 Initiator tRNAfmet binds to mRNA codon in P site IF3 released tRNAi met AUG bind Large subunit binds to complex IF1 and IF2 released EF Tu binds to tRNA EF Tu o Facilitates entry into A site 3 Chain elongation Next charged tRNA binds to A site Facilitated by EF Tu Peptide bond is formed tRNA shifts from A site to P site Uncharged tRNA falls out of ribosome Peptide bond catalyzed by peptidyl transferase 1st true step of elongation commencement Ribosomes slide 3 bases down the mRNA mRNA has shifted by 3 bases EF G facilitates translocation step Third charged tRNA has entered A site 4 Chain termination When a stop codon is reached Termination factors bind Cleave polypeptide from last tRNA Ribosome mRNA complex falls apart Example 1 How many ATP equivalents are used up in translation for each amino acid that is incorporated into a polypeptide Three 1 for tRNA charging 1 for chain initiation 1 for chain elongation Reading
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