Topic 7.4: Translation Flashcards
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 (5)
Each different tRNA molecule has a unique shape and chemical composition that is recognised by a specific tRNA-activating enzyme The enzyme (aminoacyl-tRNA synthetase) first binds the amino acid to a molecule of ATP (to form an amino acid-AMP complex linked by a high energy bond) The amino acid is then transferred to the 3’-end of the appropriate tRNA, attaching to a terminal CCA sequence on the acceptor stem and releasing the AMP molecule The tRNA molecule with an amino acid attached is thus said to be ‘charged’ and is now capable of participating in translation The energy in the bond linking the tRNA molecule to the amino acid will be used in translation to form a peptide bond between adjacent amino acids
What is the role of ribosomes?
Ribosomes are made of protein (for stability) and ribosomal RNA (rRNA - for catalytic activity)
What is contained in the small subunit of a ribosome?
The small subunit contains an mRNA binding site
What is contained in the large subunit of a ribosome?
The large subunit contains three tRNA binding sites - an aminacyl (A) site, a peptidyl (P) site and an exit (E) site
Where are ribsomes found?
Ribosomes can be either found freely in the cytosol or bound to the rough ER (in eukaryotes)
What is the difference in size of ribsomes between eukaryotes and prokaryotes?
Ribosomes differ in size in eukaryotes and prokaryotes (eukaryotes = 80S ; prokaryotes = 70S)
What are the four main steps involved in translation?
intiation, elongation, translocation, and termination
What happens during intiation? (4)
Involves the assembly of an active ribosomal complex Initiation: The small ribosomal subunit binds to the 5’ end of mRNA and moves along it until it reaches the start codon (AUG) Next, the appropriate tRNA molecule binds to the codon via its anticodon (according to complementary base pairing) Finally, the large ribosomal subunit aligns itself to the tRNA molecule at its P-site and forms a complex with the small ribosomal subunit
What happens during elongation? (2)
New amino acids are brought to the ribosome according to the codon sequence\ Elongation: A second tRNA molecule pairs with the next codon in the ribosomal A-site The amino acid in the P-site is covalently attached via a peptide bond to the amino acid in the A-site
What happens during translocation? (4)
Amino acids are translocated to a growing polypeptide chain Translocation: The ribosome moves along one codon position, the deacylated tRNA moves into the E-site and is released, while the tRNA bearing the dipeptide moves into the P-site Another tRNA molecules attaches to the next codon in the newly emptied A-site and the process is repeated The ribosome moves along the mRNA sequence in a 5’ - 3’ direction, synthesising a polypeptide chain Multiple ribosomes can translate a single mRNA sequence simultaneously (forming polysomes)
What happens during termination? (4)
At certain “stop” codons, translation is ended and the polypeptide is released Termination: Elongation and translocation continue until the ribosome reaches a stop codon These codons do not code for any amino acids and instead signal for translation to stop The polypeptide is released and the ribosome disassembles back into subunits The polypeptide may undergo post-translational modification prior to becoming a functional protein
In what direction does translation occur?
The start codon (AUG) is located at the 5’ end of the mRNA sequence and the ribosome moves along it in the 3’ direction Hence translation occurs in a 5’ - 3’ direction
Explain the process of translation (14)
Pre-Initiation: Specific tRNA-activating enzymes catalyse the attachment of amino acids to tRNA molecules, using ATP for energy Initiation: The small ribosomal subunit binds to the 5’ end of mRNA and moves along it until it reaches the start codon (AUG) Next, the appropriate tRNA molecule binds to the codon via its anticodon (according to complementary base pairing) Finally, the large ribosomal subunit aligns itself to the tRNA molecule at its P-site and forms a complex with the small ribosomal subunit Elongation: A second tRNA molecule pairs with the next codon in the ribosomal A-site The amino acid in the P-site is covalently attached via a peptide bond to the amino acid in the A-site Translocation: The ribosome moves along one codon position, the deacylated tRNA moves into the E-site and is released, while the tRNA bearing the dipeptide moves into the P-site Another tRNA molecules attaches to the next codon in the newly emptied A-site and the process is repeated The ribosome moves along the mRNA sequence in a 5’ - 3’ direction, synthesising a polypeptide chain Multiple ribosomes can translate a single mRNA sequence simultaneously (forming polysomes) Termination: Elongation and translocation continue until the ribosome reaches a stop codon These codons do not code for any amino acids and instead signal for translation to stop The polypeptide is released and the ribosome disassembles back into subunits The polypeptide may undergo post-translational modification prior to becoming a functional protein
What is the purpose of the free ribosomes synthesize proteins? (3)
Ribosomes floating freely in the cytosol produce proteins for use within the cell Ribosomes attached to the rough ER are primarily involved in producing proteins to be exported from the cell or used in the lysosome These proteins contain a signal recognition peptide on their nascent polypeptide chains which direct the associated ribosome to the rough ER
Draw and label a diagram showing the structure of a peptide bond between two amino acids
