Nonsense mutations occur whenـــــــــــــــــ is introduced into the coding sequence of a gene.

In some repair mechanisms,the parental DNA strand is used as a guide for repairing mistakes made in the newly synthesized DNA strand.The cell is able to distinguish the parental strand from the new strand because immediately following replication,the DNA isـــــــــــــــــــ .

A mutation that changes the orientation of a segment of DNA is called aan)ـــــــــــــــــــــــ.