Transcript Document

LECTURE
CONNECTIONS
15 | The Genetic Code and Translation
© 2009 W. H. Freeman and Company
15.3 Amino Acid Are Assembled into a Protein
Through the Mechanism of Translation
•
Translation takes place on ribosomes.
•
A ribosome attaches near the 5’ end of an mRNA
strand and moves towards the 3’ end, translating the
codons as it goes.
•
Synthesis begin at the amino end of the protein and
the protein is elongated by the addition of new aa to
the carboxyl end.
•
Interactions between mRNA and the rRNA that hold
the mRNA in the ribosome.
The Binding of Amino Acids to Transfer RNAs
• Aminoacyl-tRNA synthetases and tRNA charging
• The specificity between an amino acid and its tRNA is
determined by each individual aminoacyl-tRNA
synthesis.
• There are exactly 20 different aminoacyl-tRNA
syntheses in a cell. Each synthetase recognizes a
particular aa.
• Recognition of tRNAs by a synthetase depends on
the differing nucleotide sequences of the tRNAs
tRNA charging requires energy, which is supplied by
adenosine triphosphate (ATP)
The Initiation of Translation
• mRNA, small and large subunits of ribosome
• A set of 3 proteins called the initiation factors: IF-1,
IF-2 and IF-3
• initiator tRNA with N-formylmethionine attached to
form fMet-tRNAfMet
• Energy molecule: GTP
The Initiation of Translation
• The Shine–Dalgarno consensus sequence in
bacterial cells is recognized by the small unit of
ribosome.
• The Kozak sequence in eukaryotic cells facilitates
the identification of the start codon.
Elongation
• Exit site E
• Peptidyl site P
• Aminoacyl site A
• Elongation factors: Tu, Ts, and G
Concept Check 3
In elongation, the creation of peptide bonds between
amino acids is catalyzed by
.
a.
b.
c.
d.
rRNA
protein in the small subunit
protein in the large subunit
tRNA
Termination
• Termination codons: UAA, UAG, and UGA
• Release factors
15.4 Additional Properties of RNA and
Ribosomes Affect Protein Synthesis
• The three-dimensional structure of the ribosome
• Polyribosomes:
– An mRNA with several ribosomes attached
Concept Check 4
In a polyribosome, the polypeptides associated with which
ribosomes will be the longest?
a.
b.
c.
d.
Those at the 5′ end of mRNA
Those at the 3′ end of mRNA
Those in the middle of mRNA
All polypeptides will be the same length.
15.4 Additional Properties of RNA and
Ribosomes Affect Protein Synthesis
• Messenger RNA surveillance:
• Detect and deal with errors in mRNA
• Nonsense – mediated mRNA decay: eliminating
mRNA containing premature termination codons
• The posttranslational modifications of proteins