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Molecular Biology II Translation 1
Lecture 22[Page 1]
During translation multiple ribosomes (polysome) move along a single transcript
Nucleotides are read in triplets known as codons. 4nts3 = 64 combinations (enough for 20 AAs)
61 triplets encode for AAs
Nirenberg added synthetic polyribonucleotides (mRNA) to bacterial extracts and showed that they could make
polypeptides.
Crick and Brenner showed that it was a triplet by mutational analysis.
This shows that the genetic code is degenerate some AAs have more than one codonThere are 3 STOP codons that do not encode AAsUAA / UAG / UGA
All proteins begin with methionine (Met) the start codon is always AUG
Not every AUG is a start codon
Met / Trp are only encoded for by one codon each
Arg / Leu / Ser have 6 codons each
Wobble Hypothesis:
Most AAs are represented by codons that differ only in the 3rdbase
Cells usually have fewer than 61 tRNAs there is a certain degree of play between
the 3rdbase of the codon and the 1stbase of the anticodon. This allows all triplets
to be coded for even if the specific tRNA is not available. C1 in glycocidic bond is
crucial for wobble
The amount of play is defined as wobble
The 1stbase of the anticodon is often referred to as the wobble position
The wobble is possible as non Watson-Crick base pairing can occur (alternative base pairing)
Wobble Interactions
5 Anti-codon 3 Codon
G C / U
C G
A U
U A / G
I (Inosine) A / U / C
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MP
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Specifically the ACC acceptor stemtRNA (3D) structure is fairly similar L-shaped with AA at one end anti-codon at the other
RNA is flexible this allows it to self-hybridise at regions of self complimentarity
Intra-molecular base pairing and base stacking dictate the 3D structure The 3D structure is more complex than that of DNA
Secondary Structure
70-80 nucleotides long
4 self-complementary regions give rise to
a characteristic cloverleaf structure
The modified nucleotides will interact
with one another in different ways
leading to non Watson-Crick base
pairing. There are also some G U base
pairs. Both of these factors have a great
influence on tertiary (3D) structure.
Triple base pairs can occur
Tertiary Structure
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Molecular Biology II Translation 1
Lecture 22[Page 4]
Tertiary structure is very important in tRNAs.
tRNA Charging:
CCA Tail Anticodon D Arm
Acceptor Stem Anticodon Arm T (TC) Arm
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Molecular Biology II Translation 1
Lecture 22[Page 5]
Covalent link between AA and tRNA
Aminoacyl tRNA synthetases can recognise subtle differences in AAs providing specificity, as there is no base pairing.
There are 2 similar mechanisms for tRNA charging that both begin with the formation of an
AMP-AA (Aminoacyl adenylate) intermediate (1)
The AMP-AA intermediate remains bound to the active site.
Class I aminoacyl-tRNA synthetases
The aminoacyl group is transferred to the tRNA (2)
The aminoacyl group is transferred initially to the 2'-OH group of the 3' end terminal A residue.
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Molecular BiologyII Translation 1
Lecture 22[Page 6]
A transesterification reaction occurs moving the
AA from the 2' to the 3' position (3a)
Class IIaminoacyl-tRNA synthetases
2b) The aminoacyl group is transferred directly to the 3'-OH group of the 3' end terminal A residue.
tRNA Synthetases:
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