O

[2+2] photoproducts

[2+2] photoproducts

cis,syn thymidine dimer cis,syn cytidine thymidine dimer cis,syn cytidine dimer

cis,syn thymidine dimer cis,syn cytidine thymidine dimer cis,syn cytidine dimer

[6-4] photoproducts

[6-4] photoproducts

Scheme B.21.4.

Scheme B.21.4.

sible for the lethal effect of UV light. Because of the facilitated tautomeri-zation of the exocyclic amino group at C4 in C, which is caused by loss of the C5=C6 double bond, the resulting imino tautomer has the same hydrogen bonding pattern as T. Consequently, this can lead to incorporation of adenosine (A) rather than G into the daughter strand during DNA replication. In addition, C-derived dimers are known to be very susceptible to hydrolysis. The net genetic result from either of these pathways is a C ! T mutation [6].

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