Rna editing genetic information flows from dna to rna

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