Translation
© 2007 Paul Billiet ODWS
The tRNA molecule



tRNA molecules do the
final translating
At one end the have a
specific amino acid
attached by a tRNA
activating enzyme
These enzymes do the
first part of translating
At the other end they
have an anticodon
which is complementary
to the mRNA codons
© 2007 Paul Billiet ODWS
© St Edward’s University: Dept Chemistry and Biochemistry
The 3-D structure of a tRNA
© ThinkQuest.org
The genetic code
Made of 64 triplets of bases (codons)
© 2007 Paul Billiet ODWS
3rd position ↓
2nd position
1st
position
U
C
A
G
U
Phe
Ser
Tyr
Cys
U
Phe
Ser
Tyr
Cys
C
Leu
Ser
STOP
STOP
A
Leu
Ser
STOP
Trp
G
Leu
Pro
His
Arg
U
Leu
Pro
His
Arg
C
Leu
Pro
Gln
Arg
A
Leu
Pro
Gln
Arg
G
Ile
Thr
Asn
Ser
U
Ile
Thr
Asn
Ser
C
Ile
Thr
Lys
Arg
A
Met
Thr
Lys
Arg
G
Val
Ala
Asp
Gly
U
Val
Ala
Asp
Gly
C
Val
Ala
Glu
Gly
A
Val
Ala
Glu
Gly
G
C
A
G
© 2007 Paul Billiet
ODWS
Acidic
Basic
Uncharged Polar
Non-polar
The degenerate genetic code
A few amino acids are coded for by a
single codon
 Most are coded for by more than one
codon
 Some are coded for by up to six codons
 This is degeneracy in the code

© 2007 Paul Billiet ODWS
Grammar in the code?
Three codons are nonsense codons they
represent the end of the information =
STOP
 The codon for methionine found at the
beginning of the information to be
transcribed it means START
 The methionine amino acid is usually
removed from the finished protein

© 2007 Paul Billiet ODWS
1st
positi
on ↓
U
C
A
G
© 2007 Paul Billiet ODWS
2nd position
3rd position
↓
U
C
A
G
Phe
Ser
Tyr
Cys
U
Phe
Ser
Tyr
Cys
C
Leu
Ser
STOP
STOP
A
Leu
Ser
STOP
Trp
G
Leu
Pro
His
Arg
U
Leu
Pro
His
Arg
C
Leu
Pro
Gln
Arg
A
Leu
Pro
Gln
Arg
G
Ile
Thr
Asn
Ser
U
Ile
Thr
Asn
Ser
C
Ile
Thr
Lys
Arg
A
Met
Thr
Lys
Arg
G
Val
Ala
Asp
Gly
U
Val
Ala
Asp
Gly
C
Val
Ala
Glu
Gly
A
Val
Ala
Glu
Gly
G
Genetic code: characteristics

Only 61 triplets or codons code for
amino acids

3 stop codons (aka nonsense codons or
terminator codons) UUA UAG UGA
© 2007 Paul Billiet ODWS
The code is degenerative code

Several codons code for the same amino acid

The first two letters seem to be the most
important the third one tends to be
interchangeable
Codon
Amino acid
UUU
Codon
UUA
Phenylalanine
UUC
Both
pyrimidines
© 2007 Paul Billiet ODWS
Amino acid
Leucine
UUG
Both purines
Similar amino acids have similar
codons
Example
Aspartic acid codons GAU and GAC
Glutamic acid codons GAA and GAG
Both are acidic amino acids
© 2007 Paul Billiet ODWS
Punctuation?
The is no punctuation between each
codon
 The reading frame is set at the beginning
of the gene
 Frame shift mutations can be caused by
the ADDITION or DELETION of only one
or two bases. Everything downstream is
misread

© 2007 Paul Billiet ODWS
Reading the code
The reading of mRNA is always in the
same direction 5’ to 3’ (the same way as
transcription and replication)
 The polypeptide chain is constructed from
the amino end to the carboxyl end

© 2007 Paul Billiet ODWS
A universal code
The code is used by all organisms
 So it is very ancient
 Permits investigations into common
ancestry
 Permits genetically transformed organisms

© 2007 Paul Billiet ODWS
20 is the limit

Some amino acids are chemically altered
AFTER translation.
e.g. In collogen proline is converted to
hydroxyproline

Therefore the total number of amino acids found
in proteins is greater than 20 but the total used
in translation is only 20
© 2007 Paul Billiet ODWS
Translation plan
Complete protein
Polypeptide chain
TRANSLATION
Ribosomes
Stop codon
© 2007 Paul Billiet ODWS
Start codon
Translation1
AUGGGAUACACUUUUUGA
Ribosome
© 2007 Paul Billiet ODWS
mRNA
Translation 2
met
amino acid
tRNA
UAC
anticodon
AUGGGAUACACUUUUUGA
© 2007 Paul Billiet ODWS
Translation 3
gly
met
CCU
UAC
AUGGGAUACACUUUUUGA
© 2007 Paul Billiet ODWS
Translation 4
peptide bond
met
gly
UAC CCU
AUGGGAUACACUUUUUGA
© 2007 Paul Billiet ODWS
Translation 5
tyr
met
gly
AUG
UAC
CCU
AUGGGAUACACUUUUUGA
© 2007 Paul Billiet ODWS
Translation 6
met
gly
tyr
CCU AUG
AUGGGAUACACUUUUUGA
© 2007 Paul Billiet ODWS
Translation 7
thr
met
gly
tyr
UGA
CCU
AUG
AUGGGAUACACUUUUUGA
© 2007 Paul Billiet ODWS
Translation 8
phe
polypeptide chain
met
gly
tyr
thr
AAA
AUG
UGA
AUGGGAUACACUUUUUGA
© 2007 Paul Billiet ODWS
Translation: the sequence





The tRNA molecules with the correct anticodons
are lined up with their bases complementary to
the mRNA codons
Two tRNA molecules at a time can fit on the
ribosome
A peptide bond forms between their amino
acids
The first tRNA leaves the ribosome and mRNA
move along to accept a new tRNA
The process of translation proceeds in the same
direction as replication and transcription (5’ to
3’)
© 2007 Paul Billiet ODWS