衡水世纪星幼儿园收费:克隆的advantage和disvantage

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能用英语回答的尽量~~~~~~~~~谢谢
能简则简,这只是高一学生所要的答案!!3Q~~

I've decided to make cloning one of the first topics in this first issue of Science Explained because the folks who created Dolly are acquaintances of mine. (Yes, I am a name dropper, aren't I?) A few kilometers from my home lives Dolly, the world's first mammalian clone; not counting identical twins. (They're clones too.)
What makes Dolly different from identical twins is that she was grown from a cell taken from an ADULT animal! Many bright, well-respected scientists said it couldn't be done. Dr Ian Wilmut, who is in charge of the lab that created Dolly, admits that he had his doubts. However the hard work and imaginative thinking of his staff made it all possible.

How did they do it and what did they do?

First some background to teach you the basics of developmental biology.
An oocyte (pronounced "oh-oh-sight") is an unfertilized egg and it has no chance of developing into an animal unless it's fertilized. A recently fertilized egg is called a zygote (pronounced "zye-goat"). Funny how the last two letters in the alphabet describe the first stage of an individual animal. For example, a frog zygote normally divides and grows into a complete animal, a tadpole. Later that tadpole will develop into an adult frog.

A cell from a frog's gut should always remain a frog-gut-cell because it has "differentiated". Differentiation is the natural process whereby cells specialize into a certain kind of cell. As a frog embryo grows and develops its cells differentiate into nerve cells, blood cells, fat cells and many other different kinds of cells. That's what differentiation is all about. Differentiation is important because without it an animal could never be anything but a blob of unspecialized cells. As a mass of embryo cells divide and differentiate they "create" the animal. This incredible process of differentiation turns zygotes into animals and it's all controlled by the genes. Although the exact process is still poorly understood, scientists agree that differentiation must have something to do with changes in the nucleus of cells. You may recall that the nucleus is the part of the cell containing the genetic material (the DNA all coiled up in organized structures called chromosomes).

What do frog cells have to do with Dolly?

Like most scientific "breakthroughs" the earlier work done by others provided the foundation on which to try something new.
Way back in 1975 a fellow named Gurdon developed the method of "nuclear transfer". This is a two step process.
First he used delicate needles and a good microscope to suck out the nucleus from a frog oocyte, producing an "enucleated oocyte". (That's an oocyte without a nucleus.) With the genetic material removed the enucleated oocyte would not divide or differentiate even when fertilized. That was no surprise. (A cell is nothing without its nucleus.)

But the results from Gurdon's second step shocked a lot of people! He used the same equipment and skill to transfer the nucleus from a frog's gut cell into an enucleated oocyte. That's nuclear transfer, the transfer of a nucleus from one cell to another, creating a "new cell" with a different nucleus. Many of these new cells which Gurdon created behaved like a zygote. They divided and divided and divided just like a normal developing embryo, producing a ball of cells. And this ball of cells differentiated! Nerve cells, skin cells, blood cells appeared just as they would in a normal embryo. After the normal length of time Gurdon had tadpoles! Because the tadpoles had all come from the gut cells of the same adult, they all had the same genetic material. So they were all clones, identical twins of each other. But unlike normal identical twins they were made from differentiated cells.

Gurdon had proven something that many scientists had argued about. He proved that differentiation was REVERSIBLE. Gurdon's method of nuclear transfer turned back the hands of time, in a developmental sense. Gurdon's method of nuclear transfer made clones from adult cells!

Naturally this got a lot of scientists thinking about cloning. But there were two problems.

First, Gurdon's nuclear transferred tadpoles never grew into frogs! Other folks repeated his experiments and got similar results. Nuclear transfer couldn't clone frogs from frog cells; all you got were tadpoles. No one knew why. Even today, no one knows why the tadpoles made by nuclear transfer die instead of growing into frogs. Weird.

The second problem was that Gurdon's method seemed to work only with frogs (or perhaps I should say "tadpoles"). When scientists tried nuclear transfer with mice, cattle or indeed any mammal, they got nowhere. The "new cells" sometimes divided a few times, but not for long and none of them differentiated properly. You just couldn't clone mammals. By the early 1980's most scientists accepted the idea that something very special allows frogs to be (partially) cloned (into tadpoles). Whatever that process was, it was not found in mammals. The textbooks made it very clear. Differentiation was (sort of) reversible in frogs but not in mammals.

Bummer.

So what exactly did the scientists at the Roslin Institute do?

