THE LIVING WORLD
Unit Three. The Continuity of Life
13.6. Reproductive Cloning
One of the most active and exciting areas of biology involves recently developed approaches to manipulating animal cells. In this section, you will encounter three areas where landmark progress is being made in cell technology: reproductive cloning of farm animals, stem cell research, and gene therapy. Advances in cell technology hold the promise of revolutionizing our lives.
The idea of cloning animals was first suggested in 1938 by German embryologist Hans Spemann (called the “father of modern embryology”), who proposed what he called a “fantastical experiment”: remove the nucleus from an egg cell (creating an enucleated egg) and put in its place a nucleus from another cell. When attempted many years later (figure 13.13), this experiment actually succeeded in frogs, sheep, monkeys, and many other animals. However, only donor nuclei extracted from early embryos seemed to work. After repeated failures using nuclei from adult cells, many researchers became convinced that the nuclei of animal cells become irreversibly committed to a developmental pathway after the first few cell divisions of the developing embryo.
Figure 13.13. A cloning experiment.
In this photo, a nucleus is being injected from a micropipette (bottom) into an enucleated egg cell held in place by a pipette.
Then, in the 1990s, a key insight was made in Scotland by geneticist Keith Campbell, a specialist in studying the cell cycle of agricultural animals. Recall from chapter 8 that the division cycle of eukaryotic cells progresses in several stages. Campbell reasoned, “Maybe the egg and the donated nucleus need to be at the same stage in the cell cycle.” This proved to be a key insight. In 1994 researchers succeeded in cloning farm animals from advanced embryos by first starving the cells, so that they paused at the beginning of the cell cycle. Two starved cells are thus synchronized at the same point in the cell cycle.
Campbell’s colleague Ian Wilmut then attempted the key breakthrough, the experiment that had been eluding researchers: He set out to transfer the nucleus from an adult differentiated cell into an enucleated egg, and to allow the resulting embryo to grow and develop in a surrogate mother, hopefully producing a healthy animal (figure 13.14). Approximately five months later, on July 5, 1996, the mother gave birth to a lamb. This lamb, “Dolly,” was the first successful clone generated from an adult animal cell. Dolly grew into a healthy adult, and as you can see in the photo at the beginning of this chapter, she went on to have healthy offspring normal in every respect.
Figure 13.14. Wilmut's animal cloning experiment.
Progress with Reproductive Cloning
Since Dolly’s birth in 1996, scientists have successfully cloned a wide variety of farm animals with desired characteristics, including cows, pigs, goats, horses, and donkeys, as well as pets like cats and dogs. Snuppy, the puppy in figure 13.15, was the first dog to be cloned. For most farm animals, cloning procedures have become increasingly efficient since Dolly was cloned. However, the development of clones into adults tends to go unexpectedly haywire. Almost none survive to live a normal life span. Even Dolly died prematurely in 2003, having lived only half a normal sheep life span.
Figure 13.15. Cloning the family pet.
This puppy named "Snuppy" is the first dog cloned. Beside him to the left, is the adult male dog that provided the skin cell from which Snuppy was cloned. The dog in the photo on the right was Snuppy's surrogate mother.
The Importance of Gene Reprogramming
What is going wrong? It turns out that as mammalian eggs and sperm mature, their DNA is conditioned by the parent female or male, a process called reprogramming. Chemical changes are made to the DNA that alter when particular genes are expressed without changing the nucleotide sequences. In the years since Dolly, scientists have learned a lot about gene reprogramming, also called epigenetics. Epigenetics works by blocking the cell’s ability to read certain genes. A gene is locked in the off position by adding a -CH3 (methyl) group to some of its cytosine nucleotides. After a gene has been altered like this, the polymerase protein that is supposed to “read” the gene can no longer recognize it. The gene has been shut off.
We are only beginning to learn how to reprogram human DNA, so any attempt to clone a human is simply throwing stones in the dark, hoping to hit a target we cannot see. For this and many other reasons, human reproductive cloning is regarded as highly unethical.
Key Learning Outcome 13.6. Although recent experiments have demonstrated the possibility of cloning animals from adult tissue, the cloning of farm animals often fails for lack of proper epigenetic reprogramming.