1. 7.4 adapted from John Burrell http://click4biology
7.4.1 Activation of tRNA.
Activation specificity: How does the tRNA attach to the correct amino acid?
 The shape of each tRNA (defined by the loop and the helical sections. ) is
different.
 An activating enzyme adds a specific amino acid to the CCA base sequence (at
3' end of the tRNA) this requires ATP (energy).
 Each amino acid has one or more tRNA molecules. (This again reflects a
degenerate code.)
a) Amino acid which is specific to each tRNA.
(b) CCA base sequence to which the amino acid is
attached by the 'Activating Enzyme'.
(c) Complementary base pairing sequence. Helical in
shape.
(d) 8 free bases non-pairing giving one loop of RNA.
(e) 7 free bases non-pairing giving second loop of RNA.
(f) Small open loop of RNA which is variable in shape
between different tRNA.
(g) Anti-codon (3 bases) which binds to the mRNA codon
(3 bases) this is specific to the amino acids being carried.
The anti-codon is complementary to the sense DNA.
7.4.2 Ribosome structure.
Proteins and Ribosomal RNA combine in the structure
 Large sub-unit and a small sub-unit
 Large sub-unit has 3 binding sites for tRNA molecules ( E, P and
A site).
 Small sub-unit has a binding site for mRNA
Ribosome Function:
 Ribosomes contain enzymes.
They catalyze the translation of mRNA into a polypeptide.
7.4.3 Stages of translation. Translation has three parts (just as in transcription)
Initiation overview:
 The ribosome, tRNA and mRNA come together to begin the translation of the mRNA.
Elongation overview:
 tRNA molecules attach to the mRNA based on the codon-anticodon recognition. Amino
acids are brought together and polymerized into the primary structure of the polypeptide.
Termination overview:
 mRNA and the ribosomes detach from one another. The polypeptide is released and the
tRNA returns to be charged with more amino acid.

2. 7.4.4 Translation direction
Translation of the mRNA takes place from
the 5' free end to the free 3' end.
Ribosomes move along the mRNA in this
direction.
The genetic code is translated from the 5'
free end to the 3' free end.
7.4.5 Peptide bonds between amino acids.
During translation amino acids are joined
together to form polypeptides.
The specific sequence of amino acids is
called the primary structure.
Between each amino acid a peptide bond
forms to join them together.
In this example the amino acids are both
alanine in which the R group is a single
hydrogen.
The carboxyl acid end on the first amino acid
is orientated to the amino group of the
second amino acid.
The -OH group and -H are removed to form
water (condensation reaction).
The bond forms between the terminal carbon
on the first amino acid and the nitrogen on
the second amino acid.
The backbone of the molecule has the
sequence N-C-C-N-C-C
Polypeptides maintain this sequence no
matter how long the chain.
The R groups project from the backbone.
As the amino acids are added in translation
the polypeptide folds up into it specific shape.
The end of the codon sequence in mRNA
has been reached.
The ribosome encounters a termination
sequence signaling the end of translation.
The ribosome moves the alanine tRNA to
the P site.
The polypeptide is released from the
translation process.
The ribosome has no new codons read.
The two sub units move and separate.
The protein will now be further modified
in either the endoplasmic reticulum, Golgi
or secreted in a vesicle

3. 7.4.6 Translation process.
The tRNA charged with Methionine has the
anti-codon UAC. This is complementary to
the start codon (mRNA) of AUG.
The small sub unit of the ribosome
associates with the Methionine tRNA.
The small unit of the ribosome moves over
the START codon.
The large unit of the ribosome moves over
the mRNA.
There are three binding sites for tRNA on the
large sub unit.
A-(Amino acid) is the position which the new
tRNA codon-anticodon binds making sure
that the correct amino acid is in position.
P-(Polypepide) is the position in which the
amino acid on the tRNA adds to the
polypeptide.
E-( Exit) is the position the tRNA (without
amino acid) locates and is the released from
the ribosome to become re-activated.
The START codon (AUG) occupies the P
site.
The A site is free for the complementary
tRNA to bind.
Specificity is maintained by the codon-
anticodon binding which is a major
feature of the ribosome function.
In this sequence the A site has the codon
CCG.
The tRNA anticodon GGC which carried
Proline hydrogen bonds with the codon
bases.
The codon -anticodon binding has placed
the two amino acids methionine and
proline beside each other.
(a)The bond between the tRNA and
methionine is broken.
This releases free energy.
(b)The free energy is used to form the
peptide bond between methionine and
proline.
The large sub-unit then moves to three
bases (one codon) towards the 3' end of
the mRNA.

4. The anticodon tRNA for Alanine
complementary base pairs with the A site
codon.
The ribosome checks that this is the
correct tRNA and therefore amino acid.
The bond between the tRNA and Proline
is broken.
Free energy is released.
A peptide bond is formed between
Proline and Alanine.
The peptide chain will be folding and
shaping
 
7.4.7 Free and membrane bound ribosomes
Free ribosomes:
Free ribosomes in the cytoplasm are associated with the synthesis of proteins for
internal use in the cell.
Ribosomes which are attached to the wall of the endoplasmic reticulum are associated
with proteins which will be placed into vesicles and secreted form the cell.