Download DNA --> RNA --> Protein The code is a triplet code The code is a

11/10/11
DNA --> RNA --> Protein
Translation: RNA to Protein
DNA
molecule
•  Finish transcription
•  The genetic code
•  Translation to protein
Gene 2
Gene 1
Gene 3
–  Initiation
–  Elongation
–  Termination
DNA strand
(template)
3!
A
5!
C
C
A
A
A
C
C
G
A
G
T
U G
G
U
U
U
G G
C
U
C
A
TRANSCRIPTION
5!
mRNA
3!
Codon
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TRANSLATION
Protein
Gly
Phe
Trp
Ser
Amino acid
The genetic code
N terminal
C terminal
Second mRNA base
Which amino
acid is
specified by
CAG?
1st base is C
2nd base is A
3rd base is G
First mRNA base (5! end)
New amino acids added
at the carboxyl end
U
UUU
U
UUC
UUA
UUG
C
Phe
Leu
CUU
C
A
The code is a triplet code
–  the new boy saw the big cat eat the hot dog
•  The first evidence for triplet code came from
deletion experiments
–  the new boy saw the big cat eat the hot dog
–  the neb oys awt heb igc ate att heh otd og
UCC
UCA
Ser
UAC
ACU
AAU
GUC
GUA
GUG
ACC
ACA
Thr
ACG
GCC
GCA
GCG
AAC
AAA
AAG
GCU
Val
Cys
U
GAU
Ala
This is in the
mRNA sequence
(5’-3’)
C
UAG Stop UGG Trp G
AUU
Met or
start
UGC
CAU
CAG
AUA
UGU
CCU
CCG
lle
Tyr
UCG
CUG
AUC
G
UAA Stop UGA Stop A
CCC
CAC
Pro
CUA Leu CCA
CAA
GUU
mRNA is read 5’ to 3’
UAU
UCU
CUC
AUG
G
A
GAC
His
Gln
Asn
Lys
Asp
U
CGU
CGC
CGA
Arg
C
A
CGG
G
AGU
U
Ser C
AGC
AGA
AGG
A
Arg G
U
GGU
GGC
GAA
GGA
GAG Glu GGG
Gly
Third mRNA base (3! end)
Polypeptide has directionality
Glutamine
C
A
G
The code is a triplet code
•  . . . or by two more deletions to get it back
into the correct “reading frame”
–  the new boy saw the big cat eat the hot dog!
–  the neb oys awt heb igc ate att heh otd og!
–  the neb osa wte big cat eat the hot dog!
•  Partial function could be restored by an
insertion nearby
–  the new boy saw the big cat eat the hot dog
–  the neb oys aaw the big cat eat the hot dog
1
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The code is redundant
Second mRNA base
C
A
UAU
Tyr
Phe
UAC
UUC
UCC
U
UCA Ser UAA Stop
UUA
Leu
UAG Stop
UCG
UUG
U
•  Examples
–  UUUUUUUUUUU -> phenylalanine only
•  Several different
codons encode
the same amino
acid
–  UCUCUCUCUCU -> Mixture of leucine and serine
Why is it a mixture?
First mRNA base (5! end)
•  Your book tells how people used synthetic mRNAs
and in vitro translation to determine decipher the
codons.
UUU
CUG
AUU
ACU
lle
AUC
A
AUA
AAU
ACC
Thr
ACA
AUGMet or
ACG
GUU
GUC
GUA
GUG
GCU
GCC
start
Val
GCA
GCG
AAC
AAG
Ala
UGG Trp G
Lys
AGU
AGC
AGA
U
Ser C
A
AGG Arg G
GAU
GGU
Asp
GAC
GGC
GAA
GGA
Glu
GAG
GGG
U
Gly
C
A
G
Translation: the basic concept
TRANSCRIPTION
DNA
mRNA
Ribosome
tRNA serves as an
“adaptor” that
brings the correct
amino acid to each
codon.
Polypeptide
•  No punctuation between words.
–  Therefore deletions cause frameshifts
–  Start: AUG
–  Stop: UAG, UAA, UGA
Asn
AAA
TRANSLATION
•  It does have start and stop signals,
however.
G
U
Cys
UGC
C
UGA Stop A
UGU
CCU
U
CAU
CGU
His
CCC
CAC
CGC
C
Arg
Leu CCA Pro
CAA
CGA
A
Gln
CCG
CAG
CGG
G
CUU
CUC
C
CUA
G
The code is comma free
UCU
Third mRNA base (3! end)
Deciphering the Code
Amino
acids
Polypeptide
Ribosome
Trp
The ribosome is the
machine that builds
the polypeptide
Phe
tRNA with
amino acid
attached
Gly
tRNA
C
A
G
C
C
G
Anticodon
A A A
U G G U U U G G C
Codons
5!
3!
mRNA
Transfer RNA (tRNA) structure
Amino acid
attachment site
tRNA
3A!
C
C
A 5!
C G
G C
C G
U G
U A
A U
A U
U C
*
AG *
C A C A G UA
* CUC
G
*
GUGU*
G
C
CGAG
* *
AGG
U C
*
* GA
G C
Hydrogen
G C
U A
bonds
* G
A
*
A
C
*
U
A
A G
5!
3!
Amino acid
attachment site
Hydrogen
bonds
A
A G
Anticodon
Anticodon
2
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Ribosomes
TRANSCRIPTION
EPA model of a ribosome
DNA
mRNA
Ribosome
P site (Peptide site)
Growing
polypeptide
TRANSLATION
Polypeptide
E site (Exit site)
tRNA
molecules
E
Large
subunit
P A
Large
subunit
E
Small
subunit
RNA + protein.
RNA has the active site
5!
Amino end
Small
subunit
The initiation of translation
Growing polypeptide
Me
3! U
t
GTP
3!
1
Codons
5!
2
Amino end
of polypeptide
E
P
A
site site
5!
A
Small subunit binds just
upstream of start
Translation initiation complex
3
Special initiator methionine
binds to the start codon (AUG)
3!
The large subunit can now
bind to make the active
ribosome
Termination
1. tRNA with
appropriate
anticodon binds at
A site
3!
mRNA
E
5!
Small
ribosomal
subunit
mRNA binding site
t
GDP
3!
Start codon
tRNA
mRNA
Me
5! A U G 3!
mRNA
5!
Large
ribosomal
subunit
P site
5!
A C
Initiator tRNA
Next amino acid
to be added to
polypeptide chain
Ribosome ready for
next aminoacyl tRNA
A
mRNA
binding site
New amino acids are added to the
carboxyl end of the polypeptide
Fig. 17-18-1
P
3!
mRNA
Elongation
A site (Acceptor site)
Release
factor
Free
polypeptide
GTP
GDP
5!
3!
3!
E
E
P A
3!
5!
5!
Stop codon
(UAG, UAA, or UGA)
P A
GDP
3. Translocaton.
Everything moves left
one notch. Polypeptide
is now at P. Empty
tRNA leaves the E site.
1
GTP
E
P A
2. Peptide bond
formed. Growing
chain is now at A
Stop codon has
no matching
tRNA
2
Release factor
binds and
separates the
polypeptide
from the tRNA
3
The ribosome
separates
from mRNA
3