1. Translation
Chapter 9

2. Overview Occurs on ribosomes-large aggregates of rRNA and protein
tRNA acts as amino acid carriers
Prokaryotes—occurs simultaneously with transcription and mRNA degradation
Eukaryotes—occurs in cytoplasm
mRNA translated 5’3’
Protein synthesis aminocarboxy

3. Protein Synthesis
Polymerization of amino acids: condensation reaction (dehydration synthesis)

4. ~Universal Genetic Code
Codons—sets of 3 nucleotides corresponding to a single amino acid
Each codon specifies a single amino acid
More than one codon can specify the same amino acid
code is said to be degenerate
Some aa correspond to a single codon
AUG—initiator codon, methionine (Met, M)
UGG–Tryoptophan (TrP, W)
Often codons encoding the same aa differ onl;y at the 3rd nucleotide
Codons—sets of 3 nucleotides corresponding to a single amino acid
Each codon specifies a single amino acid
More than one codon can specify the same amino acid
code is said to be degenerate
Some aa correspond to a single codon
AUG—initiator codon, methionine (Met, M)
UGG–Tryoptophan (TrP, W)
Often codons encoding the same aa differ onl;y at the 3rd nucleotide

5. ~Universal Genetic Code
The Genetic Code
Chart shows aa assignments
Non-random
Tend to be clustered
Reflects similar codons specifying the same aa
Spontaneous mutations causing a single base change
May not cause an aa change
Similar aa are specified by similar codons
Greatest similarities in first two nucleotides
eg glycine – 4 codons – all GGX
Greatest variability in third nucleotide of the triplet
QUIZ—Codon for R, K, V, F
aa for UCG, CCA, GGG

6. Why~Universal? Exceptions
GUG sometimes used as a start
Mammalian mitochondria
Ciliated protozoa
Selenocysteine
NH3+
COO-
H-C-CH2SeH
GUG sometimes used as a start (bacteria)
Mammalian mitochondria- AUA start, other codons different than consensus. EX UGA-Trp (not Stop)
AGG, AGA-Stop (not Arg)
Ciliated protozoa—Ex UAA and UAG—Gln
Selenocysteine
An essential amino acid for selenoproteins
EX. Glutathione oxidase
Uses unique tRNA, initially bound to Ser
Anticodon recognizes UGA (Stop) as Sel
Signals in 3’ region determine Stop or Sel
Incorporation of selenocysteine by the translational machinery occurs via an interesting and unique mechanism. The tRNA for selenocysteine is charged with serine and then enzymatically selenylated to produce the selenocysteinyl-tRNA. The anticodon of selenocysteinyl-tRNA interacts with a stop codon in the mRNA (UGA) instead of a serine codon. The selenocysteinyl-tRNA has a unique structure that is not recognized by the termination machinery and is brought into the ribosome by a dedicated specific elongation factor. An element in the 3' non-translated region (UTR) of selenoprotein mRNAs determines whether UGA is read as a stop codon or as a selenocysteine codon.

7. Selenocysteine The 21st amino acid?
An essential amino acid for selenoproteins EX. Glutathione oxidase
Uses unique tRNA, initially bound to Ser
Anticodon recognizes UGA (Stop) as Sec
Signals in 3’ region determine Stop or Sec
Dedicated specific elongation factor
The 21st amino acid?
An essential amino acid for selenoproteins EX. Glutathione oxidase
Uses unique tRNA, initially bound to Ser
Anticodon recognizes UGA (Stop) as Sec
Signals in 3’ region determine Stop or Sec
Incorporation of selenocysteine by the translational machinery occurs via an interesting and unique mechanism. The tRNA for selenocysteine is charged with serine and then enzymatically selenylated to produce the selenocysteinyl-tRNA. The anticodon of selenocysteinyl-tRNA interacts with a stop codon in the mRNA (UGA) instead of a serine codon. The selenocysteinyl-tRNA has a unique structure that is not recognized by the termination machinery and is brought into the ribosome by a dedicated specific elongation factor. An element in the 3' non-translated region (UTR) of selenoprotein mRNAs determines whether UGA is read as a stop codon or as a selenocysteine codon.

8. Degeneracy—Wobble Hypothesis
Explains how some tRNA recognize more than one codons
tRNA molecules only need to make strong base pairs with 2 of the three codons in the nucleotide
This third loose base pairing interaction is called wobble
Note: only certain bases can substitute for others

9. Wobble Example This UCA codon was read by the
tRNA with a UGA anticodon
But if this UCA was UCG, it would still have been read by the tRNA with a UGA anticodon