- PROTEIN TRANSLATION - Weebly · 2019-10-30 · translation of information encoded in...
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- PROTEIN TRANSLATION –
Dr. suheir Ereqat 2019/2020
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Transcription and Translation in Prokaryotes vs. Eukaryotes:
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Prokaryotes
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Eukaryotes:
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Genetic code!
• The four code letters of DNA encode 20 amino acids=codon (triplet of nucleotides)
• 64 combination
• No punctuation between codons for successive amino acid residues.
• Adaptor!
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Translation requires adaptor molecules, the tRNAs,
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A minimum of 32 tRNAs
are required to translate
all 61 codons (31 to
encode the amino acids
and 1 for initiation).
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Nonoverlapping code:
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Reading Frame: all mRNAs have three potential reading frames,
a reading frame without a termination
codon among 50 or more codons =
open reading frame (ORF).
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reading frame: the series of nucleotides read in sets of 3 (codon)
only 1 reading frame is correct for encoding the correct sequence of amino acids
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The Genetic Code
Marshall Nirenberg identified the codons that specify each amino acid.
RNA molecules of only 1 nucleotide and of specific 3-base sequences were used to determine the amino acid encoded by each codon.
The amino acids encoded by all 61 possible codons were determined.
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Polynucleotide phosphorylase (do NOT need template)
(NMP)n + NDP ↔ (NMP) n+1 + Pi
lengthened RNA
The base sequences of the codons were deduced
from experiments using synthetic mRNAs of known
composition and sequence.
Cracking the Genetic Code:
Marshall Nirenberg experiment
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The Genetic Code :
- Triplet Codons Code for a Single Amino AcidUUU = Phe
- The Code is CommalessAUGUUU = Met Phe
- 1 codon for Met, Trpall other amino acids have 2-6 tripletsUCU Ser UCA SerUCG Ser UCC Ser
“Wobble base”
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• “Dictionary” of amino acid code words in mRNAs
UUU is always Phe, not sometimes.
Nonsense Codons are Stop Points
UAA, UAG, and UGA
The genetic Code is nearly Universal
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“Dictionary” of amino acid code words in mRNAs
Initiation codon=the most
common signal
for the beginning of a
polypeptide in all cells
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The genetic code is degenerate: it has multiplecodons for almost every amino acid
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Wobble base(# 1 of
tRNA)
always form
strong Watson-Crick base pairs
wobble hypothesis:
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Transfer RNAs Have Characteristic Structural Featurescloverleaf secondary structure of tRNAs.
D=dihydrouridine
methylguanylate
can carry a specific
amino acid esterified by its carboxyl
group to the 2- or 3-hydroxyl group of
the A residue at the 3 end of the
tRNA.
interacts with the large
Subunit rRNA.
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Three-dimensional structureof yeast tRNA
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translation of information encoded in mRNA=Protein; this process is carried out
by ribosomes.
Ribosomes and endoplasmic reticulum (ER):
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In eukaryotes, translation may occur on ribosomes in the
cytoplasm or on ribosomes of the RER.
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Protein synthesis: 5 stages
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Activation of amino Acid: Aminoacyl tRNA synthetases
-Amino acid + tRNA + ATP +Mg2+ aminoacyl- tRNA + AMP +PPi
This reaction takes place in the cytosol, not on the ribosome
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aminoacyl- tRNA synthesis :Charging of tRNA
Aminoacyl-AMP
which remains bound to
the active site
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The ester linkage both
activates the amino acid and joins it
to the tRNA
Aminoacyl-tRNA
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Class II: Aminoacyl group is transferred directly to the
3-OH of the A residue of tRNA
ClassI: Aminoacyl group is transferred to
2-OH of A then to 3-OH
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Proofreading by aminoacyl-tRNA synthetase:
Identity of a.a attached to tRNA not checked on ribosome.
protein synthesis fidelity relies on accuracy of this rxn.
Ile-tRNA synthetase:
Prefers Ile over Val by a factor of 200
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2nd genetic code: Specificity not only for a single a.a but also to tRNA .
recognition points that are recognized by
aminoacyl-tRNA synthetases
The interaction of aminoacyl-tRNA synthetases and
their cognate tRNAs is critical to accurate reading of the
genetic code.
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tRNAala :
same efficiency as complete tRNA.
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Initiation codon(AUG):, all organisms have two tRNAs for methionine. One is used
exclusively when (5)AUG is the initiation codon for protein synthesis. The other is
used to code for a Met residue in an internal position in a polypeptide.
N-formyl group
(Prokaryotes)
Addition of the N-formyl group to the
amino group of methionine by the
transformylase prevents fMet from
entering interior positions in a
polypeptide while also allowing
fMet-tRNAfMet to be bound at a
specific ribosomal
initiation site that accepts neither
Met-tRNAMet nor any
other aminoacyl-tRNA.
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transformylase
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How can the single (5)AUG codon determine
whether a starting N-formylmethionine (or
methionine, in eukaryotes) or an interior Met
residue is ultimately inserted?
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Shine Dalgarno sequence:
The mRNA binds to the 30S subunit. The initiating
(5)AUG is guided to its correct position by the consensus sequence; Shine-
Dalgarno sequence (initiation signal)
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