Cloned sheep and mice may represent significant technological advances, but nature began cloning a long time ago.
Cloning, scientists say, involves creating organisms with identical genetic material. Nature already makes exact replicas of many organisms. For example, bacteria, have only one chromosome containing all their DNA. They replicate by manufacturing duplicate DNA, then divide to produce two clones. Sea anemones, in transit, often leave tissue behind. This discarded tissue can create new anemones. And in humans, identical twins arise when fertilized eggs divide to produce two genetically indistinguishable embryos. But identical twins still develop individual personalities, and can have physical differences.
For cloning from adults, researchers change the natural genetic process. Children usually inherit 50 percent of their genes from their mothers and 50 percent from their fathers. If children were cloned from adults, they would get all their genes from one parental source.
Scientists say the biology of cloning animals has many variables and is still little understood. For instance, only certain cells can donate DNA to an egg. To clone people, boundaries imposed by human biology would inevitably increase the number of variables.