7.3: Translation
Ribosome review…
What are ribosomes composed of?
 How many parts are the to a ribosome
during translation?
 How many tRNA binding sites are inside
the ribosome? What are they called?

tRNA structure

Referred to as the ‘clover
leaf’ structure (fig. 3, p.g.
363)
On your diagram, highlight the anticodon and the
site for attaching an amino acid.
What bonds exist inside the tRNA molecule?
What is an advantage of having this type of bond in
this location?
Enzymes

A enzyme is a biological catalyst.
*remember* enzymes are highly specific, each
enzyme will only function at a optimum
temperature, pH and each will bond with a
specific molecule = substrate.
tRNA activating enzymes

There are 20 different tRNA activating enzymes.
Why?

Fig. 5, pg. 365. shows a tRNA molecule ‘collecting’
an amino acid via the tRNA activating enzyme.
Energy is required for this process. Energy is
supplied by ATP attaching to the enzyme at the
same time as the amino acid. The ATP is then
hydrolysed, which will provide the necessary energy
for the formation of a covalent bond between the
amino acid and the incoming tRNA. Once the
bonding is complete, the activated tRNA exits
the enzyme along with an AMP molecule.

Stages of translation
Initiation
 Elongation (some times referred to as
elongation/translocation phase)
 Termination


Use the diagrams/notes on pg. 364/5 to
define these stages.
1. Initiation - The process starting
An activated amino acid (methionine) attached to a tRNA with the
anitcodon UAC. Combines with a small ribosomal subunit and an
mRNA strand.
A large ribosomal subunit combines as well, producing a translation
initiation complex.
Initiation factors are proteins which join the complex together.
Subunits moves down mRNA strand until start codon is reached
(AUG). Hydrogen bonds form between the initiator tRNA and the
start codon.
Energy in this case does not come from ATP. It comes from Guanosine
triphosphate (GTP) which is another energy rich molecule – very
similar to ATP.
2. Elongation - the chain growing
tRNA’s bring amino acids to the mRNA-ribosomal complex.
Order specified by the codons.
Proteins called elongation factors assists binding tRNA’s to
the mRNA codons at site A.
Initiator tRNA then moves to site P.
Ribosomes catalyse the formation of peptide bonds
between amino acids.
3. Translocation
tRNA relocating within the ribosome
Takes place during elongation
tRNA moving from site A  P  E.
Occurs in a 5’ to 3’ direction so ribosomal
complex is moving along the mRNA strand
towards the 3’ end.
4. Termination
The process ending
There are three stop codons.
When one reaches site A, a release factor (protein) fills
site A.
The release factor does not carry amino acid. It causes the
bond linking the the tRNA to the P site to hydrolyse.
Releasing the polypeptide from the ribosome.
The ribosome then separates form the mRNA and splits
into it’s subunits.
More about ribosomes…
Described as either free or bound.
FREE = produce proteins for use within the cell
(cytoplasm, mitochondria, chloroplast) translation takes
place in the cytoplasm (cytosol) or the endoplasmic
reticulum.
BOUND = produce proteins which will be secreted or
used in the lysosomes. Translation occurs on ribosomes
attached to the ER.
Signal sequences on the amino acid being translated
determines the site of translation. See fig. 11, pg. 366.
Polysomes

When more than
one ribosome is
attached to a
single mRNA.
In prokaryotes, polysomes appear immediately after
transcription. Why?
In eukaryotes, polysomes appear in either the cytoplasm or
next to the ER
Protein structure
Primary organization
A simple chain of amino acids attached by peptide
bonds.
Polypeptide chains can have hundreds of amino acids.
Primary structure influences the next three
structures. Changing just one amino acids can have
drastic effects of the structure of proteins.
Sickle cell anaemia - when a single amino acid in
haemoglobin is changed. Result - RBC no longer able
to carry oxygen.
Secondary organization
Hydrogen bonds form between oxygen from the
carboxyl group on one amino acid and the
hydrogen from the amino group on another amino
acid.
Most common secondary structure are the alphahelix and beta-pleated sheets.
Regular repeating pattern.
Sketch fig. 16 pg. 370.
The Bonds
Bond
Description
Covalent
Between sulphur atoms,
forming disulphide bridges –
very strong
Between polar side chains
Hydrogen
Van der Waals
Interactions among
hydrophobic side chains of
amino acids. Strong
interactions (hydrophobic
side chains forced inwards,
when hydrophilic chains
interact with water on the
outside of the molecule)
Ionic
Between positive and negative
side chains
Tertiary organization
Polypeptide folds and coils to form a complex
molecular shape (3D)
Caused by interactions between R groups;
including H-bonds,Van der Waals, disulphide
bridges, ionic bonds and hydrophilic /
hydrophobic interactions
Tertiary structure may be important for the
function of the enzyme (e.g. specificity of active
site in enzymes)
Quartenary organization
Interactions between multiple polypeptides or prosthetic
groups that results in a single, larger, biologically active protein
A prosthetic group is an inorganic compound involved in
protein structure or function (e.g. the heme group in
haemoglobin)
A protein containing a prosthetic group is called a
conjugated protein
Quaternary structure may be held together by a variety of
bonds (similar to tertiary structure)
Protein Data Bank (PDB)
Google protein data bank, or use this link
http://www.rcsb.org/pdb/home/home.do

Search for either thermus thermophilus ribosome or
tRNA ribosome.
See if you can view the large and small subunits using
this software.
Also available on the website (companion to the
textbook!) www.oxfordsecondary.co.uk/ib-biology