Lecture 1.11 Flashcards
Why is the genetic code considered degenerate?
There are multiple codons for some amino acids, thus there are more codes than it needs
-61 codons for 20 amino acids
What is wobble base pairing?
5’ position of the anticodon tolerates mismatches or non-standard base pairing
Nonsense mutation
Change an amino acid to a stop codon
- terminates protein prematurely
- almost always alters protein function
missense mutation
Change an amino acid due to a base change
- allows protein synthesis
- doesn’t always alter protein function
What are two sources of specificity in formation of aminoacyl-tRNAs?
- high specificity
- tRNA synthetases have editing activity (hydrolyase)
What are three peptide binding sites in a ribosome and what is their function?
E-site: things exit the ribosome
P-site: growing polypeptide chain located here (also where the intiation complex attaches)
A-site: incoming tRNAs bind here
What are the general steps in translation initiation?
- protein synthesis initiates at AUG (met) codon
- specialized methionine tRNA binds to small subunit of ribosome (in the P-site)
- small subunit binds to cap of mRNA and treks along until it encounters the first AUG codon
- eIFs dissociate and large subunit binds
- aminoacyl-tRNA binds to A site and first peptide bond forms
Steps of elongation
- nascent polypeptide in P-site and a new aminoacyl-tRNA binds to the A-site
- New peptide bond is formed by peptidyl transferase (ribozyme) and conformationals change causes the tRNA to move to the E- and P- sites
steps of termination?
- stop codons mar the end of translation
- release factors bind to ribosomes wit top codon in A-site
- release factor causes peptidyl transferase to hydrolyze the last amino acid releasing the completed protein and resulting in disassociation of the ribosome
- release factors are proteins that are shaped like a tRNA
How much energy is utilized to synthesize a peptide bond in a protein?
4 high energy bonds per every new peptide bond
Where is the energy needed to synthesize a peptide bond consumed in the cell?
- 2 phosphate bonds cleaved from ATP –> AMP to charge the tRNA (put amino acid on tRNA)
- 1 phosphate bond cleaved from GTP –> GDP to dissociate the initiation factors and allow the large ribosomal subunit to bind to the initiation complex
- 1 phosphate bond cleaved from GTP–> GDP to translocate the mRNA and tRNA
suppressor
mutated tRNA
- Nonsense suppressor: mutated anticodon complements terminator sequence (can inhibit effects of nonsense mutation
- Frameshift suppressor: anticodon contains 4 residues and can insert an amino acid and restore a frame sift mutation
What differences in initiation of translation allow prokaryotes but not eukaryotes to have polycistronic mRNA?
- Prokaryotic ribosomes can bind at any ribosome-binding site (rbs - which is upstream from AUG) on the mRNA and start protein translation of multiple proteins in a single mRNA
- Eukaryotic ribosomes must attached at the 5’ cap
What is RNA editing ?
the modification of RNA after synthesis: modification of bases to change “meaning” of message
-molecular process through which some cells can make discrete changes to specific nucleotide sequences within an RNA molecule after it has been generated by RNA polymerase.
what is nonsense-mediated mRNA decay?
a pathway that acts on transcripts right after splicing to degrade RNAs containing nonsense mutations
-a surveillance pathway that exists in all eukaryotes. Its main function is to reduce errors in gene expression by eliminating mRNA transcripts that contain premature stop codons.
