Figure 1. Types of substitution mutations in a coding region: (a) synonymous, (b) missense and (c) nonsense. Reproduced with permission from Graur and Li (2000).

Figure 2. Nucleotide sequences at the borders between exon-1 and intron-I and exon-2 in the -globin gene from a normal individual and a patient with ⁺-thalassaemia, the mutated nucleotide boxed. The leader lines indicate the splicing sites. Each of the splice junctions is compared with the sequence of the consensus splice junction, and dots denote identity of nucleotides between the splice junction and the consensus sequence. Note that the nucleotide substitution in the ⁺-thalassaemia gene is synonymous, because both GGT and GGA code for the amino acid glycine. It is not, however, silent, because the activation of the new splicing site in the ⁺-thalassaemia gene results in the production of a frameshifted protein. Reproduced with permission from Graur and Li (2000) after Goldsmith et al. (1983).

Figure 3. Amino«imino and keto«enol tautomerisms. Adenine and cytosine are usually found in the amino form, but rarely assume the imino configuration. Guanine and thymine are usually found in the keto form, but rarely form the enol configuration. Thymine has two enol tautomers. All minor tautomers can assume different rotational forms. Reproduced with permission from Graur and Li (2000).