The magnitude of this dipole moment corresponds to about 0.5-0.7 unit charge at each end of the helix. These charges would be expected to attract ligands of opposite charge and negatively charged ligands, especially when they contain phosphate groups and frequently bind at the N-termini of a helices. In contrast, positively charged ligands rarely bind at the C-terminus

TLS Online TPP Program

#Id: 3257

#Unit 3. Fundamental Processes

Intercalating agents, such as proflavin, acridine, and ethidium, cause the deletion or addition of a base pair or even a few base pairs.

TLS Online TPP Program

#Id: 3258

#Unit 3. Fundamental Processes

Intercalating agents are flat molecules containing several polycyclic rings that bind to the equally flat purine or pyrimidine bases of DNA, just as the bases bind or stack with each other in the double helix

TLS Online TPP Program

#Id: 3259

#Unit 3. Fundamental Processes

 Ultraviolet light induces the formation of a cyclobutane ring between adjacent thymines

TLS Online TPP Program

#Id: 3261

#Unit 3. Fundamental Processes

g-radiation and X-rays (ionizing radiation) are particularly hazardous because they cause double-strand breaks in the DNA, which are difficult to repair. If left unrepaired, double-strand breaks can be lethal to a cell.

TLS Online TPP Program

#Id: 3263

#Unit 3. Fundamental Processes

DNA is also subject to attack from reactive oxygen species (e.g., O2- , H2O2, and OH†). These potent oxidizing agents are generated by ionizing radiation and by chemical agents that generate free radicals. Oxidation of guanine, for example, generates 7,8-dihydro-8-oxoguanine, or oxoG. 

TLS Online TPP Program

#Id: 3264

#Unit 3. Fundamental Processes

The oxoG adduct is highly mutagenic because it can base-pair with adenine as well as with cytosine. If it base-pairs with adenine during replication, it gives rise to a G:C to T:A transversion, which is one of the most common mutations found inhuman cancers.