Book 1 Reading and Speaking (1108795), страница 17
Текст из файла (страница 17)
"Thereare more out there." Dallas Swallow, a human geneticist at University College London, agrees with thatassessment. She released a study on a small Sudanese tribe in Human Genetics this past November, finding threemarkers, two of which Tishkoff had isolated in her study. Oddly enough, Swallow found no data on the Marylandstudy's primary variant. Tishkoff argues this disparity is due to geographic specificity of these mutations. Swallow,for her part, notes that Tishkoff's dramatic results may be a result of "the relative relatedness" of her sample.
"If youhave an ethnic group which is rather a small population in size but happens to migrate over geographic distancesthen they might be more related to each other than the surrounding people," she points out.(D) Working with this highly correlated locus, Tishkoff's team sequenced a broad region of the chromosomearound this nucleotide to determine whether it arose in concert with the European mutation.
"It turns out they're oncompletely different chromosome backgrounds," she explains. "So it had a completely different origin." Next, theteam tested to see if the mutation was positively selected, conferring a reproductive advantage and spreading40quickly through the population. People who had this particular SNP on both copies of chromosome 2 had identicalgenetic scripts for the next two million base pairs--a phenomenon that occurs when there is a strong benefit tohaving a particular trait." Because this section has been preserved intact without being mutated or broken up byrecombination, it indicates that it is very recent and very strong.
In addition, Tishkoff's team determined the daterange when the mutation likely occurred: 3,000 to 7,000 years ago, which matches up well with the archaeologicalrecord that places pastoralization coming to East Africa about 5,000 years ago. The European trait dates backabout 9,000 years.(E) Now, the results of a four-year, international research project find that communities in East Africa leadingtraditionally similar pastoral lives evolved their ability to drink milk rapidly and independently of the northernEuropeans.
According to University of Maryland biologist Sarah Tishkoff, the lead author of a study appearing intoday's Nature Genetics, the mutation allowing them to "get milk" arose so quickly and was so advantageous that"it is basically the strongest signal of selection ever observed in any genome, in any study, in any population in theworld."(F) Nevertheless, both researchers are pleased that their studies found at least two genetic markers incommon. Swallow concedes: "It looks jolly well as though drinking milk as an adult was good for some of us atsome time in our history, that's for sure." (From Scientific American Online, December 11, 2006)Exercise 9.
Make up a list of the 10 key facts about human evolution discussed in this unit. Prove andexplain your choice. Then summarize all the information discussed into one report.41Unit 12. AlcoholWe drink one another’s health and spoil our own.Jerome K. JeromeExercise 1. What do you know about alcohol?1. What is its main active compound?2. In what regions of the world is alcohol produced?3. How does alcohol affect the human body?4. What are the risks of alcohol overdose?5.
What causes hangover after drinking alcohol?6. What is alcoholism? How quickly does it develop?Exercise 2. Read the information about alcohol provided by NewScientist.com to check some of youranswers in Exercise 1.What is it? Ethanol produced by the action of yeast on sugars.What does it do? Ethanol is a biphasic drug: low doses have a different effect to high doses.
Small amounts ofalcohol (one or two drinks) act as a stimulant, reducing inhibition and producing feelings of mild euphoria. Higherdoses depress the central nervous system, initially producing relaxation but then leading to drunkenness characterised by poor coordination, memory loss, cognitive impairment and blurred vision. Very high doses causevomiting, coma and death through respiratory failure. The fatal dose varies but is somewhere around 500milligrams of ethanol per 100 millilitres of blood.How does it work? At low doses (5 milligrams per 100 millilitres of blood), alcohol sensitises NMDA receptors inthe brain, making them more responsive to the excitatory neurotransmitter glutamate, so boosting brain activity.These effects are most pronounced in areas associated with thinking, memory and pleasure.
At higher doses itdesensitises the same receptors and also activates the inhibitory GABA system.How long is its history?4000BC - Wine and beer making in Egypt and Sumeria3500BC - Bronze-age vessels show evidence of wine consumption in eastern Mediterranean800BC - Distillation of spirits in IndiaAD625 - Mohammed orders his followers to abstain from alcohol1850s - New York bartenders invent the cocktail1920-33 - Prohibition in the US. Alcohol was also illegal in Finland from 1919 to 1932 and in various Canadianprovinces at various times between 1900 and 1948.Exercise 3.
