Chapter 11: Inferences
Lab Activity 55: Inferences in Texts and Visuals
To apply strategies in making inferences to information from a cartoon, a biology textbook, and a news article.
Step 2: Read the following excerpt from a college biology textbook.
Much Ado About Dolly
And what is the scientific community doing? They're cloning sheep. Great! Just what we need! Sheep that look more alike than they already do!
—Dave Barry, 1997
Early in 1997 a short article in the journal Nature caused headlines throughout the world. Ian Wilmut and his associates at the Roslin Institute in Edinburgh, Scotland, had done what many scientists believed to be impossible. They cloned a mammal, using a nucleus taken from adult tissue. This type of cloning is the process of artificially creating a clone—a new individual that is genetically identical to an existing individual.
At the time of Wilmut's breakthrough, researchers had already discovered that they could replace the nucleus of an egg cell with the nucleus taken from a cell in an extremely early stage of embryonic development. After such a nuclear transfer, the introduced embryonic cell nucleus directs the development of a new individual, with no need for a fusion of sperm and egg. Until 1997, however, attempts to make clones using nuclei taken from older tissues (from embryos that had more than 16 cells) had failed. Researchers hypothesized that these attempts failed because it is impossible to undo the changes in gene expression that cause cells to specialize, to become, say, nerve cells instead of skin cells or liver cells or fat cells. Wilmut's work, however, showed that cell specialization is reversible under the right conditions. The researchers took cells from the udder of a 6-year-old ewe and grew them in culture dishes. The cells were deprived of nutrients, which forced them into the nondividing stage of the cell cycle. Apparently, this change made possible the expression of all necessary genes when the nucleus from one of the cells was transplanted into an egg cell.
Cloning is a highly artificial form of asexual reproduction, because the nucleus that directs development of the offspring was produced by mitotic division of a diploid cell and not the fusion of haploid gametes produced by meiosis. Like the hydra budding from its parent, the cloned mammal is a genetic "chip off the old block." The advantage of cloning cells from an adult rather than from an embryo is that you know what you are getting. A prized wool-producing sheep could theoretically be cloned to produce a whole herd. The same might be done with a cow that has exceptional milk production. Cloning an adult allows scientists to use asexual reproduction to take advantage of the natural variability provided by sexual reproduction. By observing the adults, the most advantageous gene combinations produced by meiosis and the union of gametes can be selected and then perpetuated in clones.
Dolly and other cloned mammals also promise to provide insights into aging. Do the genes of each species dictate a maximum life span? If so, Dolly may have been born middle-aged, because her genes came from the cell of a 6-year-old ewe. Recent results support this idea. The ends of eukaryotic chromosomes have special repetitive nucleotide sequences called telomeres. In most cells in an animal, each cell division leaves the telomeres a tiny bit shorter. Old animals, then, have shorter telomeres than young animals. Dolly's telomeres are shorter than usually seen in a 3-year-old sheep. Will her life be shorter as a result? It may take some time to know, since a sheep's average life span is 13 years.
Step 3: Read the following news article about Dolly's outcome, and then answer the questions about both readings.
The Associated Press
LONDON (AP) Dolly the cloned sheep was put to death Friday, after premature aging and disease marred her short existence and raised questions about the practicality of copying life.
The decision to end Dolly's life at age 6about half the life expectancy of her breedwas made because a veterinarian confirmed she had a progressive lung disease, according to the Roslin Institute, the Scottish lab where she was created and lived.
"We must await the results of the post-mortem on Dolly in order to assess whether her relatively premature death was in any way connected with the fact that she was a clone," said Richard Gardner, a professor of zoology at Oxford University and chair of the Royal Society working group on stem cell research and therapeutic cloning.
"If there is a link, it will provide further evidence of the dangers inherent in reproductive cloning and the irresponsibility of anybody who is trying to extend such work to humans."
Ian Wilmut, the leader of the team that created Dolly, said it was unlikely her illness was attributable to being a clone.
"The most likely thing is an infection which causes a slow progressive illness and for which there isn't an effective treatment," he said. "Sadly, we have had that in some of the sheep on the farm, so that's the most likely explanation, but we don't know."
Wilmut declined to name the disease but said it was a common respiratory infection that had been diagnosed in another of the sheep Dolly was housed with.
"The most likely thing is she caught it from that sheep and it's an unfortunate result of having to be housed in order to give her security and so that we could observe her," Wilmut said.
"Clearly, the whole group are very upset and sad."
Griffin said that Dolly had been coughing for about a week before the vet came Friday afternoon.
She was born July 5, 1996, in a research compound of the Scottish institute, and the achievement of her creationannounced Feb. 23, 1997created an international sensation.
Researchers had previously cloned sheep from fetal and embryonic cells, but until Dolly, it was unknown whether an adult cell could reprogram itself to develop into a new being.
The Dolly breakthrough heightened speculation that human cloning inevitably would become possible.
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