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Ana Luis |
Genetic code
Describe the main features of the genetic code.
Triplet code (codons of three nucleotides), non-overlapping, read continuously, nearly universal, and unambiguous (each codon specifies one amino acid or stop).
What is degeneracy of the genetic code and its biological significance?
Multiple codons encode the same amino acid; this reduces the impact of point mutations and translation errors.
Translation and tRNA
What is translation?
The process by which ribosomes synthesize proteins using mRNA as a template.
What is a tRNA and what is its function in protein synthesis?
A transfer RNA that carries a specific amino acid to the ribosome and matches it to the mRNA codon.
What are the general characteristics of a tRNA?
Small (~75 nt), cloverleaf secondary structure, L-shaped 3D structure, anticodon loop, and 3′-CCA end.
Why is the 3′ CCA region called the acceptor arm?
It is the site where the amino acid is covalently attached to the tRNA.
What is the wobble effect and which base determines it?
Flexible base pairing at the third codon position; determined by the first base (5′ end) of the anticodon.
Why are aminoacyl-tRNA synthetases the ‘true readers’ of the genetic code?
They ensure correct amino acid is attached to the correct tRNA, enforcing codon–amino acid fidelity.
How do aminoacyl-tRNA synthetases work?
Two-step reaction: amino acid activation with ATP, then transfer to tRNA; editing site removes mischarged amino acids.
Ribosomes
What is a ribosome and what are its components?
A ribonucleoprotein complex of rRNA and proteins; composed of large and small subunits.
Which ribosomal components are critical for structure and function?
rRNA forms the structural core and catalyzes peptide bond formation.
Describe the A, P, and E sites.
A-site binds aminoacyl-tRNA; P-site holds peptidyl-tRNA; E-site releases empty tRNA.
Differences between bacterial and eukaryotic ribosomes?
Bacterial: 70S (30S+50S); eukaryotic: 80S (40S+60S).
How can ribosomes be targets for antibiotics?
Antibiotics selectively bind bacterial ribosomes, inhibiting translation without affecting eukaryotic ribosomes.
Protein translation mechanism
What are the steps of protein translation?
Initiation, elongation, and termination.
Characteristics of the bacterial initiation region?
Shine–Dalgarno sequence upstream of start codon aligns mRNA with ribosome.
Why is the reading frame established during initiation?
Start codon positioning defines triplet grouping for the entire mRNA.
How does protein initiation start and what is the role of initiation factors?
Initiation factors assemble ribosomal subunits at the start codon and recruit initiator tRNA.
Role of elongation and translocation factors?
Deliver aa-tRNAs (EF-Tu/eEF1A) and move ribosome along mRNA (EF-G/eEF2).
Role of release factors?
Recognize stop codons and trigger peptide release.
What is a polysome and its significance?
Multiple ribosomes translating one mRNA simultaneously, increasing efficiency.
Differences between bacterial and eukaryotic protein biosynthesis?
Different ribosomes, initiation mechanisms, factors, and cellular localization.
How does streptomycin inhibit protein biosynthesis?
Binds 30S subunit, causes misreading of mRNA, and blocks initiation.