Frameshift mutations: If one or two nucleotides are either deleted from or added to the coding region of an mRNA, a frameshift mutation occurs, altering the reading frame. This can result in a product with a radically different amino acid sequence or a truncated product due to the eventual creation of a termination codon (Fig. 32.5). If three nucleotides are added, a new amino acid is added to the peptide. If three are deleted, an amino acid is lost. Loss of three nucleotides maintains the reading frame but can result in serious pathology. For example, cystic fibrosis (CF), a chronic, progressive, inherited disease that primarily affects the pulmonary and digestive systems, is most commonly caused by deletion of three nucleotides from the coding region of a gene, resulting in the loss of phenylalanine (Phe, or F; see p. 5) at the 508th position (∆F508) in the CF transmembrane conductance regulator (CFTR) protein encoded by that gene. This ∆F508 mutation prevents normal folding of CFTR, leading to its destruction by the proteasome (see p. 247). CFTR normally functions as a chloride channel in epithelial cells, and its loss results in the production of thick, sticky secretions in the lungs and pancreas, leading to lung damage and digestive deficiencies (see p. 174). The incidence of CF is highest (1 in 3,300) in those of Northern European origin. In >70% of individuals with CF, the ∆F508 mutation is the cause of the disease.
= guanine; U = uracil.
III. COMPONENTS REQUIRED FOR TRANSLATION
A large number of components are required for the synthesis of a protein. These include all the amino acids that are found in the finished product, the mRNA to be translated, transfer RNA (tRNA) for each of the amino acids, functional ribosomes, energy sources, and enzymes as well as noncatalytic protein factors needed for the initiation, elongation, and termination steps of polypeptide chain synthesis.
A. Amino acids
All the amino acids that eventually appear in the finished protein must be present at the time of protein synthesis. If one amino acid is missing, translation stops at the codon specifying that amino acid. [Note: This demonstrates the importance of having all the essential amino acids (see p.
262) in sufficient quantities in the diet to insure continued protein synthesis.]
B. Transfer RNA
At least one specific type of tRNA is required for each amino acid. In humans, there are at least 50 species of tRNA, whereas bacteria contain at least 30 species. Because there are only 20 different amino acids commonly carried by tRNA, some amino acids have more than one specific tRNA molecule. This is particularly true of those amino acids that are coded for by several codons.
1. Amino acid attachment site: Each tRNA molecule has an attachment site for a specific (cognate) amino acid at its 3′-end (Fig. 32.6). The carboxyl group of the amino acid is in an ester linkage with the 3′-hydroxyl of the ribose portion of the A nucleotide in the –CCA sequence at the 3′-end of the tRNA. [Note: A tRNA with a covalently attached (activated) amino acid is charged. Without an attached amino acid, it is uncharged.] 2. Anticodon: Each tRNA molecule also contains a three-base nucleotide sequence, the anticodon, which pairs with a specific codon on the mRNA (see Fig. 32.6). This codon specifies the insertion into the growing polypeptide chain of the amino acid carried by that tRNA.
C. Aminoacyl-tRNA synthetases