w6 slides fc Flashcards
substitutions/deletions in dna sequences
missense and nonsense cause big problems
nonsense= premature stop codon
3 nucleotide-pair deletion= no frameshift 1 AA missing
insertion causes STOP= immediate nonsense
2 sequential steps in ensuring fidelity
aminoacyl-tRNA synthetases
base pairing
error correction by aminoacyl tRNA synthetase
by hydrolytic editing
-able to recognize when wrong AA onto wrong tRNA and breaks bond
translation
energy stored in covalent bond b/w AA and tRNA in P site makes peptide synthesis energetically favourable
Aminoacyl (A) site - aa onto trna
Peptidyl (P) site - peptide bond formation happens
Exit (E) site
translation steps
newly bound tRNA enters A site, aminyl end translated first
new peptide bond
large subunit translocates (twists)
-tRNA in P site
eject tRNA and small subunit translocates
-peptide bond forms in A site
elongation
EF-Tu checks aminoacyl trna
-charged up–> moving by diffusion
-if base pairing NOT correct, EF-Tu not released, peptide bond can’t form
-if base-pairing correct, GTP hydrolyzed and EF-Tu released
–slight delay before formation of polypeptide bond allows one last check for accurate base-pairing
can ribosomes perform protein synthesis without the aid of elongation factors
yes but slower nd inefficient
role of elongation factors
improving speed and efficiency
error checking function
elongation factors are mediated by
gtp hydrolysis and release of EF-Tu and EF-G
ef-tu
binds aminoacyl-trna
ef-g
helps ribosome move mrna1 codon
intiation of translation in prokaryotes
shine-dalgarno sequences on mRNA base pair with rRNA in small ribosomal subunits
positioning of small ribosomal subunits to intiate AUG codons on mRNA also requires initiation factors (IFs)
fMethionine aminoacyl tRNA binds to initiator codon
-in the P site of small ribosomal subunit
large ribosomal subunit binds
initiation of translation in eukaryotes
small ribosomal subunit with translation initiation factors bound to A site
mRNA binding
small ribosomal subunit, with bound initiator tRNA, moves along mRNA searching for first AUG
translation initiation factors dissociate
large ribosomal subunit binds
charged tRNA binds to the 2nd codon (step 1)
first peptide bond forms (step 2)
translation termination
human translation release factor: protein, not tRNA
stop codon in A site
binding of release factor to the A site
termination triggers ribosome to add water molecule
ribosome dissociates
What is the significance of redundancy in the genetic code?
(A) It allows multiple amino acids to be encoded by a single codon
(B) It ensures that every codon specifies a unique amino acid
(C) It allows different codons to code for the same amino acid, reducing the impact of mutations
(D) It increases the likelihood of frameshift mutations
c
How does wobble base pairing contribute to codon redundancy?
(A) It allows a single tRNA to recognize multiple codons through flexible base pairing
(B) It prevents translation errors by restricting codon usage
(C) It increases the number of possible amino acids
(D) It requires separate tRNAs for every codon
Answer: (A) (Wobble pairing allows a single tRNA to recognize more than one codon, reducing the need for multiple tRNAs.)
How would an insertion of a single nucleotide in the coding region of a gene affect the reading frame?
(A) It would cause a frameshift, altering the downstream amino acid sequence
(B) It would replace only one amino acid and not affect the rest of the protein
(C) It would introduce a stop codon immediately
(D) It would not affect the protein at all
Answer: (A) (Single nucleotide insertions shift the reading frame, altering all downstream codons.)
Why are three-nucleotide insertions or deletions often less harmful than one-nucleotide insertions?
(A) They stop translation entirely
(B) They introduce more variability in the protein
(C) They preserve the reading frame, maintaining correct amino acid alignment
(D) They prevent translation initiation
Answer: (C) (Adding or deleting three nucleotides preserves the reading frame, maintaining most of the protein’s function.)
What ensures that a tRNA carries the correct amino acid?
(A) The ribosome proofreads aminoacylation before elongation
(B) The codon directly determines which amino acid binds to tRNA
(C) Aminoacyl-tRNA synthetases recognize the tRNA anticodon and acceptor stem before attaching the correct amino acid
(D) EF-Tu ensures the correct amino acid is added to the growing polypeptide
c
How does aminoacyl-tRNA synthetase correct mischarged tRNAs?
(A) It hydrolyzes incorrect aminoacyl-tRNA bonds using an editing site
(B) It degrades the misfolded protein
(C) It relies on ribosome error correction
(D) It prevents translation initiation
Answer: (A) (Aminoacyl-tRNA synthetases have an editing site to remove mischarged amino acids.)
How does the ribosome function as a ribozyme?
(A) The ribosome requires a separate enzyme for translation
(B) The ribosome uses ATP hydrolysis to form peptide bonds
(C) Ribosomal proteins carry out enzymatic activity
(D) The rRNA in the large subunit catalyzes peptide bond formation
d
What post-translational modification is essential for directing proteins to specific cellular locations?
(A) Polyadenylation
(B) Glycosylation
(C) Splicing
(D) DNA methylation
Answer: (B) (Glycosylation modifies proteins for secretion and membrane targeting.)
How do release factors terminate translation in bacteria?
(A) They bind to the 3′ poly-A tail and induce degradation
(B) They recognize stop codons and promote peptide chain release
(C) They act as a helicase to separate ribosomal subunits
(D) They prevent tRNA binding to the P-site
Answer: (B) (Release factors mimic tRNA structure and trigger translation termination.)
