Figure 14.3 Cloning a Plant
To demonstrate that differentiated plant cells are totipotent, clumps of cells were removed from the root of a carrot plant and grown in a nutrient medium where they were induced to dedifferentiate. The dedifferentiated cells then divided and developed into a callus. Upon planting of the callus in a special medium containing hormones and other nutrients, it developed into a reproductively functional plant. These results offered direct evidence that, under appropriate circumstances, differentiated plant cells have the ability to develop into any type of cell in the mature plant. This method of plant cloning, also referred to as plant tissue culture, makes it possible to cultivate large amounts of uniform, disease-free plant tissue in a relatively short period of time. For example, plant tissue culture is currently used to produce bananas that are free from disease. Additionally, this process is used to cultivate plants, such as seedless grapes and seedless navel oranges, neither of which can produce seeds themselves in order to reproduce. Additional advantages of plant tissue culture include the ability to produce, or rapidly duplicate, a superior variety of a particular plant, and the ability to overcome juvenility problems displayed by some fruit and nut trees. This juvenility period may last 5–20 years, during which the trees cannot flower or yield fruit. Cloning can overcome such problems, allowing quick production of profitable plants and trees. By comparison, the major disadvantage of plant cloning is that it results in complete genetic uniformity. Thus, if all of the plants in a particular region are the same, they are equally susceptible to the same diseases. In general, however, the advantages of plant cloning are believed to outweigh the disadvantages that arise from this method.
Original Papers
Stewart, F. C. 1958. Growth and Organized Development of Cultured Cells. III. Interpretations of the Growth from Free Cell to Carrot Plant. American Journal of Botany 45: 709–713.
http://www.jstor.org/stable/2439729
See also the associated papers:
Stewart, F. C., M. O. Mapes, and J. Smith. 1958. Growth and Organized Development of Cultured Cells. I. Growth and Division of Freely Suspended Cells. American Journal of Botany 45: 693–703.
http://www.jstor.org/stable/2439507
Stewart, F. C., M. O. Mapes, and K. Mears. 1958. Growth and Organized Development of Cultured Cells. II. Organization in Cultures Grown from Freely Suspended Cells. American Journal of Botany 45: 705–708.
http://www.jstor.org/stable/2439728
Links
The Naked Scientists: Science Radio & Science Podcasts: Kitchen Science - Home Cloning & Bananas
http://www.thenakedscientists.com/HTML/content/interviews/interview/817/
Answers.com: How are seedless fruits and vegetables made?
http://www.answers.com/topic/seedless-fruits-and-vegetables
Texas A&M University: Youth Adventure Program: Cloning Plants: Tissue Culture
http://generalhorticulture.tamu.edu/YouthAdventureProgram/TisueCulture/TissueCulture.html
Texas A&M University: The Many Dimensions of Plant Tissue Culture Research
http://aggie-horticulture.tamu.edu/tisscult/pltissue/pltissue.html
How Stuff Works: How Cloning Works: Producing Clones: Plant Life
http://science.howstuffworks.com/cloning1.htm
Access Excellence: Plant Tissue Culture
http://www.accessexcellence.org/LC/ST/st2bgplant.php
National Geographic News: Lack of Sex Life Threatens Banana Crops
http://news.nationalgeographic.com/news/2001/07/0726_wirebanana.html
Figure 14.4 Cloning a Mammal
Ian Wilmut and colleagues conducted the first successful cloning of a mammal as a result of a technique known as somatic cell nuclear transfer (SCNT). Briefly, mammary cells were removed from a Dorset sheep, and an unfertilized egg was removed from a Scottish blackface ewe. The nucleus of a mammary cell, which was in the G2 phase of the cell cycle, was fused with the enucleated egg from the Scottish blackface ewe. The resulting cell was stimulated to divide and form into an early embryo, which was transplanted into a second Scottish blackface ewe. The embryo developed and the ewe gave birth to Dolly, a Dorset sheep genetically identical to the sheep from which the donor mammary cells were obtained. These results demonstrated that, under appropriate circumstances, animal cells are totipotent. Aside from a number of ethical issues surrounding the idea of cloning mammals, use of the SCNT technique itself also raises other scientific and medical concerns, including the possibility of the risk of premature aging. Initially, Dolly appeared to be a healthy sheep, but over time she developed severe arthritis, a condition usually associated with older animals. In 1999, research published in the journal Nature suggested that Dolly may have been susceptible to premature aging due to the shortened telomeres in her cells. Dolly was euthanized at the age of 6 years old, approximately half of the normal life span of a sheep.
Original Paper
Wilmut, I., A. E. Schnieke, J. McWhir, A. J. Kind and K. H. S. Campbell. 1997. Viable offspring derived from fetal and adult mammalian cells. Nature 385: 810–813.
http://www.nature.com/nature/journal/v385/n6619/pdf/385810a0.pdf
Links
Giles, J. and J. Knight. 2003. Dolly’s death leaves researchers woolly on clone ageing issue. Nature 421: 776.
http://dx.doi.org/10.1038/421776a
Kimball’s Biology Pages: Can Humans be cloned?
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CloningMammals.html
The University of Utah: Learn Genetics: Genetic Science Learning Center: What is Cloning?
http://learn.genetics.utah.edu/content/cloning/whatiscloning/
Xu, J. and X. Yang. 2003. Will cloned animals suffer premature aging—The story at the end of clones’ chromosomes. Reproductive Biology and Endocrinology 1: 105.
http://www.rbej.com/content/1/1/105
National Human Genome Research Institute: Cloning
http://www.genome.gov/25020028
How Stuff Works: How Cloning Works: Dolly
http://science.howstuffworks.com/environmental/life/genetic/cloning3.htm
Ralston, A. and K. Shaw. 2008. Gene Expression Regulates Cell Differentiation. Nature Education 1(1)
http://www.nature.com/scitable/topicpage/gene-expression-regulates-cell-differentiation-931