Covers the genetic code's key features: codons of three nucleotides, 64 possible codons with the AUG start and three stop codons, and the code being unambiguous, degenerate, non-overlapping and almost universal. Then explains tRNA as an adaptor with its clover-leaf structure, acceptor, D and T arms, and anticodon pairing, including wobble pairs involving inosine. Finishes with the two-step aminoacylation reaction by aminoacyl-tRNA synthetases and their two fidelity checks.
Topic 13 of 63 in the MU Pleven syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 3 sections, about 426 words (3 min of reading), with 20 flashcards.
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Genetic code. The role of tRNA.
Genetic code
Transfer RNA (tRNA) as an adaptor molecule:
Genetic code
• Characteristics of the genetic code:
Composed of 4 nucleotides on mRNA (A, U, G, C).
Read in triplets (3 nucleotides per codon).
64 possible codons. Start codon: AUG (also codes Met). Stop codons: UAG, UGA, UAA. Amino acids: 60 codons + AUG = 61.
Genetic code is unambiguous: each codon specifies one amino acid only.
Code is degenerate: one amino acid may be specified by multiple codons (e.g., serine = 6 codons; glycine = 4; lysine = 2).
Code is non-overlapping: each nucleotide is part of only one codon.
AUG start codon defines the reading frame: 5’ — AUG — (XXX)n — STOP — 3’.
Genetic code is almost universal.
• The amino acid sequence can be unambiguously determined from the nucleotide sequence, but the nucleotide sequence cannot be unambiguously determined from the amino acid sequence.
• Translation occurs 5’ → 3’. In bacteria, translation can begin before mRNA synthesis is complete → transcription and translation are coupled.
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