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Lecture 12

BIOL 1F90 Lecture 12: chapter 11 two

Biological Sciences
Course Code
Douglas Bruce

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Nirenberg and Leder found that RNA triplets can promote the binding of
tRNA to ribosomes
-1964 found that an RNA triplet can act like a codon within an mRNA molecule
-experiment establishes relationship between triplet sequence and specic amino
-used radiolabeled amino acids bound to tRNA
-complex of tRNA, RNA triplet and ribosome could be ltered by size
-requires more components; mRNA, tRNA, ribosomes, translation factors
-most cells use a substantial amount of energy on translation
-di%erent tRNA molecules encoded by
di%erent genes
-tRNAser carries serine
-common features (cloverleaf structure,
anticodon, acceptor stem for amino acid
Aminoacyl-tRNA Synthetase
-catalyzes the attachment of amino acids to tRNA (one for each of 20 di%erent
amino acids)
-reactions result in tRNA with amino acid attached or charged tRNA or aminoacyl
-ability of aminoacryl-tRNA synthetase to recognize appropriate tRNA has been
called the 2nd genetic code
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-prokaryotes have one kind
-eukaryotes have distinct ribosomes in
di%erent cellular compartments (focus
on cytosolic ribosomes)
-composed of large and small subunits
-structural di%erences between
prokaryotes and eukaryotes exploited by
antibiotics to inhibit bacterial ribosomes
-overall ribosomes shape determined by
-discrete sites for tRNA binding and
polypeptide synthesis
-P SITE  peptidyl site; A SITE
aminoacyl site; E SITE  exit site
Comparisons of small subunit rRNAs among dierent species provide basis
for establishing evolutionary relationships
-components for translation arose in ancestor that gave rise to all living species
-all organisms have evolutionary related translational components
-gene for small subunit rRNA (SSU rRNA) found in all genomes
-gene evolution involves change sin DNA sequences
-identical sequences are evolutionarily conserved (critical function not subject to
-gene sequences more similar in more closely related species
3 Stages of Translation
1) Initiation (mRNA, rst tRNA and ribosomal subunits assemble)
-requires help of ribosomal initiation factors
-also requires input of energy (GTP hydrolysis)
-initiation in bacteria
mRNA binds to small ribosomal subunit facilitated by ribosomal
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