RNA EDITING genetic information flows from DNA to RNA to protein during gene expression this genetic information isn’t altered in the mRNA intermediary, but RNA editing has shown exceptions RNA editing processes alter the information content of gene transcripts in two ways o By 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 codon If UAA produced within coding region of mRNA it will prematurely terminate the polypeptide during translation, yielding incomplete gene product o By inserting or deleting uridine monophosphate residues
Uridine monophosphate residues inserted into gene transcripts, causing major changes in the polypeptides specified by the mRNA molecules This RNA editing process mediated by guide RNAs transcribed from distinct mitochondrial genes Editing played major role in expression of genes in the mitochondria of tryponasomes and plants EXONS AND INTRONS Primary transcript contains the complete structural gene sequence, including both exons and introns Intro sequence excised and exon sequences spliced together during processing events that convert primary transcript to mature mRNA INTRONS Increase rate at which coding sequences in different exons of a gene can reassert by recombination ; speeding up evolution Many eukaryotic genes don’t contain introns; not required for normal gene expression Noncoding introns excised from gene transcripts by RNA splicing o Mechanism joins exon sequences with accuracy to single nucleotide to assure that codons in exons distal to introns are read correctly CHAPTER 12 PROTEIN SYNTHESIS Translation Genetic 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 synthetases Ribosomes form peptide bonds: o Distributed through cells in prokaryotes o Located in cytoplasm in eukaryotes o Half protein half RNA o Composed of one large and one small subunits, which dissociate when the translation of an mRNA molecule is completed and reassociate during initiation of translation o E. coli: Small 30S subunits contains 16S and large 50S contains 5S and 23S o Mammalian ribosomes Small 18S subunit and large subunit contains 5S, 5.8S, 28S o In eukaryotes, rRNA synthesis occurs in nucleolus and is catalyzed by RNA polymerase I
TRANSFER RNAs Translation of a coded mRNA into a sequence of amino acids requires tRNA molecules anticodon of each tRNA occurs within a loop near the middle of molecule and base pairs with the codon of mRNA the amino acid is covalently attached to the 3’ end of tRNA cells contain at least 1 tRNA synthetase for each amino acid: they catalyze formation of aminoacyl tRNAs there are 3 tRNA binding sites on each ribosome o A site binds incoming aminoacyl-tRNA, the tRNA carrying the next
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- Fall '13
- DNA, RNA molecules