7.4 – Translation
7.4.1 – Explain that each tRNA molecule is recognised by a tRNA-activating enzyme that binds a specific amino acid to the tRNA, using ATP for energy
Each amino acid has a specific tRNA-activating enzyme. Before they are used in translation,
amino acids are attached to a matching tRNA molecule.
On the 3i end of the molecules, the base sequence CCA appears, which binds to the amino acid with the aid of the activating enzyme. The tRNA molecules recognise the correct amino acid due the variation in their shape. They will match to a certain enzyme.
The attachment of the amino acid requires ATP for the enzyme to induce the reaction.
The anticodon is three bases long, and uses complementary base pairing to match to the code on the mRNA molecule. Note that the DNA code is degenerate, since a single amino acid may bind to multiple tRNA molecules.
7.4.2 – Outline the structure of ribosomes, including protein and RNA composition, large and small subunits, three tRNA binding sites and mRNA binding sites
Ribosomes are primarily made up of two parts – the small subunit and the large subunit. The small subunit has the binding site for the mRNA molecule, whilst the large subunit has three binding sites for tRNA molecules. These three binding sites are called the E, P and A sites.
Ribosomes are enzymes for the translation of mRNA into a polypeptide. One ribosome may catalyse the translation of many different mRNA molecules.
7.4.3 – State that translation consists of initiation, elongation, translocation and termination
Initiation
During this stage, the mRNA molecule and the tRNA molecule both bind to the ribosome for translation to begin. The first tRNA molecule to bind to the small subunit carries the anticodon for the start codon. The small subunit then attaches to the mRNA molecule at the 5i end. The small subunit then slides along towards the 3i end until it reaches the start codon, and translation begins.
Elongation
The tRNA molecules will attach to the complementary bases on the mRNA, using codon to anticodon recognition. As this happens, the amino acids are brought together so that they can bond and form polypeptides. Once the amino acids form peptide bonds, they are detached from the tRNA by the large subunit. Throughout translation, the polypeptide will be held in position by a single tRNA molecule.
Translocation
Within the ribosome, there are three areas called the E, P and A sites. During translation, the tRNA will move through this, depending on which stage of translation it is at. The tRNA first enters at the A site. Then, as the peptide bonds form, the molecule will move to the P site. Finally, it will move to the E site to be release.
Termination
Eventually, the ribosome will reach the stop codon on the 3i end of the mRNA. Since there are no tRNA molecules with an anticodon for it, translation will stop. The mRNA and the newly synthesised polypeptide are released from the ribosome. The tRNA is also released, which will be reused and attach to another amino acid.
By this point, the polypeptide will have started to fold into the shape of the final protein. The subunits of the ribosome will also separate.
7.4.4 – State that translation occurs in a 5′ → 3′ direction
The small subunit of the ribosome moves along the large subunit and moves the three nucleotides along the mRNA. This always takes place in a 5i to 3i direction, meaning that it starts at the 5i end of the mRNA molecule and moves towards the 3i end. The genetic code is therefore translated in this direction. The start codon will always be near the 5i end.
7.4.5 – Draw and label a diagram showing the structure of a peptide bond between two amino acids
Peptide bonds form in a condensation reaction between amino acids, releasing a water molecule as a by-product. This forms the primary structure of the protein.
7.4.6 – Explain the process of translation, including ribosomes, polysomes, start codons and stop codons
As discussed before, translation begins at the stage of initiation. The mRNA molecule binds to the small subunit of the ribosome at the 3i end. The tRNA carrying Methionine binds as well, and will attach to the start codon (AUG). The large subunit will sit over it, with the tRNA in the A site until it binds and is in position.
The tRNA moves to the P site to add to the polypeptide chain. At the E site, the tRNA is then released without the amino acid. It will be activated and reused later. The anticodon specificity of the tRNA ensures that the right amino acids are added to the chain.
As the tRNA brings amino acids to the ribosome, these will form peptide bonds, making a polypeptide chain. This is the process of elongation.
Following this, the tRNA will be released from the amino acid, releasing energy in the process. The large subunit moves along the mRNA by three bases, or one codon, in the direction of the 3i end.
The tRNA in the E site is released into the cytoplasm of the cell. A new tRNA molecule is in the A site, ready to bind to the mRNA and add to the polypeptide chain. As the polypeptide chain grows, it will begin to fold and shape into the primary structure of the protein.
Once the stop codon is reached, no more tRNA molecules will bind to the mRNA, and translation stops. The process enters the termination stage, releasing the mRNA and new polypeptide chain, and the subunits will separate.
The protein is then sent to the Golgi apparatus, the endoplasmic reticulum or is secreted from the cell for use elsewhere.
7.4.7 – State that free ribosomes synthesize proteins for use primarily within the cell, and that bound ribosomes synthesise proteins primarily for secretion or for lysosomes
Within a cell, ribosomes can be found in a number of locations. Some are bound to the rough endoplasmic reticulum. These ribosomes will usually synthesise proteins that will be secreted from the cell in vesicles for use elsewhere.
On the other hand, other ribosomes may be found floating freely in the nucleus. These
normally synthesise proteins that will be used within the cell.