Cloning

A clone is a genetically identical copy of an organism (or cell). Cloning is a form of asexual reproduction — useful in agriculture, conservation, medicine and research.

Cloning in plants

Plants are easy to clone because plant cells remain capable of differentiating throughout life.

1. Cuttings (the simplest method)

• Cut a small section of stem or leaf from a parent plant.
• Dip in rooting powder (auxin — see B5.10) to encourage roots.
• Plant in compost; the cutting grows into a clone of the parent.
• Used by gardeners and to propagate fruit trees and roses.

2. Tissue culture (commercial-scale)

• Take a few cells from the parent plant (often the meristem).
• Place on a sterile agar growth medium containing nutrients and plant hormones (auxin and cytokinin).
• Cells divide and differentiate into many tiny plantlets.
• Plantlets are transferred to soil to grow into adult plants.

Why use tissue culture?

• Produces many identical plants quickly.
• Used to preserve rare/endangered plant species.
• Used to propagate commercially valuable plants (e.g. orchids).

Cloning in animals

Animal cloning is harder because most animal cells differentiate early and lose this potential.

Embryo transplants

1. Sperm from a prize bull fertilises eggs in the lab.
2. The early embryo, before its cells have specialised, is split into several smaller embryos.
3. Each embryo is implanted into a different host cow ("surrogate").
4. All offspring are genetically identical to each other (clones), but with different surrogate mothers.

This is widely used in cattle breeding to spread the genes of a single high-quality animal.

Adult cell cloning (Dolly the sheep, 1996)

1. The nucleus is removed from an unfertilised egg cell (the egg is enucleated).
2. The nucleus from a body cell of an adult is transferred into the empty egg.
3. The egg is given an electric shock to fuse the nucleus and stimulate division.
4. The resulting embryo is implanted into a surrogate mother.
5. The offspring is a genetic clone of the adult that donated the body-cell nucleus.

Dolly the sheep was the first mammal cloned this way and lived for ~6 years.

Uses of cloning

• Agriculture — multiply animals/plants with desirable characteristics.
• Conservation — clone endangered species (panda, white rhino).
• Research — produce genetically identical animals to study disease.
• Medicine — clone GM animals that produce human proteins in milk; "therapeutic cloning" of stem cells (still experimental, ethical issues).

Concerns

• Ethics of treating animals as products.
• Reduced genetic variation in cloned populations → vulnerable to disease.
• Health problems in cloned animals (Dolly developed arthritis early).
• High failure rate — many embryos do not survive.
• Cost is high.

Links

Connects to B1.2 (mitosis, stem cells, differentiation), B5.10 (auxin in tissue culture), B6.1 (asexual reproduction), and B6.7 (cloning is often used to copy a GM organism).