modification，gene mutation

What is NOT tree about genes in the process of genetic modification？

A．Genes interact with one another.

B．Normal genes may be damaged in the process.

C．The transferred gene may have different functions in different organisms.

D．The transferred trait controlled by one gene can be transferred endlessly.

A．One of the simplest and most important is through a positive feedback loop, where a master transcription regulator activates transcription of its own gene,in addition to that of other cell-typespecific genes. Each time a cell divides, the regulator is distributed to both daughter cells, where it continues to stimulate the positive feedback loop.？The continued stimulation ensures that the regulator will continue to be produced in subsequent cell generations.？Positive feedback is crucial for establishing the “self-sustaining” circuits of gene expression that allow a cell to commit to a particular fate—and then to transmit that decision to its progeny.

B．Although positive feedback loops are probably the most prevalent way of ensuring that daughter cells remember what kind of cells they are meant to be,there are other ways of reinforcing cell identity. One involves the methylation of DNA．In vertebrate cells, DNA methylation occurs on certain cytosine bases. This covalent modification generally turns off the affected genes by attracting proteins that bind to methylated cytosines and block gene transcription. DNA methylation patterns are passed on to progeny cells by the action of an enzyme that copies the methylation pattern on the parent DNA strand to the daughter DNA strand as it is synthesizeD

C．Another mechanism for inheriting gene expression patterns involves the modification of histones. When a cell replicates its DNA,each daughter double helix receives half of its parent’s histone proteins, which contain the covalent modifications that were present on the parent chromosom

E．Enzymes responsible for these modifications may bind to the parental histones and confer the same modifications to the new histones nearby. It has been proposed that this cycle of modification helps reestablish. the pattern of chromatin structure found in the parent chromosome

D．Because all of these cell-memory mechanisms transmit patterns of gene expression from parent to daughter cell without altering the actual nucleotide sequence of the DNA,they are considered to be forms of epigenetic inheritanc

E．These mechanisms, which work together, play an important part in maintaining patterns of gene expression, allowing transient signals from the environment to be remembered by our cells—a fact that has important implications for understanding how cells operate and how they malfunction in diseas

E．

A．One of the simplest and most important is through a positive feedback loop, where a master transcription regulator activates transcription of its own gene,in addition to that of other cell-typespecific genes. Each time a cell divides, the regulator is distributed to both daughter cells, where it continues to stimulate the positive feedback loop.？The continued stimulation ensures that the regulator will continue to be produced in subsequent cell generations.？Positive feedback is crucial for establishing the “self-sustaining” circuits of gene expression that allow a cell to commit to a particular fate—and then to transmit that decision to its progeny.

B．Although positive feedback loops are probably the most prevalent way of ensuring that daughter cells remember what kind of cells they are meant to be,there are other ways of reinforcing cell identity. One involves the methylation of DNA．In vertebrate cells, DNA methylation occurs on certain cytosine bases. This covalent modification generally turns off the affected genes by attracting proteins that bind to methylated cytosines and block gene transcription. DNA methylation patterns are passed on to progeny cells by the action of an enzyme that copies the methylation pattern on the parent DNA strand to the daughter DNA strand as it is synthesizeD

C．Another mechanism for inheriting gene expression patterns involves the modification of histones. When a cell replicates its DNA,each daughter double helix receives half of its parent’s histone proteins, which contain the covalent modifications that were present on the parent chromosom

E．Enzymes responsible for these modifications may bind to the parental histones and confer the same modifications to the new histones nearby. It has been proposed that this cycle of modification helps reestablish. the pattern of chromatin structure found in the parent chromosome

D．Because all of these cell-memory mechanisms transmit patterns of gene expression from parent to daughter cell without altering the actual nucleotide sequence of the DNA,they are considered to be forms of epigenetic inheritanc

E．These mechanisms, which work together, play an important part in maintaining patterns of gene expression, allowing transient signals from the environment to be remembered by our cells—a fact that has important implications for understanding how cells operate and how they malfunction in diseas

E．

Section B

Directions: There are 2 passages in this section. Each passage is followed by some questions or unfinished statements. For each of them there are four choices marked A, B, C and D. You should decide on the best choice.

Today, an estimated two-thirds of all products in supermarkets contain genetically-engineered ingredients, including tortilla chips, drink mixes, taco shells, veggie burgers, muffin mix, and baby formulas. However, only one-third of Americans are aware that their food contain genetically-engineered ingredients, despite the findings of surveys that 85 to 90% of consumers want clear labeling of all genetically-engineered foods. Because labeling is not yet required by government regulations, nor practiced by biotech companies, there is no way for consumers to know what it is that they are eating. The public has not been adequately educated of the presence of genetically-modified ingredients in their diets and the possible risks that these foods may entail, and hence, they have been disempowered in their fights to make informed choices.

Furthermore, the inadequate efforts to educate the public on genetic engineering have been sketchy at best, as some genetic scientists have been presenting the complex process of genetic modification to the public as a simple operation. They have been erroneously giving the impression that "... one gene controls one character trait, and transferring the gene results in the transfer of the corresponding trait to the genetically-modified organism, which can then pass it on indefinitely to future generations". However, the process of genetic modification is random and can cause damage in normal genes when imperfections in the genetic code of one organism are unknowingly passed to the receiving organism. Such publications promoting "public understanding" also do not include relevant findings that show that genes function in an "... extremely complex, interconnected network, so that ultimately, the expression of each gene depends on that of every other". The introduced gene may act differently when working within its new environment. For example, there are "position effects", which can lead to unpredictable changes in the pattern of gene expression and genetic function, such as production of potentially toxic products.

The credibility of these geneticists is also questionable because almost all established molecular geneticists have "... some industrial ties, thus limiting what they can research on, particularly with regard to safety". The connections between big business and genetically- modified foods is an issue in and of itself, as it seems that biotech companies have put economic gains before safety concerns. Because the current state of biotechnology is one of misinformation and lack of information, there are moral/ethical questions raised about the responsibility of these companies, as well as the government, to regulate these industries and protect the public.

There is no way of knowing the overall, long-term effects of genetically-engineered foods on human health. This unpredictability appears to be causing the most controversy over genetically-modified foods, as there is insufficient evidence and need for further research. Moreover, the current claims for genetic engineering are misleading, and there appears to be much ignorance and/or corruption at the top with the biotech corporations, as well as the government, which seem to be more interested in short-term economic gains than in properly informing and protecting the people.

Most Americans______.

A．buy genetically-engineered foods

B．know the foods they buy are genetically engineered

C．want genetically-modified foods to be clearly labeled

D．eat genetically-modified foods

共价修饰(covalent modification)

酶的化学修饰(chemical modification)

酶的共价修饰(covalent modification regulation)

组蛋白修饰(histone modification)

【名词解释】共价修饰（Covalent modification）

化学修饰(chemical modification)