Why did Dolly die after one year?

Answer 1

Dolly was "created" using cells from an adult animal. There is some speculation that these older cells had shorter telomere chains. These chains shorten with every cell division and count down the number of divisions a cell can make before it dies. Even though she was considered a lamb, her body was trying to use cells that were already quite old and couldn't provide the number of cell divisions necessary to see her have along life.