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RNA EDITINGgenetic information flows from DNA to RNA to protein during gene expressionthis genetic information isn’t altered in the mRNA intermediary, but RNA editing has shown exceptionsRNA editing processes alter the information content of gene transcripts in two waysoBy changing the structures of individual bases and substituting one base for another (rare) A C residue in pre mRNA is converted to a U, generating an internal UAA translaton-termination codonIf UAA produced within coding region of mRNA it will prematurely terminate the polypeptide during translation, yielding incomplete gene productoBy inserting or deleting uridine monophosphate residues
Uridine monophosphate residues inserted into gene transcripts, causing major changes in the polypeptides specified by the mRNA moleculesThis RNA editing process mediated by guide RNAs transcribed from distinct mitochondrial genesEditing played major role in expression of genes in the mitochondria of tryponasomes and plants EXONS AND INTRONSPrimary transcript contains the complete structural gene sequence, includingboth exons and intronsIntro sequence excised and exon sequences spliced together during processing events that convert primary transcript to mature mRNAINTRONSIncrease rate at which coding sequences in different exons of a gene can reassert by recombination ; speeding up evolutionMany eukaryotic genes don’t contain introns; not required for normal gene expressionNoncoding introns excised from gene transcripts by RNA splicingoMechanism joins exon sequences with accuracy to single nucleotide toassure that codons in exons distal to introns are read correctly CHAPTER 12 PROTEIN SYNTHESISTranslationGenetic information in mRNA molecules is translated into amino acid sequences of polypeptides according Amino acids are attached to correct tRNA molecules by a set of activating enzymes called aminoacyl-tRNA synthetasesRibosomes form peptide bonds:oDistributed through cells in prokaryotesoLocated in cytoplasm in eukaryotesoHalf protein half RNA oComposed of one large and one small subunits, which dissociate whenthe translation of an mRNA molecule is completed and reassociate during initiation of translation oE. coli: Small 30S subunits contains 16S and large 50S contains 5S and 23SoMammalian ribosomesSmall 18S subunit and large subunit contains 5S, 5.8S, 28SoIn eukaryotes, rRNA synthesis occurs in nucleolus and is catalyzed by RNA polymerase I
TRANSFER RNAsTranslation of a coded mRNA into a sequence of amino acids requires tRNA moleculesanticodon of each tRNA occurs within a loop near the middle of molecule and base pairs with the codon of mRNAthe amino acid is covalently attached to the 3’ end of tRNAcells contain at least 1 tRNA synthetase for each amino acid: they catalyze formation of aminoacyl tRNAsthere are 3 tRNA binding sites on each ribosomeoA site binds incoming aminoacyl-tRNA, the tRNA carrying the next