Well, Keith Campbell, a fellow working for Dr Wilmut, thought that maybe the cell cycle had something to do with this cloning trouble.

The cell cycle is often described as a circle of cell life and division, but I think that can be a bit confusing for some people, so let's try to remember that by "cycle" we mean it happens again and again.
A cell divides into two "daughter cells" and both of these cells live, "eat", grow, copy their genetic material and divide again producing two more daughter cells. Because each daughter cell has a copy of the same genes in its nucleus, daughter cells are "clones" of each other just like identical twins. This "twining" goes on and on with each cell cycle. This is a natural process.
The cell cycle fascinates biologists. Very fast cell cycles occur during development causing a single cell to make many copies of itself as it grows and differentiates into an embryo. Some very fast cell cycles also occur in adult animals. Hair, skin and gut cells have very fast cell cycles to replace cells that naturally die. And cancer is a disease caused by cells cycling out of control. It's no wonder that biologists think the cell cycle is so important.

But there is a kind of "parking spot" in the cell cycle called "quiescence" (pronounced "kwee-S-ence"). A quiescent ("kwee-S-cent") cell has left the cell cycle, it has stopped dividing. Quiescent cells might reenter the cell cycle at some later time, or they might not. It depends on the type of cell. Most nerve cells stay quiescent forever. On the other hand, some quiescent cells may later reenter the cell cycle in order to make more cells. (For example, when a young girl starts to develop breasts.)

Many biologists (including myself) thought that to make a clone you should transfer the nucleus from a fast dividing cell. It made sense because fast cycling cells are exactly what makes an embryo grow. Besides, the gut cells used to make the tadpole clones were fast cycling cells. Many biologists tried to make clones by transferring the nucleus from fast dividing cells but all of those experiments were unsuccessful. (I tried injecting the fast growing cells from chicken feathers into hen eggs in the hope of cloning birds, but it didn't work.)

Keith (Dr Campbell) thought about it in a different way. He wondered if a quiescent nucleus would be a better donor. True, it was not cycling (that's what makes it quiescent, by definition) but Keith thought maybe that's what the nucleus needs for it to be successfully transferred. Maybe the cell needs time to "rest" before starting to make a whole new animal. Maybe the nucleus needs time, lots of time, to get its DNA in order. Maybe...?
Maybe quiescent cells would work!

So they tried it with cells from sheep.
The folks at the Roslin Institute do a lot of work with sheep as part of their partnership with a company called Pharmaceutical Proteins Limited Therapeutics (PPL Therapeutics). Earlier they had made transgenic sheep (sheep with human genes transferred into them, but that's another story).
They used cells from an adult sheep's mammary (breast) glands for the "donor" nucleus. They grew the cells in tissue culture, an artificial situation that is commonly used in laboratories to grow large numbers of cells in bottles. Tissue culture allows scientists to fiddle with the cells and alter their characteristics. That is exactly what Dr Campbell did. He "starved" the cells of important nutrients and the cells stopped growing and dividing. They became quiescent. (Keith knew they would become quiescent when starved of nutrients because other researchers had proven that years ago; but few folks really cared because who needs quiescent cells?)

And then he made Dolly?

Yes, but creating Dolly was not easy.
Using techniques similar to those used 20 years ago by Gurdon, Bill Ritchie (a technician working with Dr Campbell) removed the nucleus from an oocyte that was collected from a Scottish Blackface ewe.
(Ewes are female sheep. The Scottish Blackface breed is a common breed of sheep in Scotland easily identified by its black face.)

Oocytes have a "shell" of proteins and fibers (called the zona pellucida) and it is through this protective coat that Bill injected the nucleus from a quiescent mammary cell into the enucleated oocyte. That cell nucleus was from a different breed of sheep called a Finn Dorset, which happens to be a pure white breed of sheep. He then used a tiny pulse of electricity to cause the new nucleus to fuse with the enucleated oocyte's cytoplasm. (Cytoplasm is the solution inside the cell.) This electricity also helps "kick start" cells into "activity" so they are more likely to divide. This new, fused cell (containing the Finn Dorset mammary cell nucleus in the cytoplasm of a previously enucleated Blackface oocyte) was transferred into the reproductive "chamber" of a Blackface ewe (the same breed that provided the oocyte).

Bill and his fellow researchers than repeated this process 276 times! That's right, 276 times.

