更多"[单选题]I soon perceived that I could "的相关试题:
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Clone Farm
Factory farming could soon enter a new era of mass production .Companies in the US are developing the technology needed to"clone"chickens on a massive scale.Once a chicken with desirable traits has been bred or genetically engineered,tens of thousands of eggs,which will hatch into identical copies,could roll off the production lines every hour. Billions of clones could be produced each year to supply chicken farms with birds that all grow at the same rate,have the same amount of meat and taste the same.
This,at least,is the vision of the US's National Institute of Science and Technology,which has given Origen Therapeutics of Burlingame,California,and Embrex of North Carolina $4 .7 million to help fund research.The prospect has alarmed animal welfare groups,who fear it could increase the suffering of farm birds.
That's unlikely to put off the poultry industry,however,which wants disease-resistant birds that grow faster on less food."Producers would like the same meat quantity but to use reduced inputs to get there,"says Mike Fitzgerald of Origen.To meet this demand,Origen aims to"create an animal that is effectively a clone",he says.Normal cloning doesn't work in birds because eggs can't be removed and implanted.Instead,the company is trying to bulk-grow embryonic stem cells taken from fertilized eggs as soon as they're laid."The trick is to culture the cells without them starting to distinguish,so they remain pluripotent,"says Fitzgerald.
Using a long-established technique,these donor cells will then be injected into the embryo of a freshly laid,fertilized recipient egg,forming a chick that is a"chimera".Strictly speaking a chimera isn't a clone,because it contains cells from both donor and recipient. But Fitzgerald says it will be enough if,say,95 percent of a chicken's body develops from donor cells."In the poultry world,it doesn't matter if it's not 100 percent."he says.
Another challenge for Origen is to scale up production .To do this,it has teamed up with Embrex,which produces machines that can inject vaccines into up to 50,000 eggs an hour. Embrex is now trying to modify the machines to locate the embryo and inject the cells into precisely the right spot without killing it.
In future,Origen imagines freezing stem cells from different strains of chicken.If orders come in for a particular strain,millions of eggs could be produced in months or even weeks.At present, maintaining all the varieties the market might call for is too expensive for breeders and it takes years to bread enough chickens to produce the billions of eggs that farmers need.
The technology of freezing stem cells from different strains of chicken can do all the following EXCEPT that______.
A.farmers can order certain strains of chicken only
B.Origen can supply all the strains of chicken the market might need
C.chicken farmers order certain strains of chicken for economic reasons
D.chicken farmers can be supplied with whatever strain they need
[单选题]The operation could prolong his life by two or three years.
A.broaden
B.worsen
C.extend
D.accelerate
[单选题]--Could I use your car tomorrow
--Sure. I _____________ a story at home.
A.have written
B.have been writing
C.will have written
D.will be writing
[单选题]Though Confucius has long gone，his powerful ideas undoubtedly__________Chinese soci-ety and tradition．
A.shaped
B.formed
C.1aid
D.made
[单选题]I could easily identifyhim among the crowd because he was wearing long hair.
A.prove
B.define
C.claim
D.recognize
[单选题]I could easily identify him among the crowd because he was wearing long hair.
A.prove
B.define
C.claim
D.recognize
[单选题]I grabbed his arm and made him turn to look at me.
A.threw
B.seized
C.broke
D.stretched
[单选题]___his help, I passed this exam in the end.
A.Thanks to
B.On all counts
C.Drawing up
D.Setting down
[单选题]I was astonished at the news of his escape.
A.amused
B.amounted
C.amazed
D.approached
[单选题]I grabbed his arm and made him turn to look at me.
A.seized
B.threw
C.broke
D.stretched
