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Basic Genetics - from an animal science perspective

 

Cloning of organisms

Cloning is a method of producing two or more genetically identical organisms by asexual reproduction. This means that there is only one parent cell, from which all the genetic information will come. Thus, the DNA sequence of cloned organisms is exactly the same as that of the parent cell. In nature, cloning occurs when identical twins, triplets, and so on are born or when a cutting from a plant is used to generate another plant. It also occurs in organisms that reproduce by asexual reproduction, such as bacteria and yeast. Recently, laboratory experiments have generated animal clones using a variety of different parent cell types.

Before 1996 and the creation of Dolly, it was thought that animal clones had to originate from embryonic cells before they differentiated into the specialized cells, such as liver, blood and bone cells, that make up an adult animal. This idea seemed logical because as a cell differentiates, it permanently turns off those genes that are not specifically needed for its function. Thus, a liver cell would no longer be able to turn on the genes necessary to become a bone cell and vice versa. In the 1950s, scientists experimentally supported this idea when they took embryonic frog cells and created frogs from them but were unable to repeat their results using differentiated cells. Thus, the 1996 news that a group of scientists at the Roslin Institute had discovered a method to clone a sheep using differentiated sheep udder cells was very surprising to many people.

The group found that if they starved udder cells that were being grown in culture, these cells could then be used to produce a sheep. The process the scientists used was technically challenging, but is easy to understand, and is highlighted by the accompanying animation.

First, an egg cell was taken from a sheep, and its nucleus was removed using a very small needle. Then a starved udder cell was inserted into the enucleated egg cell. The egg cell came from a black sheep while the udder cell, which contained all of the genetic information, came from a white sheep. The scientists made over 250 of these cloned embryos and implanted them into several different female sheep. One of these embryos developed into a white lamb, which was born on July 5, 1996 and named Dolly. Dolly had the same genetic material as the udder cell and was therefore a clone of an udder cell.

Since the birth of Dolly other experiments have been done and other animals, including mice, cows, a cat (as part of the 'Operation CopyCat' project), and even an early stage human embryo, have been cloned using different processes. Many other projects are also currently underway.

The process of cloning has many potential benefits for society. The greatest potential benefit of cloning is in the field of medicine. Cloning has been used to generate stem cells that can be used for transplants. Stem cells are cells that have not yet differentiated into specialized cells and can be used to replace unhealthy cells in an individual. For example, someone who suffers from bone marrow cancer has many cancerous bone marrow stem cells and few healthy bone marrow stem cells circulating throughout their body. Scientists and doctors have now developed processes by which these healthy cells can be removed from the patient and cloned, to produce a lot of them, in order to transplant them back into the patient. Since these cells are coming from the patient and have not been donated, the risk of the patient rejecting the cloned cells is eliminated. Stem cells can also be used to create organs for transplantation as well. The benefits from this would be two-fold because not only would the same problem of the immune system rejecting the organ be bypassed, but also the patient wouldn't have to wait for an organ donor to become available (which often can take years).

Cloned animals are also being used to maintain valuable genetic traits. Over a period of time, genes can be lost from generation to generation within a family. Therefore the offspring of transgenic animals (see section on transgenes) that carry special genes for making medicines and producing organs aren't often transgenic themselves. Scientists have solved this problem by cloning transgenic animals as a way to ensure that the genetic makeup remains the same over consecutive generations.

Despite its many benefits, cloning also presents us with some difficult ethical issues. If scientists are successfully cloning animals and human embryos right now, then how long will it be until the first adult human is cloned? Is it okay to use cloning in situations where a couple cannot have children or one member of the couple carries the genes for a heritable disease? If cloning is allowed under certain circumstances, then it could potentially be abused, such as by couples wanting their child to be a clone of an athlete, a genius or a celebrity. The possibilities would be endless.

The other problems with cloning arise from the uncertainties about the quality of life of the clones. For example, how old is a clone when it is born? If the cells used in the cloning process are ten years old then is the clone ten years old when it is born, meaning that it would have a shorter life span than average? There are two reasons that a clone may have inherited the age of the cell that it came from: telomere shortening and DNA damage.

A telomere is the region of DNA at the end of a linear chromosome that is required for replication and stability of that chromosome. Some scientists believe that telomeres determine how long an organism lives. As the organism develops, the telomere becomes progressively shorter. The cells eventually stop replicating when their telomeres reach some critical length. In the case of Dolly, her telomeres were found to be 20% shorter than a normal sheep of her age. Does this mean that Dolly will have a shortened life span?

DNA damage is due to the deterioration of DNA that occurs in a cell over time. If this damage is not fixed, then it can lead to diseases such as cancer. This would mean that if a clone were made from damaged DNA, it would be more likely to develop a disease. In fact, a recent study showed that cloned mice died of kidney failure and pneumonia much earlier than their uncloned counterparts.

Thus, society has many questions regarding cloning that it must answer. That fact is - cloning is here. The question is - what will we do with it?