背景大链接

Cloning is the asexual production of an exact copy of an original. So for example， one could use cloning to produce the exact copy of a single cell. The cell copy would be identical to the first cell and would have the same exact DNA sequence. In many cases， cloning has been used to reproduce type specific cells. In some instances， cloning of an individual organism， like the sheep， Dolly， has been possible.

Unlike reproduction that involves two “parents，” such as a male and female plants， cloning has a single parent. This is often used in reproducing certain plants. Certain plants have undergone cloning processes for thousands of years， but they do not play a part in the ethical debates that surround cloning of animals， and most particularly humans.

For example， reproductive cloning of animals was first attempted in the 1950s. Most identify the sheep Dolly， cloned in 1996. Dollys parent had DNA transferred into an egg that had its nucleus removed. This is called a somatic cell nuclear transfer. The cell was then treated with chemicals and stimulated to grow so than an almost exact replicate of the cloned sheep was born.

克隆就是通過无性繁殖，对原体的精确复制。举例来说，细胞就可以被完全复制。生成的细胞与原细胞一模一样，甚至基因链也相同。普通实验都是克隆特型细胞。当然，克隆羊多莉的成功表明有机体的克隆也是可行的。

有些植物分雌雄，其后代有父有母，而克隆产物不同于此，只有一个父源。有很多植物也是单性繁殖，数千年如此而不会有伦理问题。但一旦涉及动物甚或人类，伦理问题便凸显了。

有例可证，动物克隆的尝试始于20世纪50年代，多莉则克隆成功于1996年。将其父源基因移植到一去核卵子得以实现，这一技术称作体细胞核移植。改细胞随之采用多种化学物质进行培育，最终得到了对其父源完全克隆的多莉。