7.3 Translation Flashcards
What is the structure of a ribosome?
It has 2 sub units; one small, one large.
Three binding sites for tRNA on the surface of the ribosomes
There is a binding site for mRNA on the surface of the ribosome.
Each ribosome has three sites
E
P
A
What are the three sites in a ribosome?
E
P
A
How do you activate a tRNA molecule?
Each tRNA molecule is recognised by a tRNA activating enzyme that attaches a specific amino acid to the tRNA using ATP for energy.
The base sequence of tRNA molecules varies and this causes some variability in structure. Activation of a tRNA molecule involves the attachment of an amino acid to the 3’ terminal of the tRNA by an enzyme called a tRNA-activating enzyme.
There are 20 different tRNA activating enzymes that are each specific to one of the 20 amino acids and the correct tRNA molecule. The active site of the activating enzyme is specific to both the correct amino acid and the correct tRNA.
Energy from ATP is needed for the attachment of amino acids. Once ATP and an amino acid are attached to the active site of the enzyme, the amino acid is activated by the formation of a bond between the enzyme and the adenosine monophosphate. Then the activated amino acid is covalently attached to the tRNA. Energy from this bond is later used to link the amino acid to the growing polypeptide chain during translation.
How does translation work?
To begin the process of translation, an mRNA molecule binds to the small ribosomal sub unit at an mRNA binding site. An initiator tRNA molecule carrying methionine (amino acid) then binds at the start codon AUG.
The large ribosomal unit then binds to the small one. The initiator tRNA is in the P site (middle). The next codon signals another tRNA to bind. It occupies the A site. A peptide bond is formed between the amino acids in the P and A site.
Then elongation occurs through a series of repeated steps. The ribosome translocates three bases along the mRNA moving the tRNA in the P site to the E site, freeing it and allowing the next tRNA to occupy the vacant A site.
Termination then occurs, this happens when the stop codon is reached and the polypeptide is released.
What is the start codon?
AUG
Where is the P site?
Middle
What do free ribosomes do?
Free ribosomes synthesise proteins for use primarily within the cell. In eukaryotes proteins destined for use in the cytoplasm, mitochondria and chloroplasts are synthesised by ribosomes free in the cytoplasm.
What do bound ribosomes do?
Bound ribosomes synthesise proteins primarily for secretion or for use in lysosomes.
How do bound ribosomes become bound?
Whether a ribosome is free in the cytosol or bound to the ER depends on the presence of a signal sequence on the polypeptide being translated. It is the first part of the polypeptide translated. As the signal sequence is created it becomes bound to a signal recognition protein that stops the translation until it can bind to a receptor on the surface of the ER.
What are polysomes?
Structures visible in an electron microscope.
What is the primary structure?
The sequence and number of amino acids in the polypeptide is the primary structure.
What is the secondary structure?
The secondary structure is the formation of alpha helices and beta pleated sheets stabilised by hydrogen bonding.
Because the chain of amino acids in a polypeptide has polar covalent bonds within its backbone, it tends to fold in such a way that hydrogen bonds form between the carboxyl (C=O) group of one residue and the amino group in another part of the chain. This results in the formation of patterns within the polypeptide called secondary structures. The alpha helix and beta helix are examples.
What is the tertiary structure?
The tertiary structure is the further folding of the polypeptide stabilised by interactions between R groups.
It refers to the overall three dimensional shape of the protein. This shape is a consequence of the interaction of R - groups with one another and with the surrounding water medium. There are several different types of interaction:
- Positively charged R-groups will interact with negatively charged R groups.
- Hydrophobic amino acids will orientate themselves toward the centre of the polypeptide to avoid contact with water, while hydrophilic amino acids will orientate themselves outward.
- Polar R groups will form hydrogen bonds with other polar R groups
- The R group of the amino acid cysteine can form a covalent bond with the R group of another cysteine forming what is called a disulphide bridge.
What is quaternary structure?
The quaternary structure exists in proteins with more than one polypeptide chain.
Quaternary structure refers to the way polypeptides fit together when there is more than one chain. It also refers to the addition of non-polypeptide components. The quaternary structure of the haemoglobin molecule consists of four polypeptide chains and four heme groups.
