23. Translation Flashcards
Define codon
CODON: a triplet of bases of mRNA which codes for one amino acid in a polypeptide chain, the order of codons determines the order of amino acids in polypeptide chain
64 possible codons = 43 = 61 for am a + START + 2 STOP
Define genetic code
GENETIC CODE: set of rules which are encoded within mRNA and converted into amino acids and polypeptides by living cells
START, STOP codons
Coding starts with START codon (Met) and terminates at STOP codon (no am a)
START: AUG (Met)
Define translation
TRANSLATION: process of protein synthesis in which genetic information encoded in mRNA is translated into a sequence of am a in a polypeptide chain
Translation sequence
- INITIATION: In cytoplasm smaller ribosome subunit binds to 5’ end of mRNA - moves along until START codon reached - activated tRNA with Met and complementary anticodon binds to START codon - larger ribosome subunits aligns at P site - forms a complex
- ELONGATION: another tRNA binds to A site - tRNA at P site deacylated (no am a) - moves into E site - released - am a covalently attached by peptide bond to am a in A site - polypeptide held by tRNA in A site ribosome moves in triplets - forms polypeptide chain
Translocation: ribosome moves 5’ to 3’ end mRNA in triplets
- TERMINATION: elongation is completed in repeating cycles until STOP codon is read (no tRNA) - disassembly of ribosome releases polypeptide
Termination diagram
tRNA activation
tRNA activation - am a attached
In cytoplasm tRNA activating enzyme binds ATP to specific am a - complex formed - specific tRNA bound to am a by an energetic bond the enzyme (E used to form peptide bonds) - activated tRNA released
Explain polysomes
POLYSOME (polyribosome): a group of two or more ribosomes translating an mRNA into polypeptides simultaneously
Both in bacteria and eukaryotes
Translation in eukaryotes vs prokaryotes
EUKARYOTES: DNA and mRNA are separated by nucleus, mRNA must be transported out of nucleus - post-transcriptional modifications needed
PROKARYOTES: no nucleus - mRNA can be started to be translated while it is still transcribed, possible because both happen 5’ to 3’ end
Proteins synthesised by free vs bound ER ribosomes
FREE ribosomes: proteins for cell use - intracellular
BOUND ER ribosomes: proteins for secretion out of cell - COTRANSLATIONAL IMPORT
Protein destination is determined by initial signal sequence - binds initial signal recognition particle (SRP) - halts translation - SRP-ribosome complex docs at receptor on ER membrane - rER forms - translation re-initiated - polypeptide continues to grow into rER lumen - SRP recycled
Protein from rER transported into Golgi/lysosome
What gives tRNA its 3D L shape?
Hydrogen bonds
What enzyme matches the right tRNA with the right am a?
Aminoacyl-tRNA synthetase (ex tyrosyl-tRNA synthetase - specific for each am a) - tRNA activation completed
Prokaryotic vs eukaryotic ribosomes
Very similar yet dissimilar enough for some antibiotics to target bacterial translation and not eukaryotic
What are the different binding sites in ribosomes?
A site - holds tRNA that carries the next am a to be added to the chain (aminoacyl-tRNA site)
P site - holds the tRNA which carries the growing polypeptide chain (peptidyl-tRNA site)
E site - discharged tRNA leaves the ribosome (exit)
Explain initiation of translation
When the large subunit binds - tRNA automatically in P site
Explain elongation of translation
5’→3’
very fast process
Explain termination of translation
What is the release factor?
In termination binds instead of another tRNA - polypeptide not passed on → hydrolysis → polypeptide released from the ribosome
Explain how and why transcription and translation can happen of the same mRNA
Coupled transcription and translation - in the same compartment (not in nucleus)
