12.1 Proving That Reproductive Cloning is Possible

2. In 1938, the German embryologist Hans Spemann proposed what he called a “fantastical experiment”: Replace the nucleus of an egg cell with the nucleus of another cell Early attempts to clone animals in this way failed The breakthrough was the following insight Starvation will synchronize cells at the same point in the cell cycle 12.1 Proving That ReproductiveCloning is Possible

3. Wilmut’s Lamb • Reproductive biologist Ian Wilmut and his colleagues were able to clone the first animal in 1997 • Mammary cells were removed from the udder of a six-year old sheep • The nucleus was removed from an egg cell taken from another sheep • Both cells were synchronized to a resting state • The nucleus from the mammary cell was transferred to the enucleated egg cell • An electric shock was applied to start cell division

4. Wilmut’s Lamb • The successful embryos (about 30 in 277 tries) were transplanted into surrogate mother sheep • On July 5, 1996, “Dolly” was born • Only 1 of 277 tries succeeded • However, Wilmut proved that reproductive cloning is possible

5. Fig. 12.1 Wilmut’s animal cloning experiment

6. Since Dolly, scientists have successfully cloned sheep, mice, cattle, goats and pigs However, problems and complications arise, leading to premature death Dolly died in 2002, having lived only half a normal sheep life span 12.2 Problems withReproductive Cloning

7. The Importance of Genomic Imprinting • During gamete development, the DNA undergoes a process termed genomic imprinting • The process involves the methylation (addition of –CH3 groups) to cytosine residues in the DNA • This locks genes in either the “on” or “off” position • Normal animal development requires chemical reprogramming of the DNA • Takes months to years in adult reproductive tissues • Cloning fails because there is not enough time for the re-programming to be done properly

8. 12.3 Embryonic Stem Cells • The blastocyst, an early embryo, consists of • A protective outer layer that will form the placenta • Inner cell mass that will form the embryo • The inner cell mass consists of embryonic stem cells • These are pluripotent • Capable of forming the entire organism • As development proceeds, cells lose their pluripotency • They become committed to one type of tissue • They are then called adult stem cells

9. 12.3 Embryonic Stem Cells • Embryonic stem cells could be used to restore tissues lost or damaged due to accident or disease • Experiments have already been tried successfully in mice • Damaged spinal neurons have been partially repaired • The course of development is broadly similar in all mammals • Therefore, the experiments in mice are very promising

10. Fig. 12.2

11. Fig. 12.3 Human embryonic stem cells 12.3 Embryonic Stem Cells • The research in human embryonic stem cells is associated with two serious problems • 1. Finding a source • Harvesting them from discarded embryos raises ethical issues • 2. Immunological rejection • Implanted stem cells will likely be rejected by the immune system of the individual

12. 12.4 Therapeutic Cloning • Therapeutic cloning follows this basic approach: • A cell is obtained from an individual who lost a tissue function due to an accident or disease • It is cloned to produce an embryo • Embryonic stem cells are harvested and grown in tissue culture • The stem cells are then injected back into the same individual • There, they divide and ultimately differentiate into healthy tissue

13. Fig. 12.5 How human embryos might be used for therapeutic cloning

14. Fig. 12.5 How human embryos might be used for therapeutic cloning

15. 12.4 Therapeutic Cloning • Therapeutic cloning solves the problem of immune rejection • Cells are cloned from the individual’s own tissues, • Therefore, they pass the immune system’s “self” identity check • However, the process is still controversial • Some fear that the cloned embryo might be brought to term by inserting it into a human uterus

16. Stem cells offer enormous promise for treating a wide range of diseases However, the research involves ethical issues 1. Destruction of human embryos When does human life begin? 2. Possibility of future abuse or misuse Is human reproductive cloning next? 3. Alternative sources of stem cells Are adult stem cells equally effective? 12.5 Grappling with the Ethicsof Stem Cell Research

21. Gene therapy involves the introduction of “healthy” genes into cells that lack them It was first used successfully in 1990 12.6 Initial Attempts atGene Therapy • Two girls were cured of a rare blood disorder caused by a defective adenosine deaminase gene • The girls stayed healthy

22. Researchers then set out to apply gene therapy to cystic fibrosis In 1994, the technique was first tried on mice A normal copy of the gene, cf, was added to the vector adenovirus The virus was then squirted into the lungs of mice that carried a defective cf gene The mice had their immune systems disabled The “healthy” gene was thus introduced into lung cells And the mice were successfully cured! 12.6 Initial Attempts atGene Therapy

23. Researchers then targeted humans in 1995 The same basic approach was used as was used with mice For eight weeks, the gene therapy seemed successful However, the gene modified-cells in the patients’ lungs came under attack by the immune system The healthy genes were lost, and with them the chances for a cure 12.6 Initial Attempts atGene Therapy

24. A comprehensive 1995 review of human gene therapy trials revealed three problems 1. The adenovirus elicits a strong immune response It causes the common cold, so antibodies were formed due to previous colds 2. In rare cases, the immune response can be very severe If many patients are treated, a few may die 3. The adenovirus inserts its DNA randomly in human chromosomes This will cause mutations and potentially cancers

25. Fig. 12.6 12.7 More Promising Vectors Adenovirus • Within a few years, researchers had a much more promising vector • A tiny virus called adeno-associated virus (AAV) AAV only has two genes and thus needs adenovirus to replicate AAV

26. 12.7 More Promising Vectors • AAV has several advantages over adenovirus • 1. It inserts genes into human DNA less frequently • 2. It does not elicit a strong immune response In 1999, AAV successfully cured anemia in rhesus monkeys AAV was also used to cure dogs of a hereditary disorder leading to retinal degeneration & blindness And human trials are under way again!

27. Fig. 12.7 Using gene therapy to cure a retinal degenerative disease in dogs

28. The advent of gene therapy has raised serious ethical issues Indeed, medical ethicists tend to avoid the term “gene therapy” They prefer the term gene intervention Any procedure that alters a person’s genes, whether by modification or addition 12.8 Ethical Issues Raised byGene Therapy

29. At this time, gene intervention can only correct genetic defects in somatic tissues (body cells) Changes induced in these tissues are not inherited Gene intervention of germ-line tissue (sperm or eggs) has not yet been attempted in humans Changes induced in these tissues are inherited 12.8 Ethical Issues Raised byGene Therapy

30. In trying to assess gene intervention procedures, bioethicists apply two principles The beneficience principle The benefits of the procedure should be carefully weighed against the risks The respect-for-persons principle The persons affected by the procedure have a right to make their own informed decisions 12.8 Ethical Issues Raised byGene Therapy