Day 55

Passage 55

Mystery about Bees

①

Although all honeybees are genetically identical, they quickly take on roles within the hive; few become queens while most become workers. Once a bee is a queen or worker, the change is irreversible and they fulfill that role for life; yet, that is not the case for the subdivisions among the workers. Workers start out their lives as nurses, spending their time in the hive taking care of the queen's eggs, yet after two or three weeks they transit to a new role as foragers, traveling to find sources of nectar and pollen to bring back.

②

So how can bees with the same genetic makeup undergo such incredible transformations? The answer appears to be epigenetics—a biological process meaning roughly “above the genome”. Epigenetics refers to the chemical tags attached to genes that help regulate how those genes are expressed. Epigenetic processes can turn a gene on or off, or simply make the gene more, or less, likely to be expressed. A simpler way of looking at epigenetics is like this: while traditional genetics describes the way DNA sequences in our genes are passed from one generation to the next, epigenetics describes the way these passing on genes are used. Take an MP3 player for example. It will contain a lot of songs which are analogous to genes. But you may play them by playlist, by artist or by genre. This information—playlist, artist, or genre—determines which songs are played and in what order, and this is what epigenetics is to genetics. It is a set of processes that affects which genes are switched on, or, “expressed”.

③

Many researchers have been studying epigenetics over the past few decades, and it is currently an area of intense research activity. We now know that a part of how epigenetics work is by adding to and removing from DNA small chemical tags that are called a methyl group. You can think of these tags as post-it notes that highlight particular genes with information about whether they should be switched on or off.

④

The enzymes known as DNA methyl transferases (DNMTs) assist the adding of the methyl group to modify one of the four bases or “chemical letters”, A, C, T, and G, that make up the genetic code of our DNA. In most cases, more methylated Cs in the DNA of a gene results in the gene being switched off. Honeybees provide us with a good example of how this can work. Though the queen and workers in a hive are both female, and genetically identical, there are many differences between them, physically and behaviorally. The clue to how this comes about lies in royal jelly, a secretion that is fed to some developing larvae, which results in their becoming queens rather than workers. But a fascinating piece of research showed that if the amount of the methyl group adding DNMT enzyme was artificially reduced in bee larvae, then the larvae developed into queens, even if they weren't fed royal jelly. This means that the switch between queen and worker can be flipped by the abundance of methyl tags on the bee larvae's DNA. Fewer methyl tags lead to switching on of a special gene or genes in the developing larvae, which results in the development of the larvae into queens and not workers.

⑤

The researchers also discovered the role of epigenetics in the life-course of the worker bees by carrying out a clever experiment. First, they looked to see whether there were epigenetic differences between groups of nurse bees and forager bees. They identified epigenetic differences in over 150 genes. The researchers then played what might be considered as a cruel trick by some. When the foragers were away, they removed all the nurse bees—an act which they knew would cause some of the foragers to revert to being nurses after they returned. When the researchers analyzed the bees' epigenetic tags again, they found that around 50 of the genes they had previously identified as being epigenetically different had changed back. This suggests that the changes to the epigenome were largely responsible for the shift that occurred when a bee changed roles.

——2012年7月28日北美考试机经

The word “them” in Paragraph 2 refers to____.

A. MP3 players

B. songs

C. genes

D. epigenetics

核心词汇：

词汇练习：

阅读下列句子，用所给单词的正确形式填空。

undergo incredible sequence analogous enzyme forager modify artificially abundance result in identify identical secretion regulate

1. At seven sites he found that 57 percent of the flakes were right-oriented, and 43 percent left, a pattern almost_____to that produced today.(TPO-12:Which Hand Did They Use? )

2. For years historians have sought to _____ crucial elements in the eighteenth-century rise in industry, technology, and economic power known as the Industrial Revolution, and many give prominence to the problem of energy.(TPO-26:Energy and the Industrial Revolution)

3. At the end of the Ice Age, no more than a few thousand _____ lived along the eastern Mediterranean coast,in the Jordan and Euphrates valleys.(TPO-20:Early Settlements in the Southwest Asia)

4. The Sahara is a highly diverse, albeit dry, region that has _____ major climatic changes since 10,000 B.C.(TPO-28:Early Saharan Pastoralists)

5. At first thought it seems _____ that there can be enough space in the “solid” ground underfoot to hold all this water.(TPO-2:Groundwater)

6. The second system is the voluntary, behavioral system. Its control center is based in the forebrain, and it _____ breathing for use in speech, singing, sighing, and so on. (TPO-24:Breathing During Sleep)

7. It contained a number of channels lined by a _____ of holes, about 50 centimeters deep, all showing burn marks and filled with ashes.(TPO-27:Crafts in the Ancient Near East)

8. For example, a gap just below aluminum suggested a new element would be found with properties_____to those of aluminum.(TPO-16:Development of the Periodic Table)

9. First, at least some of the observed differences between the _____ of the Atlantic and Pacific fish populations were not random but were the result of evolutionary adaptation. (TPO-31:Speciation in Geographically Isolated Populations)

10. As a person becomes more sophisticated, its conceptions of supernatural forces and causal relationships may change. As a result, it may abandon or _____ some rites. (TPO-1:The Origins of Theaters)

11. Third, older, more dominant bucks produce more buck rubs and probably deposit more glandular_____on a given rub.(TPO-28:Buck Rubs and Buck Scrapes)

12. The water is usually naturally occurring groundwater that seeps down along fractures in the rock; less typically, the water is _____ introduced by being pumped down from the surface.(TPO-21:Geothermal Energy)

13. In the latter decades of the twentieth century there were intensified efforts to understand and record the_____of Australian rock art.(TPO-23:Rock Art of the Australia Aborigines)

14. By doing so, they could possibly secure more food, which should _____ more rapid growth or larger size,either of which is advantageous.(TPO-11:Begging by Nestlings)

参考答案：

1. identical 2. identify 3. foragers 4. undergone 5. incredible 6. regulates 7. sequence 8. analogous 9. enzymes 10. modify 11. secretions 12. artificially 13. abundance 14. result in