|MadSci Network: Cell Biology|
It is not necessary to use the nuclei of stem cells for the cloning of mammals. For the most part, all of the cells in an adult mammal are genetically equivalent, which is to say that they contain the exact same set of DNA sequences that was present in the fertilized egg. The differences between a nerve cell, a muscle cell, a skin cell, and so on, are due to differences in the genes that are expressed, not differences in the genes that are present. This is evident from the cloning of mice from differentiated cells like neurons; the cloned mice have all cell types, and are not merely a bundle of neurons. The exception to this general rule is that some cells of the immune system undergo DNA rearrangements to generate new antibody types; the DNA of these cells is no longer equivalent to that of the fertilized egg. It is actually more important to use cells that are at the right part of the cell cycle. The 'cell cycle' is the series of events that cells go through when they grow and divide. You are probably familiar with mitosis, the dramatic series of events that partitions replicated chromosomes into the two daughter cells. Mitosis occurs during M phase of the cell cycle. Another phase of obvious activity is S phase, during which DNA replication occurs (S is for 'synthetic'). Two other phases are 'gap' phases, during which it is not apparent that the cell is doing very much: G1 and G2. The series of the phases is: G1, S, G2, M. You can read more about the cell cycle at: http://www.ncbi.nlm.nih.gov:80/books/mboc/mboc.cgi?code= 1701011618232369 There is another state that cells can exist in (G0) when they are no longer going to divide. Cells enter G0 following the last M phase and then just live out their lives, never dividing again. For the cloning of mice, it was found to be important to use nuclei from G0 cells. Nuclei from Sertoli cells, neuronal cells, and cumulus cells were used successfully. About 90% of cumulus cells that surround recently ovulated oocytes are in G1/G0. Sertoli cells and neurons do not normally divide in adults, so they are in G0. A good paper to read about mouse cloning is: Wakayama T, Perry AC, Zuccotti M, Johnson KR, Yanagimachi R Full-term development of mice from enucleated oocytes injected with cumulus cell nuclei. Nature 1998 394:369-74 You might find this glossary useful in reading this answer: http:// www.informatics.jax.org/userdocs/glossary.shtml Thank you for an interesting question. Yours, Paul Szauter Mouse Genome Informatics
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