Translation Flashcards
(11 cards)
Translation
• Translation is the conversion of information in the mRNA into protein information
• Results in the synthesis of new protein
• Machinery involved is the ribosome
– Complex organization of proteins and ribosomal RNA (rRNA)
tRNA
• Small RNA molecule
• Can fold up and form base pairs
• tRNA deciphers the code in mRNA
• Key features
– Anticodon loop
– Amino acid attachment site
Charging the tRNA
• Amino acid and ATP bind to the enzyme
• There are many varieties of the aminoacyl-tRNA synthetase enzyme
• At least one for each of the 20 amino acids
• Adenine Phosphate transferred to amino acid
• This is an input of energy
• The specific tRNA binds
• The AMP is released
• Amino acid is coupled to the tRNA
• There are many different tRNA
• Which one binds depends on the amino acid bound and the particular enzyme
• Charged tRNA is released
• The overall process is critical in translation
• The right amino acid and tRNA have to be joined
• Makes use of the exquisite selectivity of enzymes
The ribosome
• Machinery of synthesis
• Complex of many proteins and rRNA acting together
• Tunnel for the mRNA between the subunits
• Tunnel for the protein in the large subunit
• 3 binding sites for tRNA
Location of tRNA binding sites
• mRNA binds to small subunit
• A site, for incoming charged tRNA
• P site, for tRNA with growing peptide attached
• E site, exit point for used tRNA, now uncharge
Starting translation.
• mRNA binds to free small subunit
• Lined up in a very particular way
• Initiator tRNA-Met base pairs to start codon
• Large subunit binds, energy required
• Note tRNA-Met is now in P site
Cycle of synthesis
• Second tRNA binds into A site
• Binds in by base pairing to the mRNA
• In this way it reads the sequence of the mRNA
• The sequence is read in groups of 3 bases
• Each 3 bases is a codon
• Each codon specifies which tRNA comes in
• This means the amino acid coming into the A site is
related to the sequence of the bases in the mRNA
• The genetic code is deciphered, and translated
• Input of energy in the form of GTP
• The growing peptide is moved from tRNA in P site on to the free end of the amino acid on the tRNA in the A site
• New peptide bond is formed
• Synthesis occurs from N-terminus to C-terminus direction
• Now tRNA in A site has the growing peptide
• P site has uncharged tRNA
• Now the growing peptide is in the P site
• Uncharged tRNA is released from E site
• Uncharged tRNA in E site leaves
• Growing peptide on tRNA in P site
• Another input of energy
• Ribosome shunts along by one codon (3 bases)
Termination of synthesis
• Certain codons signal the ribosome to stop
• Stop codon comes into A site
• Release factor binds
• Energy input
• Complex dissociates
• New protein is released
Eukaryotic protein export
• Some proteins are for export
• They have a signal in their amino acid sequence (1o seq)
• With the help of SRP, ribosome can associate directly with export apparatus
• Protein is exported directly as it is synthesized
• Signal is removed as it goes through the membrane
• In prokaryotic cells this results in the release of the protein to the external environment
• In eukaryotic cells the ribosome is in association with the endoplasmic reticulum (ER), specifically the rough ER
• Proteins are initially transported into the lumen of the ER vesicles
Exocytosis
• In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents
• Many secretory cells use exocytosis to export their products
Polyribosomeof eukaryotes
Many ribosomes can translate an mRNA at the same time in a 5’ to 3’ direction. The resulting complex of many ribosomes simultaneously making many copies of a protein is called a polysome
