Cytotoxic Drugs

Hey guys, I'm really struggling with this.

I've read that alkylating agents - a particular type of cytotoxic drug - work through the pairing of alkylguanine and thymine, which leads to the substitution of AT for GC and therefore defective replication.

I'm confused as to why this automatically leads to this substitution?

Authored By Gemma Loach on Wednesday 31st October 2012