mbii - l22 - translation 1

8/8/2019 MBII - L22 - Translation 1

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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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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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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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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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mbii - l22 - translation 1

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