Answer:
DNA contains four base pairs: adenine, thymine, cytosine and guanine. Adenine forms bond with thymine, its a double bond, and cytosine forms triple bond with guanine.
Cytosine in natural process does not forms bond with thymine, but if it happen then many errors could occur in replication. As DNA is the basic material of cell and all genetic information lies in it, a single mutation can lead to fatal disorders, and when cytosine forms bond with thymine instead of guanine then the backbone of DNA would be distorted, RNA being formed from this DNA would also be have errors that would result in mutated proteins that can cause serious illness and destroy many natural processes.