Cloning

Explained

Cloning plants and animals

A clone is an organism genetically identical to another. Plants are cloned by taking pieces of tissue and growing them on; animals are cloned by moving a nucleus from one cell into another. The methods are different, but both rely on mitosis, because mitosis is the division that produces identical cells.

Micropropagation in plants

Take small pieces of tissue, called explants, from a plant with the features you want. Place them on sterile nutrient agar containing plant growth hormones. Each explant divides by mitosis and grows into a plantlet, which can then be planted out.

Sterile conditions are not a detail. Without them, bacteria and fungi grow far faster than the plant tissue and the culture is destroyed, so aseptic technique is usually worth a mark on its own.

The advantage is that thousands of identical plants can be produced quickly from one parent, all carrying the desired characteristic. The drawback is that they are also identical in their weaknesses. A disease that can kill one can kill the whole crop, because there is no genetic variation to give any of them resistance.

Cloning a mammal by nucleus transfer

The method that produced Dolly the sheep works in four steps.

• Take a body cell from the adult to be cloned, and remove its diploid nucleus.
• Take an egg cell from another animal and remove its nucleus, leaving an enucleated egg cell.
• Insert the diploid nucleus into the enucleated egg cell, then give it an electric shock to start division.
• Implant the developing embryo into the uterus of a surrogate mother.

The clone is genetically identical to the animal that supplied the nucleus, not to the egg donor and not to the surrogate. That is the point questions test most often, because three animals are involved and only one is the genetic parent.

Transgenic animals, and why cloning them is useful

A transgenic animal contains genetic material transferred from another species. Such an animal can be engineered to produce a useful protein, for example a human protein in its milk.

Producing one is difficult and uncertain. Once you have a successful animal, cloning lets you make more of it without repeating the genetic engineering, so the value of cloning here is reliability rather than novelty.