A clone is a genetically identical copy of a living organism. Human clones are nothing new; they occur naturally in the recognizable form of identical twins. Not until the successful cloning of a sheep called Dolly, however, has the possibility of intentionally producing an identical copy of a human been considered seriously.
Production of a human clone would most likely utilize the same basic methodology that resulted in Dolly. A human egg would be retrieved from an individual, and its genetic material (DNA) would be removed and replaced with DNA derived from any adult human cell type. This would bypass the need for fertilization of the egg by the sperm in order to obtain a full complement of DNA. In a laboratory dish, the egg would then undergo several cell divisions. Placed into a uterus, the resulting embryo would grow and, with luck, develop until birth.
Although this cloning technique is conceptually and procedurally simple, its success rate has been extremely low. The birth of one Dolly, for instance, required the preparation of 277 re-nucleated eggs, followed by the implantation of 29 resulting best embryos. The low success rate can be attributed to the difference between the young DNA of a normally fertilized egg and the genetic material of the re-nucleated egg, which is mature and of defined destiny – it has already committed itself to a particular physiological role. In order for proper fetal development to occur, mature DNA must be coaxed into reverting to its youthful state, a complex process that will be difficult to achieve for the human species.
Beyond the practical difficulties of intentional human cloning, scientists, politicians, and others have raised serious ethical concerns about this practice. For example, there is a chance that the cloning procedure would adversely affect the developing embryo. There also might be deleterious effects on the long-term health of the clone. Proponents of human cloning counter that human sexual reproduction is not a risk-free affair either. Even if human cloning could be made safe, the motivations behind producing human clones do not fall clearly on one side of the ethical divide. While proponents stress potentially appealing motivations – a man and a woman who are both carriers of a genetic disease can use cloning to assure that their child will not be afflicted with the disorder – critics argue that the practice could and likely would be used for unethical ends, such as to select for certain traits, such as hair/eye color or sexual orientation, for example.
Question for Practice Test: Reading Comprehension - 28
Try yourself:According to the passage, which of the following is a potential obstacle to human cloning?
Explanation
The third paragraph states "The low success rate can be attributed to the difference between the young DNA of a normally fertilized egg and the genetic material of the re-nucleated egg, which is mature and of defined destiny – it has already committed itself to a particular physiological role." This means that human cloning faces difficulties in overcoming the fixed roles of mature DNA.
(A) The difficulty lies in overcoming the fixed roles of mature DNA; this choice does not address the issue.
(B) CORRECT: "Predetermination" reflects the fixed rolls of DNA found with "mature" or adult cells.
(C) The passage does not mention funding issues.
(D) The passage does not mention a need to determine "biological relationships within an egg" or the equivalent. The issue revolves around the different between young DNA and mature DNA.
(E) The passage does not mention any issues surrounding chemical reactions in the uterus. The issue revolves around the different between young DNA and mature DNA.
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Question for Practice Test: Reading Comprehension - 28
Try yourself:The primary purpose of the passage is to
Explanation
The question asks for the primary purpose of the passage. The correct answer must take the entirety of the passage into account without misrepresenting its focus. The passage focuses on the process by which cloning occurs and gives a brief discussion of the debate surrounding the ethics of the practice; in this debate, the author presents both sides from the points of view of others (critics and proponents).
(A) The passage does not dismiss cloning as entirely unethical, it merely raises the issue of ethics.
(B) Cloning is not a medical procedure, and furthermore the passage does not defend cloning. It gives a more neutral presentation of the topic.
(C) CORRECT: This passage explains cloning and discusses its ramifications.
(D) Cloning is not a hypothesis, as the passage states – it has been successfully accomplished with sheep. Furthermore the passage does not focus only on the negative consequences.
(E) Cloning is not a new medical approach. Also the passage doesn't analyze possible outcomes, or results, of cloning; it discusses the positive and negative ramifications of cloning.
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Question for Practice Test: Reading Comprehension - 28
Try yourself:Which of the following is NOT given in the passage as a reason not to pursue human cloning?
Explanation
In this "double-negative" question, we have to be very careful to make sure that we don't get confused or turned around. The "true-false" technique will be useful here:
answer choices get a True if they are mentioned as a reason not to pursue human cloning and a False if they are not mentioned for this reason. The correct answer will be labeled False.
(A) True. Paragraph four states that there "might be deleterious effects on the longterm health of the clone." This translates into unknown effects on adult human clones.
(B) True. Paragraph four states "there is a chance that the cloning procedure would adversely affect the developing embryo."
(C) True. Paragraph four states that cloning could be used to "select for certain traits, such as hair/eye color..."
(D) True. Paragraph three states that "mature DNA must be coaxed into reverting to its youthful state, a complex process that will be difficult to achieve for the human species."
(E) CORRECT: False. While it may be true that cloning is exorbitantly expensive, these costs are not mentioned anywhere in the passage
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Question for Practice Test: Reading Comprehension - 28
Try yourself:The author of the passage mentions Dolly most probably in order to
Explanation
In the first paragraph, the author states "Not until the successful cloning of a sheep called Dolly, however, has the possibility of intentionally producing an identical copy of a human been considered seriously." This indicates that the author believes Dolly's success has encouraged researchers to pursue human cloning.
(A) CORRECT: If researchers have been encouraged by Dolly's success to pursue human cloning, it must be because they believe there is a chance of success.
(B) Dolly is mentioned in the first paragraph; the ethical concerns are discussed in the fourth paragraph. In addition, the author does not show that ethical concerns are misguided; he merely presents both sides of the argument.
(C) The author does not argue, or even discuss, anywhere in the passage the idea that one type of cloning is less efficient than another type.
(D) The author does not argue for (or against) human cloning anywhere in the passage.
(E) The author does not refute anything in the passage. In the fourth paragraph, the author does present the viewpoints of proponents and critics, but the author does not weigh in on the debate.
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Question for Practice Test: Reading Comprehension - 28
Try yourself:The passage suggests which of the following?
Explanation
The second paragraph states "A human egg would be retrieved from an individual, and its genetic material (DNA) would be removed and replaced with DNA derived from any adult human cell type. This would bypass the need for fertilization of the egg by the sperm in order to obtain a full complement of DNA." This implies that the DNA from the adult cell already contains DNA from the egg (mother) and the sperm (father).
(A) The passage does not address anything about the time table for success with human cloning.
(B) The passage directly contradicts this statement by noting, in paragraph three,
how difficult it was to clone Dolly. In addition, the word "always" is extreme.
(C) The passage does not imply this; if anything, it implies that the similarity between the two types of egg is part of the reason why the success of cloning Dolly may translate into success with human cloning.
(D) CORRECT: As noted above, the passage implies that an adult cell contains DNA from both parents.
(E) The passage does not imply this; if anything, it implies that the similarity between the two types of DNA is part of the reason why the success of cloning Dolly may translate into success with human cloning.
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