Book 2 Listening (1108796), страница 14
Текст из файла (страница 14)
Sadly, reality is not quite so clear-cut. In a paperin Science, Sandra Garrett and Joshua Rosenthal of the University of Puerto Rico illustratehow the instructions in the DNA are not always followed faithfully. The RNA message can berewritten before it is read. And that provides an extra opportunity for evolution to occur.Dr Garrett and Ms Rosenthal were studying octopuses, looking for differences betweenthose that live in warm, tropical water and those that inhabit the poles. They concentrated onthe make-up of the ion channels in the animals' cell membranes. These channels arecylindrical assemblages of protein molecules which help to control such things as theelectrical activity of nerve cells and the release of hormones.
The two researchers suspectedthat the channels found in warm-water species would not work well in the freezingtemperatures that their polar cousins endure.That turned out to be correct. What was odd was that the genes for the proteinsinvolved were almost identical in warm- and cold-water animals.
This surprised Dr Garrettand Ms Rosenthal, who had expected that natural selection would have changed the DNA,and thus the composition of the resulting protein.Instead, differences in composition between warm-water and cold-water ion channelswere the result of a phenomenon called RNA editing, in which special enzymes alter thestructure of the RNA messenger, and thus of the final protein.Though RNA editing has been observed before, in animals ranging from humans tonematode worms, this is the first time an edit has been tied to a clear evolutionary differencecaused by a feature of the environment - in this case ambient temperature.
Of course, it isnot strictly a departure from Crick's dogma. Enzymes, too, are proteins, and so are thetranscription factors that regulate their production. Eventually, when the chain of causation istraced in full, the chances are that the underlying difference between polar and tropicaloctopuses will be in the DNA itself. RNA editing of this sort does, however, provide anotherway to drive evolution, and may help explain why animals (as opposed to, say, bacteria) areso complex. (From The Economist, January 7, 2012)Script 22. The nature of manThe nature of manLarge-scale genetic studies are throwing light on what makes humans humanHumans are peculiar as a species, so what makes them so must be hidden in theirgenome.
To an almost disconcerting extent, though, the human genome looks similar to thegenomes of other primates, especially when it comes to the particular proteins it allows cellsto make.The powerful new ways of looking at the genome being pioneered by the ENCODEconsortium, though, provide ways to seek out the subtle species-specific signals.
Lucas Wardand Manolis Kellis of the Massachusetts Institute of Technology report on the results of suchsleuthing in a paper just published in Science.The two researchers used data from ENCODE to identify the bits of the genome thatactually do things and data from the 1,000 Genomes Project, which has studied human56genome variation across hundreds of people, to discover how much these functionalelements vary from person to person. In particular, they looked for telltales that an element isbeing maintained by natural selection. If something is evolutionarily important then randomvariations in its DNA sequence will be slowly eliminated from the population, keeping it on thefunctional straight and narrow in a process known as purifying selection.Dr Ward and Dr Kellis found that, in addition to the 5% of human DNA that is conservedbetween mammals, an additional 4% of human DNA appears to be uniquely human in thesense that it is prone to purifying selection in humans but not in other mammals.
Much of thisproprietary DNA is involved in regulating gene activity - for example, controlling how much ofa protein is produced, rather than changing the nature of the protein itself. This finding is inline with modern thinking that a lot of evolutionary change is connected with regulatoryelements rather than actual protein structure.
The researchers also found that long noncoding segments that are not conserved in other mammals are in fact highly constrained inhumans, suggesting they have human-specific functions.Some areas identified as particularly human are the regulation of the cone cells of theretina (which are involved in colour vision) and the regulation of nerve cell growth. Theseprocesses evolved rapidly in man's primate ancestors but are now under strong purifyingselection to maintain their beneficial functions. The implications of that, given humanity'smain distinguishing feature - its huge brain - are obvious. Dr Ward and Dr Kellis have thuscreated a powerful tool for investigating in detail just what it is that makes a human beinghuman.
(From The Economist, September 8, 2012)Script 23. Tibetan geneticsLife at the topThe secret of Tibetans' success lies with ancestors who were not quite humanResearchers have known for a while that many people alive today carry genes fromhuman species other than Homo sapiens - the result of ancient interbreeding withNeanderthals and Denisovans.
They have even worked out that this admixed DNA mustoften be doing something particularly useful, because its pattern suggests natural selection isactively retaining it. The specifics, though, have not been clear. But in one case they now are,for it is because of these occasional Denisovan ancestors that Tibetans thrive in Tibet.The plateau of Tibet is one of the most hostile places people inhabit. The air is thin andthe weather cold.
