【克隆羊多利】求一篇关于克隆羊多利的影响的英文文章一定要是...

【克隆羊多利】求一篇关于克隆羊多利的影响的英文文章一定要是...

【答案】 Cloning Dolly
　　Cloning Dolly the sheep was not an easy task.To understand how she was cloned,you must first know some of the basics of developmental biology.Here's a crash course:
　　An 'oocyte' (ooh-oh-sight) is an unfertilized egg that has no chance of becoming an animal unless it becomes fertilized.An egg that has just become fertilized is called a 'zygote' (zye-goat).For example,a frog zygote will split and grow into a tadpole.This tadpole will become a frog and the process continues.'Differentiation' is when cells change into certain kinds of cells.These cells can be differentiated into blood cells,nerve cells,fat cells,and many other types of cells.As a mass of embryo cells split and differentiate,they create an animal.
　　A nucleus is the main part of a cell,where all of the information is stored.
　　An 'enucleated oocyte' is an oocyte without a nucleus.
　　'Nuclear transfer' is when the nucleus of of one cell is transferred into another cell,creating a 'new cell' with a different nucleus.
　　A 'quiescent' (qwee-S-cent) cell is a cell that has departed from the cell cycle and has stopped dividing.It may or may not re-enter the cell cycle at a later time,because it depends on the type of cell.
　　Diagram thanks to www.synapses.co.uk/science/clone.html
　　The 'cell cycle' is often depicted as 'a circle of life and division',but here it will be referred to as 'the cycle',meaning it repeats.
　　Got it?Good.
　　In 1975,a man by the name of Gurdon came up with the nuclear transfer.This two step process uses fine needles and a powerful microscope to take the nucleus out of a frog oocyte (I guess frogs were the guinea pigs at this time),creating an enucleated oocyte.He discovered that the enucleated oocyte wouldn't divide or differentiate,even when fertilized,which isn't a surprise,as a cell can't do anything without the nucleus.
　　Gurdon's second step surprised many.He moved the nucleus from the frog cell into an enucleated oocyte.These nuclear transferred cells behaved a lot like a zygote.They divided and divided just as they were supposed to,until a great ball of cells was produced.Then this ball differentiated!The process continued on as a normal embryo,and the tadpoles were right on time.The tadpoles were clones,as they had come from the cells of the same adult and all had the same genetic make-up.Gurdon had proved that differentiation was reversible,because normal identical twins aren't made from differentiated cells.
　　Naturally,scientists got excited when they saw how Gurdon had cloned the tadpoles.Unfortunately,there were two problems,the first being that,no matter how hard scientists tried,nuclear transfer only worked on frogs.They tried it on mice,cattle,and many other mammals,but nothing happened.The 'new cells' would divide occasionally,but it didn't last and none of them properly differentiated.Secondly,Gurdon's nuclear transferred tadpoles never grew up!His experiment was repeated,but always with the same outcome.Even today,scientists can't explain why the tadpoles made by nuclear transfer never lived to become adults.By the 1980's most scientists accepted the fact that frogs have something special in them that allows them to be cloned.Whatever this ingredient is,it's not found in mammals.Therefore,differentiation is somewhat reversible in frogs (as they always stay tadpoles) but not in mammals.
　　It seemed as though cloning animals had come to a halt.There weren't many options left,but Roslin Institute scientist Keith Campbell had the idea that the cloning trouble may have had to do with the cell cycle.
　　A cell divides into two more cells,referred to as 'daughter cells',and both of these cells grow regularly,copy their hereditary material and again split,making two more daughter cells and repeating the process.The daughter cells are clones of each other,because both nuclei (plural of nucleus) have the same genes in them.This natural process goes on and on with each cycle.
　　The cell cycle intrigues biologists.Very fast cells occur during development causing a single cell to make many copies of itself as it grows and differentiates into an embryo. (Dr.Jamie Love,Issue 1-Science Explained).Hair,gut,and skin cells are fast cycling cells so that cells that have naturally died can quickly be replaced.This is why your hair grows so fast or why that cut on your arm is healed within the week.Skin cells divide to make more skin cells to replace the ones that died.Cancer is caused by cell cycles that are out of control.
　　Many biologists thought that,in order to clone something,the nucleus from a fast dividing cell should be transferred.This was quite logical because quickly cycling cells are just what makes an embryo grow.Thinking back,the gut cells used to clone the tadpoles were fast cycling.Many scientists tried this theory,but all attempts failed.
　　Dr.Keith Campbell tried a different route.He thought that a quiescent nucleus would be a better candidate.Although it wasn't cycling (which is what makes it quiescent),Dr.Campbell figured that was what the nucleus needed for it to be properly transferred.Perhaps the cell needed a 'rest' before beginning to create a whole new animal.Maybe the nucleus needed some time to get its DNA in order.So maybe quiescent cells would work!
　　They used cells from an adult sheep's mammary glands for the 'donor' nucleus.The cells were grown in 'tissue culture',An artificial situation that is commonly used in laboratories to grow large numbers of cells in bottles. (Dr.Jamie Love,Issue 1-Science Explained).Tissue culture allows scientists to tinker with the cells and change their characteristics.This is precisely what Dr.Campbell did.He 'starved' them of vital nutrients and the cells ceased to grow and divide.They became quiescent.Soon Dolly would be created.
　　Using methods like ones used 20 years before by Gurdon,technician Bill Ritchie (who worked with Dr.Campbell) took the nucleus from an oocyte that was accumulated from a Scottish Blackface female sheep (the Scottish Blackface is a breed of sheep common in Scotland and recognizable by its black face).
　　Oocytes have a 'coat' of fibres and proteins (known as the zona pellucida) and it is through this guarded cover that that Mr.Ritchie injected the nucleus from the quiescent mammary cell into the enucleated oocyte.The nucleus was from a separate kind of sheep called the Fin Dorset.He then used a very minute pulse of electricity to make the nucleus fuse with the cytoplasm of the enucleated oocyte.This pulse also helped to start the cells into 'activity' so that they will more likely split or divide.This new cell was put into the reproductive track of a Blackface ewe (this was the same breed that gave the oocyte).
　　Diagram thanks to www.synapses.co.uk/science/clone.html
　　Scientists at the Roslin Institute repeated this operation 276 times!This was definitely no easy task.
　　After a normal length of time for a Fin Dorset,148 days,Dolly was born.
　　Dolly is a normal,healthy looking Fin Dorset and is a true clone,because Dolly's Blackfaced mother couldn't have given birth to a white faced sheep no matter who the father was.But the scientists double checked anyway,and 'fingerprinted' dolly and her mom and were able to prove that Dolly's DNA matched that of the cells from the tissue culture,and not the cells of her birth mother.Although some newspapers have said so,Dolly has not eaten her owner or any fellow sheep.She doesn't shoot laser beams from her eyes or talk.
　　The process failed 276 times.The 'new cells' (enucleated oocytes with donor nucleuses) were inserted into many ewes.The scientists checked their process after a few days and only recovered 247 cells.Some may have been lost because they are so hard to find.Others died early and decomposed.When examined under a microscope,88% of the 'new cells' that had been transferred had not developed.The researchers put the 29 remaining embryos into 13 ewes.Some ewes got only one embryo,some two,and some three.If the embryo and the ewe are not in synch,then the embryo won't become implanted in the ewe's womb.This is difficult to do and Roslin researchers said in their paper Not all recipients were perfectly synchronized.This may be why only one of the 13 ewes became pregnant.Obviously,this was the one to give birth to Dolly.
　　But now,after only five years in this world,Dolly has run into a problem.What's happened?Go to the Dolly Dilemma page to find out how it happened.

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