Part B Directions: In the following text, some sentences
have been removed. For Questions 41-45, choose the most suitable one from the
list A-G to fit into each of the numbered blanks. There are two extra choices,
which do not fit in any of the blanks. Mark your answers on ANSWER SHEET 1.
Most people would not object to living a few years longer than
normal, as long as it meant they could live those years in good health. Sadly,
the only proven way to extend the lifespan of an animal in this way is to reduce
its calorie intake. Studies going back to the 1930s have shown that a
considerable reduction in consumption ( about 50% ) can extend the lifespan of
everything from dogs to nematode worms by between 30% and 70%. Although humans
are neither dogs nor worms, a few people are willing to give the
calorie-restricted diet a try in the hope that it might work for them, too. But
not many—as the old joke has it, give up the things you enjoy and you may not
live longer, but it will sure seem as if you did. Now, though,
work done by Marc Hellerstein and his colleagues at the University of
California, Berkeley, suggests that it may be possible to have, as it were, your
cake and eat it too. Or, at least, to eat 95% of it. Their study, to be
published in the American Journal of Physiology—Endocrinology and Metabolism,
suggests that significant gains in longevity might be made by a mere 5%
reduction in calorie intake. The study was done on mice rather than people.
But the ubiquity of previous calorie-restriction results suggests the same
outcome might well occur in other species, possibly including humans. However,
you would have to fast on alternate days. (41)______ Cancer
is the uncontrolled growth of cells. For a cancer to develop efficiently, it
needs multiple mutations to accumulate in the DNA of the cell that becomes the
tumor’s ancestor. (42)______ A slower rate of cell division
thus results in a slower accumulation of cancer-causing
mutations. (43)______ Heavy water is heavy because the
hydrogen in it weighs twice as much as ordinary hydrogen (it has a proton
and a neutron in its nucleus, instead of just a proton). Chemically, however, it
behaves like its lighter relative. This means, among other things, that it gets
incorporated into DNA as that molecule doubles in quantity during cell
division. (44)______ Dr Hellerstein first established how
much mice eat if allowed to feed as much as they want. Then he set up a group of
mice that were allowed to eat only 95% of that amount. In both cases, he used
the heavy-water method to monitor cell division. The upshot was that the rate of
division in the calorie-restricted mice was 37% lower than that in those mice
that could eat as much as they wanted--which could have a significant effect on
the accumulation of cancer-causing mutations. (45)______
[A] To stop this happening, cells have DNA-repair mechanisms. But if a
cell divides before the damage is repaired, the chance of a successful repair is
significantly reduced. [B] Bingeing and starving is how
many animals tend to feed in the wild. The uncertain food supply means they
regularly go through cycles of too much and too little food ( it also means that
they are often restricted to eating less than they could manage ff food were
omnipresent). [C] But calorie-reduction is not all the
mice had to endure. They were, in addition, fed only on alternate days: bingeing
one day and starving the next. So, whether modern man and woman, constantly
surrounded by food and advertisements for food, would really be able to forgo
eating every other day is debatable. [D] Why caloric
restriction extends the lifespan of any animal is unclear, but much of the smart
money backs the idea that it slows down cell division by denying cells the
resources they need to grow and proliferate. One consequence of that slow-down
would be to hamper the development of cancerous tumors. [E] So,
by putting heavy water in the diets of their mice, the researchers were able to
measure how much DNA in the tissues of those animals had been made since the
start of the experiment (and by inference how much cell division had taken
place), by the simple expedient of extracting the DNA and weighing
it. [F] The second reason, according to Elaine Hsieh, one of Dr
Hellerstein’s colleagues, is that cutting just a few calories overall, but
feeding intermittently, may be a more feasible eating pattern for some people to
maintain than making small reductions each and every day. [G]
At least, that is the theory. Until now, though, no one has tested whether
reduced calorie intake actually does result in slower cell division. Dr
Hellerstein and his team were able to do so using heavy water as a chemical
"marker" of the process.