Download Translation and the Genetic Code

Translation and the Genetic Code
12.1
PROTEIN STRUCTURE
p. 313
For general review
12.2
PROTEIN SYNTHESIS: TRANSLATION
p. 316
For general review.
Be sure you understand what you see in Fig. 12.17. I'm not going to be holding you
responsible for nit picky details like "How many proteins are there in the small subunit of
a eukaryotic ribosome?"
The process of translation can be divided into three main phases:
initiation, during which the ribosomal subunits join the mRNA and locate the AUG
initiator (start) codon;
elongation, during which amino acid codons are decoded and the bulk of the polypeptide
is made; and
termination, during which a termination (stop) codon directs the release of the
completed polypeptide from the ribosome.
• There are no tRNA’s with anticodons comnplementary to termination codons.
•
Release factor protein goes to A site.
•
Hydrolysis of peptide•tRNA bond
12.3
THE GENETIC CODE
p.329
Properties of the Genetic Code: An Overview
Three Nucleotides per Codo
Deciphering the Code
Initiation and Termination Codons
A Degenerate and Ordered Code
A Nearly Universal Code
12.4
CODON-tRNA INTERACTIONS
Recognition of Codons by tRNA's: The Wobble Hypothesis
The appearance of Inosine in
the anticodons of some
tRNA’s is an example of
post-transcriptional base
modification.
p. 334
Be sure that you can list ALL POSSIBLE tRNA's and the MINIMUM SET of tRNA's for
any amino acid.
EXAMPLE
Amino Acid
Isoluecine
All Possibe
3' UAA 5'
3' UAG 5'
3' UAI 5'
Minimunm Set
3' UAI 5'
3' UAI 5'
5' AUU 3'
5' AUC 3'
5' AUA 3'
Met
5' AUG 3'
!!!
3' UAA 5'
3' UAG 5'
3' UAU 5'
SYNONOMOUS CODON PREFERENCE ("Codon Bias")
Synonomous codons are NOT generally used with equal frequency.
Synonomous codon bias influences the %GC content of DNA.
Synonomous codon usage is correlated (particularly in highly expressed genes) with the relative
abundance of isoaccepting tRNA's.
Some examples of codon usage:
Thr
ACU
ACA
ACG
ACC
E. coli
.14
.09
.30
.46
TERM
UAA
UAG
UGA
.67
.07
.27
H. sapiens
.25
.18
.10
.47
CODON BIAS IS CORRELATED WI TH tRNA ABUNDANCE
Rel ative Abun dance of Yeast L eucin e
Co dons in Highly Expressed Genes
5' UUA
3'
AAU
UUG
AAC
CUU
CUC
GAG
CUA
CUG 3'
GA U
Rel ative abundance o f Yeas t Leu cine t RNA's
5'
Effect of Mutations on Translation Product
DNA Change
BP substitution
Protein Change
Silent: replaces a codon with a synonomous codon
Missense: replaces a codon with a non-synonomous codon
Insertion or deletion of 1n or 2n
bases
(where n is a small integer)
Nonssense: replaces a codon with a termination codon
Frameshift
What circumstance may cause a "silent" mutation to exert a strong phenotypic effect?