Читать книгу Algorithms in Bioinformatics - Paul A. Gagniuc - Страница 22

In both eukaryotes and prokaryotes, mRNA molecules, which contain the information structure for protein synthesis, are stochastically encountered by two ribosomal subunits that initiate the translation step. Once bound to an mRNA transcript, the two subunits form the ribosome. The ribosome is a ribonucleoprotein (made of RNA and proteins) organelle that facilitates the formation of chemical bonds between amino acids in the order specified by the information encoded in the mRNA molecule. Life evolved a molecular scheme for translation, known as the “genetic code” [47]. In this scheme, groups of three nucleotides are associated with different amino acids used for polypeptide synthesis. Each set of consecutive and nonoverlapping nucleotide triplets on the mRNA transcript is known as a codon. Polypeptide synthesis begins from a start codon, which initiates the position of the reading frame. Usually, the start codon is represented by the “AUG” triplet (representation with the highest frequency across all life). However, other triplet combinations (non-AUG start codons) can take the role of a start codon (with a lower frequency) [48]. Post initialization, the mRNA transcript slides in between the two ribosomal subunits by one codon at a time following the reading frame set by the start codon [49, 50]. Different versions of tRNAs present in various concentrations in the cytoplasm are each linked to an amino acid. The type of amino acid connected to a tRNA is associated with an anticodon, a special nucleotide triplet region from the tRNA destined for a temporary bind to an mRNA transcript. Thus, tRNAs are the temporary links between the mRNA transcript and the nascent amino acid chain. An assembled ribosome contains three “openings” (A, P, and E sites) for tRNA–mRNA interactions (Figure 1.3.b). The smaller subunit of the ribosome allows for a complementary between three nucleotides (the codon) on the mRNA transcript and three nucleotides (anticodon) of a tRNA molecule (Figure 1.3.b). Once the mRNA–tRNA binding has been facilitated by the smaller subunit, the amino acid transfer from a tRNA to the nascent amino acid chain is facilitated by the larger subunit of the ribosome [51]. The tRNA molecules with appropriate anticodons come into contact through complementary with the mRNA transcript.

The amino acid chain is passed from the previous tRNA to the amino acid of the next incoming tRNA, increasing the growing peptide by one amino acid on each switch. Thus, the amino acid chain remains attached to the most recently bound tRNA and is not released until a termination codon appears in the mRNA transcript (UAA, UAG, UGA) [56]. Since it is an evolved/evolving scheme, small variations of the genetic code exist above different kingdoms of life, and these variations are central to the ultimate goals of bioinformatics (i.e. how life works).