Flashcards in Exam 4: Lecture 2 Deck (13)
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1
Terminating Translation
-ribosome must discriminate between sense codons and stop codons
-stop codons not recognized by tRNA only recognized by release factors
-at every codon ribosome is inserting tRNAs and release factors
-stop codon lined up within A pocket, no tRNA properly base pairs and all tRNAs expelled
-when release factor brought into A pocket, recognizes stop codon and stimulates release of protein from ribosome
-codon in A pocket not stop codon, release factors expelled and ribosome continues
-different release factors recognizes stop codons in prokaryotes
-all stop codons recognized by single release factor in eukaryotes
2
Ribosome Recycling
-disassociation must happen to ribosome after polypeptide/protein is released from ribosome
-required for ribosome reuse
3
RRF
-release of protein chain stimulates insertion of ribosome recycling factor (RRF) into A pocket (which still contains stop codon)
-RRF bound to elongation faction G which stimulates release of two tRNAs in E and P pockets
-RRF and EF-G displaced from ribosome
-mRNA releases and two halves of ribosome disassociate
4
Why ribosomal disassociation?
-allows for free small ribosomal subunit to be bound by IF1 or IF3 (pro) or elF1 or ElF1A (euk)
-block E and A pockets and are important in building pre-initiation ribosome complex
5
Antibiotics
-naturally occurring used to combat bacterial infections
-many target components of ribosomal and translation machinery
-some specifically target prokaryotic cells (bacterial) while others inhibit translation in both pro and euk. (side effects)
6
Puromycin
-can function similar to tRNA and can insert into A side of ribosome
-can serve as target for formation of peptide bond
-growing polypeptide chain added to the antibiotic
-prevents additional amino acids from being attached to protein
-causes disassociation of truncated protein (usually non-functional) from ribosome
-overall translation levels reduced, cell dies
7
Exon Junction Complex
-set of proteins that are deposited on mRNA transcript ~20 nucleotides upstream of each exon-exon junction during splicing
-don't recognize specific sequence, but interact with splicing factors at exon-intron junctions
-introns spliced out, EJC proteins deposited near remaining exon-exon junction which serve as guideposts for advancing ribosome
-contact by ribosome with EJC signals there are more exonic sequences to translate
8
Nonstop Mediated Decay
-no stop codon so ribosome continues to translate whole transcript including 3' UTR and Poly A tail
-many lysines added which is interpreted as a signal to degrade both protein and mRNA template
9
Nonsense Mediated Decay
-change of a sense codon into a stop codon (nonsense mutation)
-ribosome figures out if stop codon is correct or nonsense mutation
-achieved through interactions with EJC
-ribosome reaches stop codon, determines if EJC proteins are ahead, if no ribosome terminates translation normally, if yes mutant mRNA degraded by RNA specific exonnucleases
10
Nonsense-suppressor tRNAs
-arise when mutations within tRNA genes change anticodon sequence
-study example in notes
-nonsense suppressor mutations results in fully functional protein and wild type animal in forced experiments in lab
11
Nonsense Supression
-suppress nonsense stop codons by mutation sin eukaryotic release
-release factors do not recognize stop codons and in rare instances ribosome will accept insertion and pairing of amino acid bearing tRNA with stop codon
-means in rare instances mutation within release factor can result in rescue of a loss of function mutant
12
Drosophila Example
-ventral side of embryo patterned with alternating zones of naked cuticle and denticle belts
-formation is result of careful gene expression patterns and reflect segmentation of animal into anterior and posterior segments
-wingless (wg) gen critical in process
-if you remove wg you lose all naked cuticle segments and have embryo half the size only with denticle belts
-overexpose wg along ventral surface, eliminate all denticle belts and left with only naked cuticle
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