http://www.sciencenet.org.uk/database/Biology/9609/b00588d.html
All cloning means is making a copy of a plant or animal so that the copy contains the same genetic information (the blueprint that tells the organism how to grow) as the original. Cloning is actually very abundant in nature. Practically all organisms that reproduce without sex, like bacteria and protozoa, many plants (e.g. the potato) and, in part, insects like aphids, produce offspring that are clones. Cloning by humans sounds a bit like science fiction until you realise that people have been cloning plants for centuries. You can clone a plant by taking a leaf and a bit of stem from an adult plant and putting it in sand or water until roots begin to grow and then planting it in soil. This new plant is genetically identical to the old. Cloning people has been done too; identical twins are clones!
In order to produce a clone of an animal or plant, you need to remove a small part of that being and carefully place that part in conditions that will enable it to grow into a copy of its parent.
Plants lend themselves to this because of the unique way in which they grow, from shoot tips and root tips. Also, plants tend to have some of these growing tips in reserve, ready to grow out just in case one is broken off or chewed by a plant-eater. The easiest way to clone plants is by taking cuttings which contain a shoot tip or a bud, which is a dormant growing tip. To make large numbers of plant clones quickly, however, you need to persuade plant parts that are not shoot tips to grow into shoot tips. This s done in two stages.
Stage 1 - small parts of the plant, frequently leaves, are cut and placed on a nutrient medium that can encourage growth in general. The wounding at the cut edges stimulates some cells to start dividing and continue to grow but in an unstructured manner, giving rise to blobs of tissue called calluses (singular = callus). You can see callus formed naturally on the cut base of a carrot, cauliflower or cabbage kept for a few weeks in the cupboard. The callus cells have lost many of the specialised features that their parent cells had in the plant and are now ready for stage 2.
Stage 2 - the callus is moved to a medium that promotes shoot growth because of the presence of plant growth hormones. Many areas on the callus will begin to form tiny shoots. These new shoots can in turn be cut off and carefully nurtured until they grow large enough to be transferred to soil. The process does not work equally well with all plants. Some varieties of potatoes, tomatoes, tobacco, carrot and maize can be easily cloned in this way. Wheat and some tree species as eucalyptus are much more difficult to clone.
Animals are more difficult to clone. Until recently, the only way that you could clone an animal was to take an embryo that was very, very, young and contained just a few cells (8-16), and to split it into two parts. Each part could grow into a separate animal with identical genetic information. This is actually what occurs naturally in animals including humans when a female becomes pregnant with identical twins. The recent cloning that has been reported is very exciting because it lets scientists grow up a new animal from a single cell (the small building blocks that make up your body) taken from an adult. It would be as if I took a cell from your skin and from it grew up your identical twin.
Scientists never thought that this would be possible. Although all the cells in your body contain the same genetic information, as you grow from a fertilised egg into a baby, your cells take on separate jobs (differentiate). For example, cells in your brain do different work than cells in your blood. In order to differentiate, the cells need to turn off some of the genetic information that they don't need and turn on other bits that they will need. It would be like if each of your cells was a library and all the cells in your body contained the same books but as they took on separate jobs, they put some books in storage and others were left on the shelves to be used. The cells that the scientist used to grown up the sheep, Dolly, was an udder cell which is differentiated. Now in order for cloning to work, all of the genetic information in the cell needs to be available (i.e. All the books have to be on the shelves) to tell the animal how to form. Scientists found that it was possible to make all the genetic information from a differentiated cell available, if they put it into a new environment. This new environment was an egg cell which has had its own genetic information removed. Once the scientist put the udder cell's genetic information into the egg, it became reorganised (i.e. All the books in storage were put onto the shelves) so that all of it was available to be used. This egg cell containing udder cell DNA then started dividing and formed an embryo which was put into the womb of a sheep to develop into a baby lamb.
The idea of cloning animals and maybe humans sounds exciting but it needs to be put into perspective. First, although the procedure appears simple it is actually very difficult and time consuming. The scientists that cloned Dolly had to try 277 times before they succeeded. That means that they had to isolate 277 mature eggs from sheep ovaries in order to get one clone. In humans, doctors are very lucky if they can harvest 6 mature eggs from a woman. With time however, technical improvements will no doubt be made. Second, just because it worked in sheep doesn't mean that it will work in humans. The early stages of embryo formation are different between the two species. The genetic information is used in human embryos when they are only 2 cells big but in sheep when they are 8-16 cells big. This means that the genetic information from the differentiated cell will have more time to reorganise in sheep than in humans before it is needed. Third, many governments have prohibited any research on human cloning. Finally, Dolly's genetic information is older than she is and no one quite knows how Dolly will age.