How Dolly the sheep was cloned

The scientists who produced Dolly used a technique that transferred the nucleus from a cell of an adult sheep (the donor cell) into an egg cell (the recipient cell). They obtained donor cells from the udder of an adult sheep and recipient cells from other sheep.

Before the nucleus was transferred from the donor cell to the recipient cell, the scientists used a high-powered microscope and a very fine micropipette to suck out the recipient cell's nucleus. (Inside the nucleus are chromosomes, the packages that contain the cell's DNA.)

Each donor cell was then forced into a state of 'quiescence' - where the DNA stops dividing – and placed alongside a recipient cell. The two cells were then encouraged to fuse by way of an electric pulse. The recipient egg cell now had 'new' DNA – that of the donor cell – with which to begin the process of cell division and growth. It was implanted in the uterus of yet another sheep and its progress monitored. Of the 277 original donor cells, only 29 made it to the stage of being implanted, and of those only one – Dolly – went full term.

In 1998, several laboratories announced the successful cloning of other species, although the rate of success remained low in all cases. A University of Hawaii lab has produced dozens of cloned mice, using a variation of the nuclear replacement technique used with Dolly. Instead of fusing the donor and recipient cells, the researchers inserted the nucleus of the donar cell directly into the recipient cell. The cells from which the donor nucleus was extracted are naturally quiescent cells found in the ovary.

According to Ian Wilmut, the leader of the team that produced Dolly, getting donor cells (or donor DNA) into a quiescent state is essential because it allows the reprogramming of the adult DNA. Nevertheless, scientists in a US lab claim they have produced cloned calves without establishing quiescence beforehand.

Performed by: Jorge Borbinha
Posted by: Rui Cóias