I told you this wasn't easy. After 148 days, a normal length of time for the Finn Dorset breed of sheep, Dolly was born.
As you can see she is a healthy, normal looking Finn Dorset. (Dolly's the wee one on the left) born to a Blackface ewe (her mom's on the right). This proves that Dolly wasn't the product of a sneaky mating; Dolly's Blackface mom could not produce a white faced sheep no matter who was the father. (It has to do with the genetics of sheep breeds.) But just to be sure, the scientists DNA "fingerprinted" Dolly and her "mom" and proved that Dolly's DNA matched the cells from the tissue culture, not the cells from the ewe that gave birth to her.

Dolly is a normal (Finn Dorset) sheep. Contrary to the reports in some of the trash newspapers, she has not eaten her keeper or her fellow sheep. She does not shoot laser beams out of her eyes or talk. Dolly is a friendly, normal, healthy sheep who enjoys being petted, especially if you have some food in your hand!

This amazing research was published in Nature and you can read all the technical details there. You may be surprised to learn that clones had been made at the Roslin Institute before, but those clones were made from the nucleus of embryo cells not adult cells.

Want to create a flock of cloned sheep?
Here's your chance! Visit the Esheep Homepage and download the silly, little program that creates entertaining sheep on your desktop. Each time you run Esheep.exe a cute cartoon sheep appears. Create as many as you want! They will wander around your desktop interacting with each other and entertaining you. Double click on the sheep to call up the Esheep options box with the "remove" feature that causes the sheep to go away. You can always call them back by running Esheep.exe again (and again, and again)!

Ok, I'm not being very scientific here but these things are fun. The free version is enough entertainment for me but you might want to register yours and get the extra features.

Hani writes from South Africa ...

Why did the process have to be repeated 276 times? Don't the cells divide or is it because the experiment failed 276 times?

The short answer is that the experiment failed 276 times. To understand the failures we need to know more about the way the researchers did their work.

The Roslin team knew from previous research on embryo transfers (moving embryos into a different mother) that the transfers don't always work. The embryos may die for various reasons. They also knew that the nuclear transfer itself might introduce new ways for things to go wrong. Remember, they had to inject the nucleus through the enucleated oocyte's protective coat (zona pellucida), zap it with electricity and hope that the nucleus would have become quiescent during its time in tissue culture. Any of these new steps might cause the embryo to die or never develop. In order to better understand what might be going on the researchers introduced a step I failed mention.

First a little more background information about developmental biology. A few days after normal fertilisation a zygote would be expect to have divided approximately four times, producing a ball of 16 cells. (Think about the math. That's 1---> 2---> 4---> 8--->16) At this point in its development the embryo looks like a tiny mulberry and is called a morula. ("Morula" is Latin for "mulberry".) Soon after this stage the zona pellucida (the "egg shell") starts to disappear and as the cells continue to divide they allow fluid to enter the center of the mass of cells. This forms a hollow ball, with cells on the outside and fluid inside. This is called a blastocyst. All mammals (including you and me) developed from zygote, to morula and then to blastocyst, before implanting into the wall of mother's uterus. These are very important steps in our development and that of a lamb.
Anyway, a few days after placing the "new cells" into ewes' oviducts, the researchers collected them to see how well they had developed. As you might imagine it is not easy to find a tiny morula or blastocyst in a sheep's oviduct. Of the 277 they put into ewes they recovered only 247. That means over 11% of the embryos were lost in the first few days. Some may have been lost because they are so difficult to find. Others may have died early and decomposed. Unfortunately, when they examined each of them under the microscope only 29 of the 247 recovered were either a morula or a blastocyst. Or to put it another way, 88% of the "new cells" transferred had not developed. That's a significant loss in the experiment.

The researchers then placed the remaining 29 "good embryos" into 13 ewe's. Some ewes got only one embryo, some got two (which is the average number of lambs that ewes have at a time) and some ewes got three embryos. The exact number depended upon availability of the embryos at the time and the availability of "receptive" ewes. Ewes, like all mammals, must have the right balance of hormones in order to "adopt" an embryo at its particular stage of development. If the embryo and ewe are not "synchronized" (the phrase used for this balance) the embryo will not implant in the ewe's uterus. This is not an easy thing to do and the Roslin researchers explain in their paper "Not all recipients were perfectly synchronized." Perhaps that's why only one of those 13 ewes actually became pregnant. Of course, that was the one that gave birth to Dolly!

If you take a look at their paper you'll see that they have all this data in Table 1.
Note: The table and their paper also report data from parallel experiments in which the donor nuclei were from fetal or embryo cells. Some of the "new cells" made with the fetal a