PCB3063 Final Exam Study Guide Chapter 12 The Genetic Code and Transcription 12 1 12 2 The genetic code is unambiguous degenerate comma less nonoverlapping and nearly universal Research by Crick using T4 phage and frameshift mutations showed that a triplet code provides 64 codons to specify the 20 amino acids 12 3 Studies by Nirenberg Matthaei and Others Deciphered the Code RNA Heteropolymers two or more different ribonucleotides used to decipher more codons o This methodology determined the composition of the triplet code words for all the 20 amino acids but not the sequence of the triplet Triplet binding assay used to determine other specific codon assignments o Ribosomes bind to a single codon of three nucleotides o The triplet acts like a codon in mRNA and attracts a complementary sequence within tRNA anticodon Two major conclusion drawn from studies o The code is degenerate one amino acid can be specified by more than one triplet o The code is unambiguous a single triplet specifies only one amino acid 12 4 12 5 12 6 Wobble hypothesis proposes a more flexible set of base pairing at the third position of the codon o The anticodon of a single form of tRNA can pair with more than one triplet in mRNA Ordered genetic code the genetic code shows order in that chemically similar amino acids often share one or two middle bases in the triplets encoding them N Formylmethionine fmet modified form of methionine start codon o AUG is the only codon to encode for methionine sometimes called the initiator codon o When AUG appears internally in mRNA an unformylated methionine is inserted into the Termination codons UAG UAA UGA do not code for any amino acid o Mutations producing any of these three triplets internally will result in termination nonsense polypeptide chain mutation o Only a partial polypeptide is synthesized and prematurely released Studies with bacteriophage MS2 confirmed that the genetic code is a triplet code degenerate unambiguous and commaless with start and stop signals o The genetic code in the virus is identical to that in bacteria Recombinant DNA technology reveals that eukaryotic genes can be inserted into bacterial cells which are then transcribed and translated o Mitochondrial DNA revealed some exceptions to the universal genetic code o Example UGA normally specifies termination but encodes tryptophan in yeast and human mitochondria 12 7 Different Initiation Points Create Overlapping Genes The genetic code is nonoverlapping each ribonucleotide in an RNA is part of only one codon o A single mRNA may have multiple initiation points for translation which then creates different reading frames within the same mRNA This specifies more than one polypeptide meaning there are overlapping genes In some viruses overlapping genes open reading frame or ORF have been identified in which initiation at different AUG positions out of frame with one another leads to distinct polypeptides Chapter 13 Translation and Proteins tRNAs are small in size and are very stable o Composed of 75 90 nucleotides identical in structure in prokaryotes eukaryotes o tRNAs are transcribed from DNA and contain posttranscriptionally modified bases and enhance hydrogen bonding efficiency during translation o Amino acids have to be chemicall joined with the appropriate tRNA Initiation o Initial binding of mRNA by IF1 and small subunit occurs through interacts with Shine Delgarno sequence upstream of start codon o 2nd amino acid joined to carboxy terminus first Elongation lengthening of the growing polypeptide chain o Requires both ribosomal subunits assembled with the mRNA to form the P peptidyl site and A aminoacyl site o Directionality of elongation amino to carboxy end of protein o The charged tRNAs enter the A site and peptidyl transferase catalyzes peptide bond formation between the amino acid at the A site and the growing peptide chain bound to the tRNA in the P site The uncharged tRNA then moves to the E exit site The tRNA bound to the polypeptide chain moves to the P site the sequence of elongation and translocation is repeated over and over o The elongation process requires several elongation factors as well as energy derived from the hydrolysis of GTP Termination o Signaled by a stop codon UAG UAA UGA in the A site o GTP dependent release factors cleave the polypeptide chain from the tRNA and release it from the translation complex o tRNA is released from the ribosome which then disassociates into its subunits Polyribosomes are mRNAs with several ribosomes translating at once 13 5 Early observations by Bateson and Garrod 1900 o Alkaptonuria is now a well characterized metabolic disorder o Garrod noted related individuals more likely to exhibit similar disease phenotype and heritability patterns Beadle and Tatum 1940s Neurospora experiments o Mutating DNA changes biochemical capacity of organism o one gene one enzyme Most but not all proteins are enzymes o Some proteins have two or more polypeptide chains quaternary structure because of this it is now thought to be one gene one polypeptide chain 13 8 Variation in Protein Structure Is The Basis of Biological Diversity Following translation polypeptides fold up and assume higher order structures and may interact with The polypeptides of proteins are linear non branching polymers with 20 amino acids that serve as All amino acids have a central carbon covalently bound to a carboxyl group an amino group and an other polypeptides subunits R group For some proteins correct folding is dependent on molecules called chaperones o These are ubiquitous proteins that function by mediating the folding process by excluding the formation of alternative incorrect patterns and do not become part of the final product Misfolded proteins may be nonfunctional and can accumulate and be detrimental to cells Prions are aggregates of misfolded proteins causing transmissible brain disorders in mammals 13 9 Proteins Function in Many Diverse Roles Proteins are the most abundant macromolecules found in cells and play diverse roles in the body o Hemoglobin and myoglobin transport oxygen which is essential for cellular metabolism o Collagen and keratin and structural proteins associated with the skin connective tissue and hair of organisms o Actin and myosin are contractile proteins found in abundance in muscle tissue o Tubulin is the basis of the function of microtubules in mitotic and meiotic spindle fibers Domains are associated with unique functions in proteins o 50 300 amino
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