CH15:
TRANSLATION
mRNA stability
•
The steady-state level: the amount of mRNA in the cell
•
combination of transcription rate and rate of mRNA
degradation
•
Determines the amount of mRNA available for translation
•
Regulated in response to cellular needs
.
RNAi gene silencing
•
RNA interference uses Dicer to cleave
double-stranded RNA molecules into
small interfering RNAs (siRNAs) and
micro RNAs (miRNAs)
• siRNAs and miRNAs
repress mRNA
translation and
trigger degradation
Why translate?
Amino acids are joined together by peptide bonds.
The Genetic Code: RNA to Amino Acids
Second base of RNA codon
UUU
U
UUC
UUA
First base of RNA codon
UUG
UCU
Phenylalanine
(Phe)
UCC
Leucine
(Leu)
CUU
C
CUC
CUA
A
G
C
Leucine
(Leu)
A
UAU
Serine
(Ser)
UAC
UAG
Stop
UGG Tryptophan (Trp) G
CCU
CAU
CCC
CCA
Proline
(Pro)
CAC
CAA
ACU
AAU
ACC
AUA
ACA
AAC
Threonine
(Thr)
AAA
AUG
Met or start ACG
AAG
GUU
GCU
GAU
GUA
GUG
C
UCG
AUU
Valine
(Val)
U
UGA Stop
CAG
GUC
UGC
Cysteine
(Cys)
Stop
CCG
Isoleucine
(Ile)
UGU
UAA
UCA
CUG
AUC
Tyrosine
(Tyr)
G
GCC
GCA
GCG
Alanine
(Ala)
GAC
GAA
GAG
Histidine
(His)
CGA
Glutamine
(Gln)
CGG
AGU
Asparagine
(Asn)
AGC
Lysine
(Lys)
U
CGU
CGC
AGA
AGG
GGU
Aspartic
acid (Asp) GGC
GGA
Glutamic
acid (Glu) GGG
A
Arginine
(Arg)
C
A
G
Serine
(Ser)
Arginine
(Arg)
U
C
A
G
U
Glycine
(Gly)
C
A
G
Third base of RNA codon
U
.
Figure 14.1
tRNA
•
tRNAs are small in size and very
stable
•
•
Composed of 75–90 nucleotides
Transcribed from DNA and contain
post-transcriptionally modified
bases
•
•
Modified bases enhance hydrogen
bonding efficiency during translation
The two-dimensional structure of
tRNAs is a cloverleaf
Figure 14.3
Wobble may exist in the pairing of a codon and anticodon.
The mRNA and tRNA pair in an antiparallel fashion.
Proteins in 3 “Easy” Steps!!
•
Initiation
•
Elongation
•
Termination
Initiation
Met
Large
ribosomal
subunit
Initiator
tRNA
P site
mRNA
2
1
Start codon
Small
ribosomal
subunit
A site
Prokaryotes: Shine-Dalgarno Sequence
The poly(A) tail
of eukaryotic
mRNA plays a
role in the
initiation of
translation.
Eukaryotes
Elongation
•
The charged tRNAs enter the A site,
and peptidyl transferase catalyzes
peptide bond formation
•
The uncharged tRNA moves to the E
(exit) site
•
The tRNA bound to the peptide chain
moves to the P site
•
The sequence of elongation and
translocation is repeated over and
over
Termination
•
Termination: signaled by a stop codon (UAG,
UAA, UGA) in the A site
•
GTP-dependent release factors cleave the
polypeptide chain from the tRNA and release
it
Figure 14.17
Figure 14.18
Figure 14.19
Figure 14.20
Post-translational modifications