Translation Flashcards
What are the key elements required for translation?
- Genetic Code
- Messenger RNA
- Transfer RNA
- Ribosome
What is the genetic code?
Specifies the relationship between the sequence of nts in the mRNA and the amino acids in the protein
What is the genetic code read in and what are these called?
Code is read in groups of 3 nucleotides known as codons
How many codons is there in the code?
64 different codons including 3 stop codons - more codon potential than needed for amino acids
Code is almost universal and is degenerate
What does the messenger RNA have?
-Bacterial mRNA has a 5’ untranslated region (5’UTR) where the ribosome binding site is and a 3’ untranslated region (3’UTR) which is after the stop codon at the end of coding sequence
-Start codon (methionine) is a few nts from RBS - ribosome binding site
Where is the start and stop codon found in mRNA?
-Start codon (methionine) is a few nts from (after) RBS
-Stop codon is at end of coding sequence followed by the 3’UTR
If a start codon was missing from a gene, how would that affect transcription and translation?
-Transcription wouldnt be affected
-Translation wouldnt take place
What is the structure of tRNA?
Structure of pro and euk tRNA have common fts
-3 stem loops and a fourth stem with a single stranded region
-Anticodon at end of stem-loop 2 (3 base sequence complimentary to codon in mRNA)
-3’ single stranded region is the acceptor sites for amino acid when tRNA is charged
What catalyses the attachment of amino acids to tRNA
Enzymes called aminoacyl-tRNA synthestases catalyse the attachment of amino acids to tRNA
20 different enzymes for the different amino acids e.g Alanyl-tRNA synthetase
ATP binds to enzyme aswell
What is an anticodon?
A 3 base sequence complementary to a codon in mRNA
Process of charging tRNAs
- Amino acid and ATP bind to enzyme
- Enzyme catalyzes coupling of amino acid to AMP to form aminoacyl-AMP. 2 phosphates are lost in the reaction
- Uncharged tRNA binds to the enzyme
- Enzyme transfers amino acid from aminoacyl-AMP to tRNA to form aminoacyl-tRNA (aa-tRNA)
- The aa-tRNA and AMP are released from the enzyme
- Enzyme returns to its original state
Ribosome in bacteria vs eukaryotic cells
-Bacteria have one type of ribosome which translates all mRNA in cytoplasm- ribosomal proteins and rRNA are made and asembled in cytoplasm
-Euk cells have distinct ribosomes found in various cell compartments
-Ribosomal proteins are made in cytoplasm and moved to nucleus to assemble w rRNA to make subunits
-Assembly is then in cytoplasm after export of subunits out
During bacterial translation where does the mRNA lie?
Within the space between the 30S and 50S subunit
-The synthesised polypeptide exits through a site in the 50S subunit
What are the 3 site model of ribosome structure?
-Peptidyl (P site)
-Aminoacyl (A site)
-Exit (E site)
What are the 3 stages of Translation?
- initiation
- elongation
- termination
What happens in initiation?
-mRNA binds to the small ribosomal subunit- occurs due to a short ribosomal binding sequence (RBS) near the 5’ end of mRNA pairing w a short sequence in 16S rRNA
-Initiator tRNA binds to the start codon in mRNA (start codon few nts downstream of RBS)
-Large ribosomal subunit binds and initiator tRNA is in the P site
Where will the start codon be found?
In the P site
How does initiation in euks differ from in bacteria?
- Instead of RBS, euk mRNA have a 5’ guanosine cap that is recognised by cap binding proteins to promote binding of mRNA to the small ribosomal subunit
- The start codon can be a variable distnace from RBS
- The starting tRNA is methionine as opposed to formyl-methionine in bacteria
What happens in Elongation?
Involves covalent bonding of amino acids one at a time to create a polypeptide
-the polypeptide that has been synthesised is occupying the P site attached to a tRNA (peptidyl-tRNA)
-a charged tRNA binds to the mRNA in the A site of the ribosome (energy as GTP and elongation factors required)
-a peptide bond is formed between the amino acid in the A site and the growing polypeptide chain
-the polypeptide is removed from the tRNA in the P site and transferred to the amino acid in the A site (peptidyl transfer rxn catalysed by a region of 50S subunit known as peptidyltransferase centre)
-translocation of ribosome towards the 3’ end of mRNA by exactly 1 codon
-the tRNA in P and A sites shift to E and P sites respectfully
-next codon in mRNA is exposed in unoccupied A site (back to start)
-uncharged tRNA exits the E site
-next charged tRNA enters the empty A site and same series of elongation steps take place
-elongation continues until stop codon moves into A site
What happens in termination?
Which 3 stop codons does tRNA not recognise?
-Stop codons UAA, UAG, UGA are not recognised by a tRNA
-These codons are recognised by release factors (release factor binds to stop codon at the A site)
-The completed polypeptide is in the P site and is released from the tRNA
-The mRNA, ribosome subunits and the release factors dissociate
