Book 2 Listening (1108796), страница 15
Текст из файла (страница 15)
Hethen removed the nuclei of both. Those from the healthy cells, he discarded. Those from thediseased cells, he transplanted into the healthy cells. He then fertilised the result with spermand allowed the fertilised eggs to start dividing and thus begin taking the first steps on thejourney that might ultimately lead to them becoming full-fledged human beings.Nearly all of the experimental eggs survived the replacement of their nuclei, and threequarters were successfully fertilised. However, just over half of the resulting zygotes - as theballs of cells that form from a fertilised egg's early division are known - displayedabnormalities. That compared with an abnormality rate of just an eighth in control zygotesgrown from untransplanted, healthy eggs.
This discrepancy surprised - and worried - DrMitalipov. The abnormality rate he observed was much higher than those seen when theprocedure is carried out on other species. That, though, could be because this is the first timeit has been attempted with human eggs. Each species has its quirks, and if mitochondrial58transplants were to become routine, the quirks of humans would, no doubt, quickly becomeapparent. With tweaks, they could be fixed, Dr Mitalipov predicts.However, turning this experiment into a medical procedure would be a long road, andnot just scientifically.
Dr Mitalipov has little doubt that his zygotes could be brought to term ifthey were transplanted into a woman's womb. That experiment, though, is illegal - and, in theview of some, rightly so. But the fact that it now looks possible will surely stimulate debateabout whether the law should be changed. (From The Economist, October 27, 2012)59Unit 10. Animal adaptationsScript 25. Radiation and evolutionSurviving falloutBirds can evolve to cope with the lingering effects of nuclear incidentsThe disaster last year at the Fukushima Dai-ichi nuclear power plant, caused by anearthquake and tsunami, scored seven on the International Nuclear and Radiological EventScale (INES). No worse rating exists. Radiation is harmful to living things, yet the long-termeffects of persistently high levels of background radiation on ecosystems are poorlyunderstood. With this in mind, a team led by Timothy Mousseau of the University of SouthCarolina and Anders Moller of the University of Paris-Sud set out to compare bird speciesdwelling near the Fukushima plant with those living at the site of another nuclear incident thatscored a seven on the INES: the Ukrainian town of Chemobyl, where disaster struck in 1986.Remarkably, they found that some species seem to develop a tolerance for radioactivity overtime.Fukushima and Chernobyl are more than 7,000 km (4,350 miles) apart, but DrMousseau and his colleagues soon realized that the two sites had much in common.
Bothare in areas that have a temperate climate with species that have similar habits and needs.And both are surrounded by a mixture of farmland and forest. Upon closer examination theresearchers found that 14 species of bird lived in both regions, including the barn swallow,great tit, great reed warbler, buzzard and Eurasian jay. With so many similarities between thetwo places, a comparison of the biological responses to radiation in each (recent inFukushima; long-term in Chernobyl) would surely be illuminating.To do this, during July 2011, the researchers counted and identified birds at 300locations near Fukushima that had radiation levels as low as 0.5 microsieverts per hour andas high as 35 (for comparison, dental X-rays rarely expose patients to more than 0.05microsieverts).
Then they compared these results to bird data collected in areas that had thesame range of radiation levels near Chernobyl between 2006 and 2009.Their results, published in Environmental Pollution, show that as radiation levels in anarea rose to 35 microsieverts per hour, the average number of birds dropped by almost athird compared with the areas where radiation levels were only 0.5 microsieverts per hour.This makes sense: in those areas with a high level of radiation, living things would tend to dieor sicken and fail to reproduce.
However, when researchers looked at the 14 bird speciesthat lived in both regions, they found that the same level of radiation was associated withtwice as large a drop in bird numbers in Fukushima as in Chernobyl.The reasons for this are not clear. It is possible that the composition of the radionuclidesare proving more dangerous to the Fukushima birds than they are to the birds nearChernobyl. But Dr Mousseau suggests a more likely explanation is that evolution has alreadybeen at work near Chernobyl, killing off individual birds that cannot cope with the backgroundradiation and allowing the genes of those that have some tolerance to be passed on. Thebirds at Fukushima are only beginning to face the evolutionary challenge of living in aradioactive world. (From The Economist, March 03, 2012)Script 26.
EvolutionSomething glowing onThe nuclear accident at Chernobyl has created a natural laboratoryCATCHING evolution in action is hard. The best-known examples are those wherehuman action, in the form of pesticides, herbicides or drugs, has intentionally made the worlda nastier place for some specific group of creatures, and natural selection has pushed backto create resistance. But a group led by Timothy Mousseau of the University of SouthCarolina, Anders Moller of the CNRS, in France, and Ismael Galván of the Doñana ResearchStation in Spain has now, in a paper in Functional Ecology, provided an example of selection60responding to a human action thatwasmost definitely unintentional: the explosion and fire at anuclear reactor in Chernobyl, Ukraine, 28 years ago.Dr Mousseau and Dr Moller knew from previous work that birds living near Chernobylhave better survival rates than those living near Fukushima, in Japan, where a seriousreactor accident happened in 2011.
They suspected that was because the Ukrainian birdshad had time to evolve resistance. They therefore sent blood and feather samples from 120birds of 13 species they collected from both high- and low-radiation regions around thedefunct reactor at Chernobyl to Dr Galván, who looked for genetic damage in them and alsoanalysed their levels of glutathione, an antioxidant that mops up highly reactive (andtherefore harmful) molecules created when radiation hits biological tissues.In those birds taken from low-radiation zones the average concentration of glutathionewas 450 micrograms per gram of body mass; in high-radiation areas it was 725 microgramsper gram. Moreover, the higher a bird’s glutathione level, the lower the amount of geneticdamage Dr Galván could spot in its cells.
Birds in high-radiation zones, then, seem to haveevolved to deal with the threat, just as Darwin would have predicted. (From The Economist,May 09, 2014)Script 27. PaleontologyTwo wingsMicroraptor's existence raises two questions: exactly how did it deploy its four wings,and why does it have no living descendants? The answers, according to Michael Habib andJustin Hall of the University of Southern California, are linked.Until recently, there were two schools of thought about how Microraptor carried its wingswhen in flight. One school proposed that it looked like a biplane, holding one set of wingsabove the other when it flew.
The second school suggested that all four wings were coplanar.Dr Habib and Mr Hall, however, think both are wrong.They see a division of labour between the two sets of wings. The front pair, they agree,provided lift. But they believe that the back pair were for steering. Microraptor's hind wingswere radically different in shape from its forewings. Rather than being lithe and graceful, theywere short and stubby. But they would have made good rudders, as Dr Habib and Mr Hallhave demonstrated using a computer model of them.
This suggests they would have allowedthe animal to reduce the radius of its turning circle by 40% and almost triple the speed of aturn.That makes sense. The rocks Microraptor fossils are found in are also full of trees.Clearly, it was a forest animal. Predatory birds that dwell in modern forests, such as thesharp-shinned hawk, are masters of making tight, quick turns around trunks and overbranches as they pursue their prey.
That Microraptor was a predator is known because onespecimen has a bird in its belly. It would surely have benefited from a similar capability.Why, then, was such an aerial paragon not ancestral to any modern creature? Onepossibility is bad luck. The mass extinction at the end of the Cretaceous was caused by acollision between Earth and an asteroid or comet. Though some groups of animals did betterthan others, survival was often at random. However, the fossil record does not prove that anyMicroraptor-like animal actually made it to the end of the Cretaceous.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
