Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 182

Each 3-nucleotide sequence, or codon, in the mRNA encodes a specific amino acid, and the assignment of the codons is known as the genetic code (Table 2.2). Because there are 3 nucleotides in each codon, an mRNA can be translated in three different frames in each region. Initiation of translation at a specific initiation codon establishes the reading frame of translation, so that in most cases only a single reading frame is utilized. Once translation has begun, the ribosome moves 3 nucleotides at a time through the coding part of the mRNA. If the translation is occurring in the proper frame for protein synthesis, we say the translation is in the zero frame for that protein. If translation is occurring in the wrong reading frame, it can be displaced either back by 1 nucleotide in each codon (the –1 frame) or forward by 1 nucleotide (the +1 frame). In a few instances, translational frameshifts that change the reading frame even after translation has initiated can occur. Frameshift mutations (see chapter 3) cause incorrect reading of an mRNA because of the insertion or deletion of nucleotides in the DNA sequence, which is then copied into mRNA. Translational frameshifting occurs when the ribosome shifts its position on the mRNA without a change in the mRNA sequence itself.