Science / period 6
How dolly the sheep was cloned
Most people have always wondered how dolly the cloned sheep came to be. There were scientific break throughout the whole process, such as the two step process called nuclear transfer and the man who made this process was Gurdon. This got a lot of people going on the nuclear transfer there for spread throughout the country having tons of scientists trying it
First he decided to use delicate needles and a good microscope to suck out the nucleus from an animal oocyte, producing an “enucleated oocyte”. (That’s an oocyte without a nucleus.) Now with the genetic material that has removed the enucleated oocyte would not divide, even when fertilized. Now that he did that it was pointless because a cell is nothing without a nucleus.
But the results from Gurdon’s second step stunned a lot of people. He used the same equipment and strategy to transfer the nucleus from an animals gut cell into an enucleated oocyte. And that’s how nuclear transfer began, the transfer of a nucleus from one cell to another, creating a “new cell”. They divided and divided and divided just like a normal developing embryo, producing a ball of cells. Nerve cells, skin cells, blood cells appeared just as they regularly should in a normal embryo.
Nearly all animal cells have a nucleus, with the only exception being the red blood cell. The nucleus has two major functions, which are housing the DNA and controlling the cell’s activities. In the centre of the nucleus is the nucleolus. This doesn’t have a membrane, but holds itself together. In the nucleolus, ribosomes are created through the mixture of RNA and proteins. These proteins are ...
But there were two problems with this whole process. First, Gurdon’s nuclear transferred animals never grew into grown adults. Nuclear transfer couldn’t clone animals to animals; all you got was the first stage of life. No one knew why, even today, no one knows why the animals made by nuclear transfer didn’t die instead of growing into adults.
The second problem was that Gurdon’s method seemed to work only with certain animals. 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 very long and none of them differentiated properly. You just couldn’t clone mammals.
Very fast cell cycles occur during development causing a single cell to make many copies of it 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.
The only problem there is kind of a gap “in the cell cycle called “quiescence”. A quiescent 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.
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 year ago; but few folks really cared because who needs quiescent cells?)
?The cell cycle, or cell-division cycle, is the series of events that take place in a cell leading to its division and duplication that produces two daughter cells. In cells without a nucleus, the cell cycle occurs via a process termed binary fission. In cells with a nucleus, the cell cycle can be divided in three periods: interphase—during which the cell grows, accumulating nutrients needed for ...
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 pellucid) and it is through this protective coat that a scientist 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).
You may be surprised to learn that clones had been made at the Roslyn Institute before, but those clones were made from the nucleus of embryo cells not adult cells.