By Sly Tutor – Updated Aug 30, 2022
When a gene is expressed, the DNA sequence is first transcribed into messenger RNA (mRNA). Transfer RNA (tRNA) then decodes this mRNA, attaching the appropriate amino acid to a growing polypeptide chain. The variety of tRNA species is essential for faithfully translating the genetic code into functional proteins.
DNA is composed of four nucleotides—adenine, guanine, cytosine, and thymine. These nucleotides form triplets called codons, and with four possible bases in each position there are 43 = 64 theoretical codons. However, several codons encode the same amino acid, a feature known as “wobble.” This redundancy means the cell needs fewer than 64 distinct tRNAs, but still a diverse set to cover all codons.
Each codon specifies a single amino acid. tRNA molecules bridge the genetic code and the amino acid repertoire by binding a codon on one end and carrying the corresponding amino acid on the other. Humans employ 20 standard amino acids, and the tRNA repertoire must accommodate every codon that directs each of these amino acids.
Three codons—UAA, UAG, and UGA—serve as stop signals, terminating polypeptide synthesis. Although they do not encode amino acids, the translation machinery requires specialized tRNA-like factors to recognize these stop codons and release the completed protein.
Some organisms incorporate additional amino acids beyond the standard 20. A notable example is selenocysteine, the 21st amino acid, which is inserted at UGA codons. The unique selenocysteine‑tRNA initially pairs with serine and is later modified to selenocysteine. Dedicated translational elements ensure that UGA is read as selenocysteine rather than as a termination signal, allowing proteins to include this essential trace element.
