Figure 1. Types of mutation. (a) Wild-type original sequence; (b) transition from C to T; (c) transversion from G to C; (d) deletion (Del) of the sequence ACCTA, the ^Ú sign indicates from where it has been removed; (e) insertion (Ins) of the sequence AAAGC, the two ^Ú signs indicate where the sequence has been inserted.

Figure 2. Types of substitutions in a protein-coding region: (a) synonymous, (b) missense, and (c) nonsense. In each case, the top sequence is wild type and the bottom sequence is mutated.

Figure 3. Examples of frameshifts caused by deletion or insertion. (a) A deletion of a G causes premature termination. (b) An insertion of a G obliterates a stop codon. Termination codons are underlined. In each case, the top sequence is wild type and the bottom sequence is mutated.

Figure 4. Generation of duplications or deletions by slipped-strand mispairing between contiguous repeats (bold red). Small arrows indicate the direction of DNA synthesis. Dots indicate base pairing. (a) A two-base slippage in a TA repeat during DNA replication. Slippage in the 3¢ 5¢ direction results in the insertion of one TA unit (left panel). Slippage in the other direction results in the deletion of one repeat unit (right panel). The deletion shown in the right panel results from excision of the unpaired repeat unit (asterisks) at the 3¢ end of the growing strands, presumably by the 3¢ 5¢ exonuclease activity of DNA polymerase. (b) A two-base slippage in a TA repeat in nonreplicating DNA. Mismatched regions form single-stranded loops, which may be targets of excision or mismatch repair. The outcome (a deletion or an insertion) will depend on which strand is excised or repaired and which strand is used as template in the DNA repair process.