Question

    In 1977, a University of Oxford statistician named Richard Peto pointed out a simple yet puzzling biological fact: we humans should have a lot more cancer than mice, but we don’t.

Dr. Peto’s argument was simple. Every time a cell divides, there’s a small chance it will gain a mutation(突变) that speeds up its growth. Cells that accumulate (聚集) several of these mutations may become cancerous. The bigger an animal is, the more cells it has, and the longer an animal lives, the more times its cells divide. We humans undergo (经历) about 10,000 times as many cell divisions as mice—and thus should be far more likely to get cancer.

A number of scientists have assumed that large, long-lived animals must have evolved extra cancer-fighting weapons. Otherwise, these species would die out.

Dr. Joshua D. Schiffman, an oncologist (肿瘤学家）at the Huntsman Cancer Institute, reviewed zoo records on the deaths of 644 elephants and found that less than 5 percent died of cancer. By contrast, 11 percent to 25 percent of humans die of cancer.

To understand the elephants’ defenses, the scientists investigated a gene that is vital to preventing cancer called p53. The protein encoded by the gene monitors cells for damage to the DNA they contain. Dr. Schiffman and his colleagues found that elephants have evolved new copies of the p53 gene. While humans have only one pair of p53 genes, the scientists identified 20 pairs in elephants.

To see whether these extra copies of p53 made a difference in fighting cancer, they ran experiments on elephant cells. They treated elephant cells with damaging radiation, chemicals and UV light.

“In all cases, instead of trying to repair the damage, the elephant cells simply committed suicide” said Dr. Schiffman. He said he thought the way the elephant cells responded was a very effective way to block cancer.

Patricia Muller, an oncologist at the MRC Toxicology Unit at the University of Leicester, who was not involved in the studies, said the results, though convincing, didn’t firmly establish exactly how elephants use p53 to fight cancer. One possibility is that the extra copies don’t actually cause cells to commit suicide. Instead, they may act as decoys (诱饵) for enzymes (酶) that destroy p53 proteins.

Dr. Muller said it was especially important to understand precisely how elephants fight cancer before trying to copy their strategies with drugs for humans.

1.What does the 2nd paragraph want to tell us?

A.Human should be more likely to develop cancer than mice.

B.Cell division can increase the speed of growth of mutations.

C.The bigger an animal is, the smaller chance of getting cancer it has.

D.Dr. Peto’s argument was not simple.

2.According to the passage, p53 is ______.

A.a protein to prevent cancer B.a gene to defend against cancer

C.a protein to destroy damaged cells D.a gene to change cells

3.What does Dr. Schiffman say about the extra copies of p53 fighting cancer?

A.They cause the cells to repair the genes.

B.They stop cells from dividing further.

C.They act as decoys for enzymes that destroy p53 proteins.

D.They cause the cells to commit suicide.

4.Which of the following is TRUE?

A.Dr. Schiffman pointed out that we humans don’t have more cancer than mice.

B.The cancer death rate of humans is five percent higher than that of elephants.

C.The scientists found there are 20 pairs of p53 genes in elephants.

D.Dr. Muller also took part in the studies.

Answer

1.A

2.B

3.D

4.C

【解析】

这是一篇说明文。文章讲述了，动物越大，患癌地可能性就越大，但是长寿的大型动物很多，本文以大象为研究对象，研究出大象身上有一种对预防癌症至关重要的基因p53，但并不能确切地确定大象是如何利用p53来对抗癌症的，人类能否使用p53来抗癌还需进一步研究。

1.主旨大意题。由第二段的句子“We humans undergo (经历) about 10,000 times as many cell divisions as mice—and thus should be far more likely to get cancer.”可知，我们人类经历的细胞分裂次数是老鼠的10000倍，因此患癌症的可能性要大得多。所以第二段想告诉我们人类比老鼠更容易患癌症。故选A项。

2.细节理解题。由第五段的句子“To understand the elephants’ defenses, the scientists investigated a gene that is vital to preventing cancer called p53.”可知，为了了解大象的防御能力，科学家们研究了一种对预防癌症至关重要的基因p53。所以p53是抵抗癌症的基因。故选B项。

3.细节理解题。由题干extra copies of p53定位到第六、七段。第六段讲对大象额外的p53基因做实验；由第七段的句子“In all cases, instead of trying to repair the damage, the elephant cells simply committed suicide”可知，在所有情况下，大象额外的p53基因细胞并没有试图修复损伤，而是自杀了。故选D项。

4.细节理解题。由第五段的最后一句“While humans have only one pair of p53 genes, the scientists identified 20 pairs in elephants.”可知，虽然人类只有一对p53基因，但科学家在大象身上发现了20对。故选C项。