Slippage

Template strand of DNA

T T C G G A|C A G|C A G|C A G|C A G|C A G|C A G|C A G| C A~G A T A C G G

Slippage during synthesis

FIGURE 13.18 Strand Slippage of Trinucleotide Repeat

Multiple trinucleotide repeats, such as CAG, may cause strand slippage during DNA replication. In the case illustrated, looping out has occurred in the newly synthesized strand of DNA. The result will be an insertion of six trinucleotide repeats.

FIGURE 13.18 Strand Slippage of Trinucleotide Repeat

Multiple trinucleotide repeats, such as CAG, may cause strand slippage during DNA replication. In the case illustrated, looping out has occurred in the newly synthesized strand of DNA. The result will be an insertion of six trinucleotide repeats.

become misaligned (Fig. 13.17). Depending on which strand slips, a base may be inserted or omitted during replication.

Slippage may also occur in regions of DNA where there are multiple repeats of a short sequence, perhaps two or three bases (Fig. 13.18). In this case, a whole repeat unit of several bases will be added or deleted. Well known cases occur in the human trinucleotide repeat expansion diseases, such as fragile X syndrome and Huntington's disease. Here copies of a three-base repeat are added or lost due to slippage.

Spontaneous Mutation Can Be Caused by Inherent Chemical Instability 353

Mutations Can Result from Mispairing and Recombination

Recombination may occurs between closely related sequences of DNA, such as two alleles of the same gene. Many DNA rearrangements, including deletions, inversions, translocations and duplications may result from mistaken pairing of similar sequences followed by recombination. The mechanism of recombination is dealt with in Ch. 14; here, the overall result of mispairing will be considered. If the similar sequences are in the same orientation, mispairing followed by crossing over will generate a duplication on one molecule of DNA and a corresponding deletion on the other (Fig. 13.19).

If two copies of a sequence are on the same DNA molecule but face each other (i.e., are in opposite orientations), mispairing followed by crossing over will generate an inversion (Fig. 13.20). For example, the chromosome of E. coli contains seven copies of the genes for ribosomal RNA. Strains of E. coli are known in which the whole segment of the bacterial chromosome between two of these rRNA operons has been inverted. Such strains grow slightly slower but nonetheless, are viable.

Spontaneous Mutation Can Be the Result of Tautomerization

However sophisticated DNA polymerase may be, there are chemical limits on the accuracy of DNA replication. Even if DNA polymerase inserts the correct base, errors may still occur. This is due to the tautomerization of the bases that constitute DNA. Each base may exist as two possible alternative structures that interconvert. Such structural isomers that exist in dynamic equilibrium are known as tautomers. In each case, one isomer is much more stable and the vast majority of the base is found in this form. However, the less stable alternative tautomer will appear very rarely. If this happens just as the replication fork is passing, the rare tautomer may cause incorrect base pairing.

Thymine has keto and enol tautomers (Fig. 13.21). The common, keto-form pairs with adenine, but the rare enol-tautomer base pairs with guanine. Guanine also has keto and enol tautomers. In this case the rare enol-guanine base pairs with thymine rather than cytosine. Similarly, adenine equilibrates between common amino and rare imino tautomers. The rare imino-adenine base pairs with cytosine instead of thymine. Cytosine alone does not have the potential to introduce mismatches. Although it does have amino and imino tautomers, both pair with adenine. As the temperature increases, the probability that a base is in the incorrect tautomeric state also increases and so, therefore does the mutation frequency.

Spontaneous Mutation Can Be Caused by Inherent Chemical Instability

Although DNA is relatively stable, some of its components do show a low level of spontaneous chemical reaction. Several bases undergo slow but measurable loss of their amino group; i.e., deamination. Adenine, guanine and cytosine may all spontaneously deaminate, but by far the most frequent is the deamination of cytosine to give uracil (Fig. 13.22). In addition, the modified base, 5-methyl-cytosine, is especially prone to deamination, so giving "methyl-uracil;" in other words, thymine. The result, in both cases, is the replacement of C by T. Deamination of A (to hypoxanthine) and G (to xanthine) occurs at only 2 to 3 percent of the rate for cytosine. Both hypoxanthine and xanthine usually (but not exclusively) base pair with C, so mutations may be introduced in some cases.

deamination Loss of an amino group tautomerization Alternation of a molecule, in particular a base of a nucleic acid, between two different isomeric structures

Original DNA

Direct repeat

Gene 1

Gene 2

Direct repeat

One molecule of dna loops

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