Protein Synthesis Flashcards
What is the genetic code?
- Determine how RNA code is converted to amino acid sequences
What are the rules for the genetic code?
- Code is degenerate and unambiguous
- Code is non-overlapping
- Start codon AUG sets reading frame and ends with stop codon UGA, UAG, UAA
- Order of codons in mRNA determines amino acid sequence
What does it mean for the genetic code to be degenerate and unambiguous?
- Amino acids - more than 1 codon
- Each codon specifies only 1 amino acid
What does it mean for the code to be non-overlapping?
- Each nucleotide only read once
- Codons read sequentially
What causes mutations and what can they cause?
- Occur because of DNA damage and DNA replication errors
- Transcribed to mRNA - results in proteins with abnormal sequences
What are the three types of point mutations?
- SILENT - Single base change - produces same amino acid
- MISSENSE - SIMILAR TO SILENT - produces different amino acid
- NONSENSE - Change forms premature stop codon
What are the two types of frameshift mutations?
- INSERTION - ADDITION of 1 or more bases - protein with more amino acids
- DELETION - REMOVAL of 1 or more bases - protein with fewer amino acids
- Cause shift of reading frame - amino acid sequence altered
Outline initiation
- mRNA migrates to and binds to ribosome
- tRNA carries specific amino acid to ribosome
- Anticodon on tRNA binds to mRNA codon
TWO TY
Outline elongation and terminastion.
- Ribosome catalyses amino acid bonding to adjacent aa to form polypeptide chain
- tRNA releases
- Ribosome reaches stop codon - translation ends/polypeptide chain released
What is tRNA?
- Adaptor molecule - carries amino acid to ribosome and binds to codon
What are the 4 key regions of a tRNA molecule?
- Acceptor stem - carries amino acid at 3’ CCA end
- Anticodon - associate with mRNA coodon by complementary base pairing
- T arm - associate with ribosome at E,P and A sites
- D arm - associate with tRNA activating enzyme
Outline eukaryotic ribosomal structure.
- Made up of rRNA: 60S and 40S subunits
- A site - binds tRNA carrying amino acid
- P site - binds tRNA attached to growing peptide chain
- E site - releases tRNA carrying last amino acid
Outline chain initiation. PART 1
- Initiator tRNA-methionine (UAC) loaded onto 40S subunit, with eukaryotic initiation factors (eIFs)
- 40S binds to 5’ end of mRNA by recognizing 5’ cap
- mRNA scanned from 5’-3’ for start codon
- Translation begins at start codon AUG, eIFs dissociate
Outline chain initiation. PART 2
- 60S assembles to complete ribosome
- Initiator tRNA-Met is bound to P-site, A-site vacant
- Aminoacyl tRNA binding
Outline chain elongation. PART 1
- 2nd complementary tRNA binds to vacant A site
- Met in P site covalently attach to amino acid in A site (catalyzed by peptidyl transferase in 50S)
- tRNA in P site is deacylated and peptide chain is in A site
- Ribosome translocate (5’- 3’) and shift tRNAs to E and P sites (via EF and GTP hydrolysis)
Outline chain elongation. PART 2
- tRNA is released from E site
- Peptide chain is in P site
- A site is open for next aminoacyl-tRNA
- Polypeptide is built from N-terminal to C-terminal direction
Outline chain termination.
- Stop codons signal to ribosome to stop translation
- Release factor (RF) bind to A site on stop codon
- Carboxyl end of the polypeptide chain is freed from tRNA
- Completed chain is released into the cytoplasm
- Ribosome releases mRNA and dissociates into its subunits
What are polysomes?
- Structures in prokaryotes where multiple ribosomes simultaneously translate a single mRNA
Describe protein folding.
- N-terminal folds whilst C-terminal still synthesising
- Molecular chaperons bind to hydrophobic residues and make folding more efficient
Describe post-translational modifications.
- Cleavages e.g of Met at N-terminal activate protein
- Increase protein stability
- Signal sequence added to protein to direct it to a cellular component
Give examples of post-translational modifications.
- PHOSPHORYLATION
- HYDROXYLATION
- METHYLATION
- GLYCOSYLATION
Describe proteasome-mediated protein degradation.
- Abnormal/misfolded proteins tagged with ubiquitin degraded by proteasomes
Describe the process of transporting signal peptides to their desired destinations.
- SRP binds to signal peptide and translation pauses
- Polypeptide transported to ER where translation continues
- Protein transported to Golgi apparatus and sent to target destinations.
