Biol 1411 1st Edition Lecture 24 Outline of Last Lecture I Transcription II Genetic Code Outline of Current Lecture I Gene Expression a Translation b Gene Regulation Current Lecture Translation RNA ribonucleic acid differs from DNA o Usually a single polynucleotide strand o The sugar is ribose o Contains uracil U instead of thymine T Transfer RNA o tRNA the adapter molecule associates information in mRNA codons with specific amino acids o Three functions of tRNA it binds to an amino acid and is then charger it associates with mRNA molecules it interacts with ribosomes o The conformation of tRNA results from base pairing within the molecule o 3 end is the amino acid attachment site binds covalently o Anticodon at the midpoint of the tRNA sequence site of base paring with mRNA Unique for each species of tRNA Aminoacyl tRNA synthetases o Activiating enzymes aminoacyl tRNA synthetases charge tRNA with the correct amino acids o Each enzyme is highly specific for one amino acid and its correcsponding tRNA the process of tRNA charging is called the second genetic code o The enzymes have 3 part active sites they bind a specific amino acid a specific tRNA and ATP Codon anticodon wobble These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute o Wobble specificity for the base at the 3 end of the codon is not always observed o Wobble allows cells to produce fewer tRNA species but does NOT create any ambiguity in the genetic code Ribosomes the workbench holds mRNA and charged tRNAs in the correct positions to allow assembly of polypeptide chain o Ribosomes have 2 subunits large and small o In eukaryotes the large subunit has three molecules of ribosomal RNA rRNA and 49 different proteins in a precise pattern The small subunits has one rRNA and 33 proteins Stages of Translation o Like transcription translation also occurs in three steps Initiation formation of initial complex a charged tRNA and small ribosomal subunit both bound to mRNA Prokaryotes rRNA binds to mRNA recognition site upstream of start codon Eukaryotes small subunit bnds to the 5 cap on the mRNA and moves until it reaches the start codon Elongation charged tRNAs enter A site large subunits acts as peptidyl transferase When the first tRNA has released its methionine it moves to the E site and dissociates from the ribosome can then become charged again Elongation occurs as the steps are repeated assisted by proteins called elongation factors Termination stop codon enter the A site Translation ends when a stop codon enters the A site Stop codon binds a protein release factor allows hydrolysis of bond between polypeptide chain and tRNA on the P site Polypeptide chain separates from the ribosome C terminus is the last amino acid added UTR untranslated region Gene Regulation o Gene expression begins at the promoter where transcription is initiated o In selective gene transcription a decision is made about which genes to activate o Otherwise constant gene transcription is know as constitutive expression o Regulatory proteins control expression of other genes most gene are under the control of multiple regulatory proteins o General purpose genes needed by all cells but not expressed at all times of cell cycle o Specialty function genes needed for response to specific environmental changes or for specialized cell tissue functions o Regulation of protein levels regulatory control of transcription is a primary determinant of protein level produced from gene expression in eukaryotes and prokaryotes Two types of regulatory protein activity repressors and activators o Negative regulation binding of a repressor protein to DNA prevents transcription initiation can occur in the absence of the repressor protein o Positive regulation activator protein binds to DNA and stimulates transcription transcription initiation low in the absence of the activator protein
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