The locals, nevertheless, do well. And Rasmus Nielsen of the University ofCalifornia, Berkeley, and his colleagues at BGI, a Chinese DNA sequencing laboratory,suggest in this week's Nature that one of the genes which lets them do so is Denisovan.The Denisovans are a mysterious branch of Homo. They were identified in 2010 by ananalysis of the DNA of a bone discovered in a cave (occupied in the 18 th century by a hermitcalled Denis) in the Altai Mountains in Russia.
This bone was thought, when found, to beeither Neanderthal or modern human, but the analysis showed it was neither. In the wake ofthat finding, a small percentage of Denisovan DNA has been discovered in various groups ofpeople in Asia and the Pacific islands, Tibetans among them.The gene Dr Nielsen has been investigating is a version of EPAS-1. This encodes partof a protein called hypoxia-inducible transcription factor 2-alpha. Transcription factorsactivate other genes, and this one (as its name suggests) does so in response to low oxygenlevels.
When that happens, it is responsible for stimulating the production of red blood cells,the growth of capillaries and the production of proteins involved in energy generation.Everybody has some version of EPAS-1, and so everybody can acclimatise to highaltitude. But such acclimatisation comes at a price: the extra red cells make blood stickierand more likely to clot, which increases the risk of thrombosis.
Except, curiously, in Tibetans.They are well acclimatised without having noticeably raised red-cell counts. And that effecthas been tracked down to the particular version of EPAS-1 in their chromosomes.Dr Nielsen and his team wanted to study the Tibetan version more closely, so theysequenced both it and the area around it in detail. When they did this they discovered that57the block of DNA it inhabits is so similar to its Denisovan equivalent that it must originallyhave come from a mating (maybe more than one) between a Denisovan and Homo sapiens.Moreover, it is pretty much only Tibetans who have this version of the gene.
Dr Nielsenestablished in 2010 that about 90% of them do, compared with fewer than 10% of their HanChinese neighbours. He estimated that the altitude-friendly version became this widespreadin Tibet in a mere 3,000 years.His latest study also looked at other groups with Denisovan genes, and could find notrace at all in them of the Denisevan form of EPAS-1. Probably, it is useless or worse at lowaltitudes, so natural selection has removed it. But, though the Altai Mountains are not as highas Tibet, it might well have been useful there )From The Economist, July 5, 2014)Script 24. Gene TherapyHello mothers, hello fatherA technique intended to eliminate mitochondrial diseases would result in people withthree genetic parentsIs it possible for a child to have three parents? That is the question raised by a paperjust published in Nature by Shoukhrat Mitalipov and his colleagues at Oregon Health andScience University.
And the answer seems to be "yes", for this study paves the way for thebirth of children who, genetically, have one father, but two mothers.The reason this is possible is that a mother's genetic contribution to her offspring comesin two separable pieces. By far the largest is packed into the 23 chromosomes in the nucleusof an unfertilised egg. In that, she is just like the child's father, who provides another 23through his sperm. But the mother also contributes what is known as mitochondrial DNA.Mitochondria are a cell's power-packs. They convert the energy in sugar into a form usableby the cell's molecular machinery.
And because mitochondria descend from a bacterium that,about 2 billion years ago, became symbiotic with the cell from which animals and plants aredescended, they have their own, small chromosomes.In people, these chromosomes carry only 37 genes, compared with the 20,000 or so ofthe nucleus. But all of the mitochondria in a human body are descended from those in theegg from which it grew. The sperm contributes none. And it is that fact which has alloweddoctors to conceive of the idea of people with two mothers: one providing the nuclear DNAand one the mitochondrial sort.
The reason for doing this is that mutations in mitochondrialDNA, like those in the nuclear genes, can cause disease. These diseases especially affectorgans such as the brain and the muscles, which have high energy requirements. Eachparticular mitochondrial disease is rare. But there are lots of them. All told, there is about onechance in s,ooo that a child will develop such an inherited disease.
That rate is similar, forexample, to the rate of fragile-x syndrome, which is the second-most-common type ofcongenital learning difficulty after Down's syndrome. Mitochondrial disease is thus not a hugeproblem, but it is not negligible, either.To find out whether mitochondrial transplantation could work in people (it has alreadybeen demonstrated in other species of mammal) Dr Mitalipov collected eggs from the ovariesof women with mutated mitochondria and others from donors with healthy mitochondria.
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