Chapter 13 Gene Expression RNA link between DNA and protein RNA copy made from info in DNA used to direct polypeptide synthesis polymer of nucleotides usually single stranded sugar ribose similar to deoxyribose has hydroxyl group at 2 base uracil substituted for thymine template strand DNA strand used in transcription takes info from one kind of nucleic acid DNA and copies it as another RNA mRNA messenger RNA carries info for making protein tRNA transfer RNA folds back on itself to form specific shape carries to ribosome rRNA ribosomal RNA globular form important in ribosome structure catalytic function during protein synthesis 1 2 3 after transcription translation mRNA used to specify amino acid sequence of polypeptide nucleic acid language amine acid language codon 3 consecutive bases 1 amino acid triplet code genetic code assignment for amino acid and stop start signals transfer RNA adapters connect amino acids and nucleic acids 1 link with specific amino acid 2 recognize appropriate mRNA codon for that particular amino acid anticodon sequence of 3 bases tRNA hydrogen bonds with mRNA codon by comp base pairing 5 UUC 3 codon anticodon Translation Ribosomes 1 2 3 AAG 5 mRNA each tRNA anticodon to be H bonded to complementary mRNA codon amino acid carried by tRNA to be linked in order specified by sequence of codons in Ribosomes site of translation organelles composed of 2 different subunits each contain protein and rRNA attach to 5 end of mRNA and travel along it allow tRNA to attach sequentially to the codons of mRNA reading frame fixed starting point for genetic message Nirenberg protein sythesis outside living cells derived from e coli bacteria 1st experimental evidence assignment of triplets to specific amino acids constructed artificial mRNA molecules with known base sequences determined which amino acids would be incorporated into protein TRANSCRIPTION step 1 transcription of DNA nucleotide sequence to RNA nucleotide sequence Eukaryotic RNA synthesized by 1 of 3 RNA polymerases 1 2 3 RNA Polymerase I RNA Polymerase II RNA Polymerase III catalyse synthesis of rRNA part of ribosomes production of protein coding mRNA synthesis tRNA and 1 rRNA molecule synthesis in 5 3 direction use nucleotide w 3 phos groups as substrates for rxn as link 2 phos get removed initiation elongation and termination promoter RNA binds to not transcribed nucleotide sequence in DNA does not need primer does need additional protein cleotide at 3 end 2 phos removed exergonic reaction leaves 3rd P to become backbone RNA polymerase skips to begin transcription of protein coding DNA sequence 1 after identifying promoter RNA polymerase unwinds DNA and initiates transcription 2 first nucleotide a 5 end keeps triphosphate group elongation and each new nu 3 continues until termination reaches termination sequence of DNA template RNA polymerase seperates bacteria stops at end of termination sequence eukaryotic adds nucleotides to mRNA molecule after term sequence has recognition sites for ribosomes binding properly position ribosomes to translate start codon signals beginning of coding sequence actual polypeptide message leader sequence mRNA 5 end message end of leader sequence signals end of protein bacteria common to have more than one polypeptide per mRNA stop codon UGA UAG UAA both bacteria and eukaryotic followed by trailing sequence Promoter sequence of bases RNA polymerase binds to TATA Box promoter Transcription Factors bind DNA enhancers and inhibitor silence regions TRANSLATION conversion of triplet nucleic acid code to 20 amino acid alphabet ensures both peptide bonds form AND link in correct order tRNA link between mRNA and protein each binds to specific amino acids use ATP as energy aminoacyl tRNA resulting complex bind to mRNA coding sequence to align amino acid in correct order to from polypeptide chain tRNA 70 to 80 nucleotides long complimentary binding sequence for correct mRNA codon each with unique base sequences and common ones anticodon recognized by aminoacyl tRNA synthetase that add correct amino acid has attachment site for specific amino acid specified by anticodon recognized by ribosomes 1 2 3 4 is doubled back and folded 3 loops 1 contains anticodon amino acid binding site 3 end carboxyl group 3 hydroxyl group covalent bond Ribosomes consist of 2 subunits both protein and rRNA rRNA has catalytic function doesn t transfer info 3 D Structure large subunit has depression on one surface where small subunit fits A P E binding sites where transfer RNA attach to 1 2 3 Peptidyl sites tRNA holding the growing polypeptide binds here Aminoacyl aminoacyl tRNA the next amino acid binds here Exit site RNA that have delivered their amino acid to chain exit P site A site E site Chapter 14 Gene Regulations Bacteria vs Eukaryotes Bacteria simple exist inependently each performs all own essential functions grow rapidly short time between cell division carry fewer chemical Primary Requirement most efficient production of enzymes and transcriptional level control related genes grouped turn on and off as needed only produce needed at a time requires rapid turn over of mRNA to prevent messages from being translated when not rarely regulate enzymes levels by degrading protein only when cells are starved deprived essential amino acids will recycle amiac by break needed ing down proteins Eukaryotes complex do have transcriptional level control also have control at other levels especially in multicellular organisms division of labor long life span respond to many stimuli active inactive state ex mRNA of egg active at fertilization differential expression of genes in cells in various tissues Gene Regulation in Bacteria bacteria efficient functionally related genes ex glycolysis enzymes constitutive genes genes constantly transcribed enzymes to efficiently use available organic molecules controlled by 1 regulating activity of enzymes 2 regulating of enzymes bacteria respond efficiently functionally related genes regulated together in complexes called operons operons in bacteria facilitate coordinates control of functionally related genes e coli use lactose as energy Jacob Monod concluded DNA coding sequence for all 3 enzymes are linked as unit on bacte rial DNA and controlled by common mechanisms Structural Gene each enzyme coding sequence operons a gene complex consisting of a group of structural genes with related function and sim ilar DNA sequence Gene Regulation in Eukaryotic Cells more complicated cells