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List the components required for translation
Outline the process of activation of an amino acid to form an
amino acyl tRNA
Describe the 3 stages of translation in prokaryotic cells
Identify ways in which eukaryotic translation differs from
prokaryotic translation

The amino acid sequence of a protein is determined by the sequence of bases on the
DNA strand


How do 4 bases code for 20 amino acids?
• Pairs: 42 = 16 possibilities --> not enough
• Triplets: 43 = 64 possibilities --> more than enough (degenerate code)

How is the code translated?
1
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g
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Find out which amino acids incorporated into protein using a cell-free system -->
phenylalanine only
























• Shows there is no punctuation in the genetic code





























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The genetic code is read in triplets
Code degenerate
o 64 codons for 20 amino acids
o 3 special termination codons
o All proteins start with the initiation codon AUG (methionine)


Reading frames
If you start at 1 different position it changes the whole
reading frame --> changes the protein completely













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A point mutation leads to sickle cell anaemia
o 1 triple code changes from GAG to GTG (glutamate to valine)


Translation: the players
• Messenger RNA
o Carries sequence information to make protein
• Transfer RNA (tRNA)
o Bring individual amino acids to sites of protein synthesis
• Ribosomes
o Contain ribosomal RNA (rRNA) and ribosomal proteins
o Machinery of protein synthesis
o Bind to mRNA and tRNA

tRNA molecule

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The anticodon is the complementary part that
attaches to mRNA
Are 60 different types made up of 75-90
nucleotides
There is extensive internal base-pairing which gives
a cloverleaf structure
Contains unusual bases
CCA-OH sequence at 3’ end where the amino acid
attaches
The anticodon is on the central loop
The flexible CAA arm is needed to allow the amino
acid to be transferred to the amino acid chain

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Anticodon is a triplet of bases
complementary to a codon
3 bases are unpaired and available
for hydrogen bonding
61 amino acid codons but less than
61 different tRNA molecules
therefore some tRNAs can
recognise more tapenade one
codon
Wobble pairing allows this to occur
as it means the same tRNA can
attach to different codons with a
different 3’ end base












Wobble mechanism
• Last base on anticodon loop can form non
Watson-Crick base pairs
• Doesn't alter amino acid sequence of a protein
• Allows a single tRNA species to recognise more
than 1 codon
• Can still fit together
• Only happens for certain amino acids
















Amino acids are attached to the 3’ end of the tRNA
• Speck fix aminoacyl-tRNA-synthetases load tRNA
molecules with amino acids
• Each tRNA can accept only a single amino acid
that is appropriate for its anticodon sequence

Activated amino acids
Amino acid + tRNA + ATP --> aminoacyl-tRNA + PPi +
AMP
• Energy for addition of amino acid to tRNA comes
from hydrolysis of ATP
• 20 different types of tRNA classed by their amino
acid e
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tRNAphe
• When they are charged they are sometimes called
activated amino acids and named differently e
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Phe-tRNAphe

Double selectivity: amino acid – tRNA – codon

The ribosome: E
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How start codon is found is
the ribosome
unclear
fmet
Initiator tRNA is fmet-tRNA
Initiator tRNA is met-tRNAmet
Polycistronic mRNA code for more than
mRNA codes for 1 protein - monocistronic
one protein
Polyribosomes are free in cytoplasm
Polyribosomes are free in cytoplasm or can
be bound to RER






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