7.4.1 Activation tRNA.
7.4.2 Ribosome structure.
7.4.3 Stages of translation.
7.4.4 Translation direction.
7.4.5 Peptide bonds between amino acids.
7.4.6 Translation process.
7.4.7 Free and membrane bound ribosomes.
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.
Activation specificity:how does the tRNA attach to the correct amino acid.
shape of each tRNA is different.
shape of the tRNA is defined by the loop and the helical sections.
shape of the tRNA selects a specific enzyme (aminoacyl-tRNA synthetase).
the 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 is an example of a degenerate code.
Proteins and Ribosomal RNA combine in the structure
Large sub-unit and a small sub-unit
Large sub-unit has three binding sites for tRNA molecules ( E, P and A site).
Small sub-unit has a binding site for mRNA
Ribosome Function:
Ribosomes are enzymes.
The catalyse the translation of mRNA into a polypeptide.
Their substrate is mRNA.
Each ribosome can catalyse the transcription of different mRNA.
There are three stages in translation ( just as in transcription)
Initiation:
In which the ribosome, tRNA and mRNA come together to begin the translation of the mRNA.
Elongation:
tRNA molecules attach to the mRNA based on the codon-anticodon recognition. Amino acids are brought together and polymerised into the primary structure of the polypeptide.
Termination:
mRNA and the ribosomes detach from one another. The polypeptide is released and the tRNA return to be charged with more amino acid.
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.
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 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-(Polypeptide) 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.
Both units of the ribosome are now located over the second and third codons
(a) The tRNA for methionine is on the E site and is released from the ribosome. It will beceom recharged with methionine in the cell cytoplasm.
The tRNA for proline is in the P site.
(b) The A site for the next tRNA is free and holds the codon base sequence GCU.
The tRNA for Alanine has the anti-codon CGA which is complementary to the codon on the A site.
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.
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.
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 ne placed into vesicles and secreted form the cell.