Download DNA, RNA and Protein

Translation
How the Genetic Information Is
Used to Build a Protein
Information Flow From DNA
replication
DNA
transcription
initial transcript RNA
processing
mature RNA
translation
Protein
Properties of the Genetic Code
• Triplet
– 3 nucleotides code for 1 amino acid
• Non-overlapping
– Codons are adjacent to each other
• Degenerate
– Some amino acids have more than one
codon
• “Almost” universal
– A few exceptions occur in mitochondrial,
bacterial and protist genes
5’3’ Sequence of CODONS
Accurate Protein Synthesis Depends
on Codon-Anticodon Recognition
tRNA Charging
• A specific tRNA synthetase catalyzes
the attachment of the appropriate
amino acid to each tRNA
• Energy from conversion of ATP to AMP
drives the reaction
Overview of Translation
At the ribosome, codons in mRNA are
recognized by tRNA anticodons to place
amino acids in the specific sequence
determined by the DNA.
Three Stages of Translation:
Initiation- assemble components to start process
Elongation- add amino acids in repeated cycles
Termination- release protein product
Initiation (Prokaryotic)
Formation of initiation complex containing:
Small ribosomal subunit
mRNA
Initiator (f-Met) tRNA
Large ribosomal subunit
f-met
UAC
5’-------------AUGUUUCUCUGA---3’
mRNA
Elongation
a. next tRNA binds to mRNA at the
A site
E site P site A site
f-met phe
UAC AAA
5’ -------------AUGUUUCUCUGA---3’
mRNA
Elongation
b. amino acids are joined with
peptidyl transferase
peptide bond
f-met phe
UAC AAA
5’-------------AUGUUUCUCUGA---3’
mRNA
Elongation
c. ribosome moves by one codon (translocation)
--growing peptide is now in P site
--first tRNA is in E site
f-met
phe
UAC AAA
5’-------------AUGUUUCUCUGA---3’
mRNA
Elongation
d. first tRNA is released from E site
f-met
UAC
phe
AAA
5’-------------AUGUUUCUCUGA---3’
mRNA
Elongation (second cycle)
a. next tRNA binds to mRNA at the
A site
f-met
UAC
phe leu
AAA GAG
5’ -------------AUGUUUCUCUGA---3’
mRNA
Elongation (second cycle)
b. amino acids are joined with
peptidyl transferase
peptide bond
UAC
f-met
phe
leu
AAA GAG
5’ -------------AUGUUUCUCUGA---3’
mRNA
Elongation (second cycle)
c. ribosome moves by one codon
--growing peptide is now in P site
--second tRNA is now in E site
UAC
f-met
phe
leu
AAA GAG
5’-------------AUGUUUCUCUGA---3’
mRNA
Elongation (second cycle)
d. second tRNA is released from E site
UAC
f-met
phe
leu
AAA
GAG
5’-------------AUGUUUCUCUGA---3’
mRNA
Termination
a. release factor binds to stop codon
UAC
f-met
AAA
phe
leu
RF
GAG
5’-------------AUGUUUCUCUGA---3’
mRNA
Termination
b. protein chain is released
other components separate
UAC
f-met phe leu
GAG
Large Subunit
RF
Small Subunit
AAA
5’-------------AUGUUUCUCUGA---3’
mRNA
Applying Your Knowledge
If the mRNA sequence for codons 5, 6, and 7 of
a protein is 5’-AAG-AUU-GGA-3’, what is
the amino acid sequence in the protein?
1. Gly-ile-lys
2. Arg-leu-glu
3. Glu-leu-arg
4. Asn-met-gly
5. Lys-ile-gly
Control of Initiation in Prokaryotes
A. Small Ribosomal Subunit binds to mRNA
a. IF-3 binds to small ribosomal subunit
b. Small ribosomal subunit binds to
Shine-Dalgarno sequence on mRNA
A sequence in the 16S rRNA
of the small ribosomal subunit
is complementary to the
Shine-Dalgarno sequence on
mRNA
IF-3 = Initiation Factor 3
Control of Initiation in Prokaryotes
B. Formyl-methionine tRNA binds to mRNA
a. IF-2 +GTP + f-met-tRNA join
b. f-met-tRNA binds to the first codon
c. IF-1 joins to small subunit
d. IFs dissociate, GTP is hydrolyzed to GDP
C. Large Ribosomal Subunit binds to mRNA
Control of Initiation in Eukaryotes
Binding of Small Subunit assisted by proteins bound to
-5’-methyl guanine cap
-Poly-A tail
Control of Elongation by Elongation Factors
EF-Tu joins GTP
and a charged
tRNA to form a
complex that binds
to the A site
EF-Ts regenerates
the EF-Tu + GTP
complex
EF-G and GTP are
required for ribosome
translocation
Control of Termination by Release Factors
RF1 binds to UAA and UAG
RF2 binds to UAA and UGA
RF3 forms a
complex with GTP
that binds to the
ribosome
Energetics of Translation
Number of Phosphate Bonds
Required (use of ATP or GTP)
Initiation
1 for assembly of ribosomal
subunits
2 for activation of f-met tRNA
Elongation 2 for activation of each tRNA
2 for addition of each amino acid
Termination 1 for dissociation of ribosomal
subunits
Energetics of Translation
How many phosphate bonds are required to
build a protein of 100 amino acids?
First Amino Acid
Next 99 Amino
Acids
Termination
Total
3
99x4=396
1
400