Now read more detailed information about the effects of alcohol on the brain provided byAnthony Dekker D.O., Director of Ambulatory Care and Community Health at Phoenix Indian MedicalCenter.What are the effects of alcohol on the brain?The product of the oldest chemical reaction studied by man, alcohol, continues to challenge researchers.Since the original work on alcohol's neurological effects in the early 20th century, new theories have regularlyemerged. What we have learned is that alcohol is a sedative-hypnotic in the acute intoxication phase for mostpatients.
But it diminishes the quality of sleep. Individuals with sleep apnea often experience longer and moresevere apneic episodes and hypoxia, or oxygen deprivation, after drinking alcohol.In other individuals, though, alcohol may act as a stimulant. Indeed, its association with violent and selfabusive behavior is well documented.
At intoxicating levels, alcohol is a vasodilator (it causes blood vessels torelax and widen), but at even higher levels, it becomes a vasoconstrictor, shrinking the vessels and increasingblood pressure, exacerbating such conditions as migraine headaches and frostbite. Researchers have alsothoroughly documented the effects of alcohol on the developing fetus. Approximately one third of all babies born toalcoholic mothers will develop Fetal Alcohol Syndrome or Effects (FAS or FAE), causing central nervous systemdysfunctions including Attention Deficit Disorder (ADD) and impaired IQ.
There are also growth and facialabnormalities associated with these infants.In the early 1900s, H. Meyer and Charles Ernest Overton originally theorized that the effect of alcohol wasachieved by altering the lipid environment of cell membranes. This theory, however, requires much higherconcentrations of alcohol than are clinically observed. A recent theory, supported by several researchers, pinsalcohol's effect on voltage and ligand-gated ion channels that control neuronal activity. Two distinct ligand-gatedchannels have been identified, inhibitory ones (GABA receptors and strychnine-sensitive glycine receptors) andexcitatory ones (N-methyl-D-aspartate (NMDA) and non-NMDA glutamate-activated channels and the 5HT3subtype of serotonin receptors).The inhibitory aspect occurs due to a hyperpolarization of neurons, secondary to an influx of chloride ions.The neuron becomes less likely to achieve the threshold membrane potential.
The excitatory receptor is dependenton the NMDA and non-NMDA glutamate receptors that control the influx of sodium and calcium, which bind to42endogenous neurotransmitters (glutamate or aspartate) and depolarize the neuronal membrane. The NMDAreceptor seems to have a high permiability to calcium, which acts as a catalyst to several intracellular events.Chronic exposure to alcohol seems to alter the NMDA receptors and this may play a role in the clinicalsymptoms of alcohol withdrawal. In vitro studies have demonstrated an increase in the binding sites for MK801(dizocilpine) in neurons chronically exposed to alcohol. This rise may account for the acclimation process, in whichgreater concentrations of alcohol are needed to cause experimental and clinical symptoms of intoxication.
NMDAcan cause seizure activity. Mice that have been exposed to chronically elevated levels of alcohol reveal increasednumbers of NMDA receptors and NMDA related seizure activity. The NMDA antagonist MK801 has been shown todecrease the severity of seizures in these mice during withdrawal. Through a complex process of cell membraneion pumps and neurotransmitter stimulation, the multi-faceted effects of alcohol and alcohol withdrawal arebecoming better understood.Under the influence of alcohol, the brain experiences impairments in the following regions:•Frontal Lobe - Loss of reason, caution, inhibitions, sociability, talkativeness and intelligence.•Parietal Lobe - Loss of fine motor skills, slower reaction time, shaking.•Temporal Lobe - Slurred speech, impaired hearing.•Occipital Lobe - Blurred vision, poor distance judgement.•Cerebellum - Lack of muscle coordination and balance.•Brain Stem - Loss of vital functions.Exercise 4.
Do you agree with the statements below? Give reasons for your choice.1. Alcohol may have sedative-hypnotic and stimulatory effect on people.2. Alcohol affects circulatory system.3. So far scientists have not completely uncovered the mechanism of its activity.4.
The effect of alcohol is most pronounced in the brain.5. Chronic exposure to alcohol produces no changes of brain structures.Exercise 5. Some scientists argue that alcohol can produce significant positive effect on one’s health. Thearticle below provides evidence for this point of view. Read the article and find answers to the questions:1.
What disorders can alcohol protect from?2. What are the mechanisms involved?Drink to Your Health?By Arthur L. KlatskyThree decades of research shows that drinking small to moderate amounts of alcohol has cardiovascular benefits.A thorny issue for physicians is whether to recommend drinking to some patients.America has always had trouble deciding whether alcohol is a bad thing or a good thing.
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