Download The Genetic Code

THE GENETIC CODE
© 2016 Paul Billiet ODWS
The tRNA molecule
• tRNA molecules do the final
translating
• A specific amino acid is
attached at one end by a tRNA
activating enzyme
= first part of translating
• An anticodon is at the other
end, complementary to the
mRNA codons
= second part of translation.
www.cs.stedwards.edu/.../CHEM43/tRNA/trna1.gif
© 2016 Paul Billiet ODWS
The 3-D structure of a tRNA
library.thinkquest.org/.../content/_thb_tRNA.jpg
© 2016 Paul Billiet ODWS
The genetic code
Made of 64 triplets of bases (codons).
© 2016 Paul Billiet ODWS
1st
position
U
C
A
G
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.
© 2016 Paul Billiet ODWS
1st
position
U
C
A
G
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
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.
© 2016 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
© 2016 Paul Billiet ODWS
The 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
U & C are both
pyrimidines
© 2016 Paul Billiet ODWS
Amino acid
Leucine
UUG
A & G are both
purines
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
Acidic
Basic
Uncharged Polar
Non-polar
Similar amino acids have similar
codons
Example
Aspartic acid codons GAU and GAC
Glutamic acid codons GAA and GAG
Both are acidic amino acids.
© 2016 Paul Billiet ODWS
1st
position
U
C
A
G
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
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,
means START
• The methionine amino acid is usually removed
from the finished protein.
© 2016 Paul Billiet ODWS
Punctuation?
• There 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.
© 2016 Paul Billiet ODWS
Reading the code
• mRNA is always read in the same direction 5’ to
3’ (same as transcription and replication)
• The polypeptide chain is constructed from the
amino end to the carboxyl end.
© 2016 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.
© 2016 Paul Billiet ODWS
Well almost universal
• Mitochondrial gene expression is a bit different
• Mitochondria use UGA to code for tryptophan
rather than as a stop codon
• Animal mitochondria use AUA for methionine
instead of isoleucine
• All vertebrate mitochondria use AGA and AGG as
stop codons
• Yeast mitochondria use all codons beginning with
CU for threonine instead of leucine.
© 2016 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.
© 2016 Paul Billiet ODWS
Well almost 20
• There are two codons that operate under
exceptional circumstances
• The stop codon UGA codes for selenocysteine
• The stop codon UAG codes for pyrrolysine in
species of archaea and bacteria.
© 2016 Paul Billiet ODWS