Re: How did they clone that lamb????

Well i guess what you are talking about is our four legged scottish friend dolly.

Most of the information that I will give has come from lasts weeks issue of the NewSccientist. Basically Dolly has arisen not by a mating but by isolation of a single cell taken from a 6 year old Ewe. This is quite remarkable since effectively what has been achieved by the researchers at the Roslin institute in Edinburgh is a reversal of cellular memory. The researchers took the cell from the udder of the 6 year old Ewe and were able to shut off its 'Udder' identity. In order to understand how they did that we have to consider the life cycle of the cell. When a new cell begins dividing, certain genes will be turned on and subsequently turned off at specific periods of the cell's development. It is believed that this regulation of gene expression during different times of cellular development are controlled by modification of the DNA primarily by Methylation. It is by this selective turning on and off that a cell or a group of cells develops what is referred to as the cellular memory. Up until recently it was basically believed that this cellular meory was very difficult or even impossible to reverse. Dolly the sheep shows that in fact it is possible to cause a reversal and take the cell back to its 'unmemorised' state. The way that Dolly was produced seems to be quite simple in principle. The udder cell that was isolated from the mature ewe was cultured in a salt solution that carried just about the right amount growth factors to keep the cell alive. Under such conditions the continous expression of many genes is energetically impossible for the cell and so the cell expresses only those genes that are essential for the survival of the cell. In essence the cell is therefore placed in a restart mode whereby it can then grow and divide as an undifferentiated cell to to givfe a new individual with exactly the same genetic make up as that of the adult ewe from which the udder cell was taken. Similar experiments had already been carried out at the Roslin institute by Wilmut and his team. But this is the first time that a specialised cell from a specific tissue has been reprogrammed. Last year cells were taken from enbryos and foetuses and these were successfully used to produce genetic clones of individual animals. However, whilst these are also amazing feats, they would in theory be easier to achieve since the so called cellular memory is perhaps much weaker in the earlier stages of develompent. The generation of Dolly didnt only involve the isolation of the specialised cell from the adult Ewe. Subsequently oocytes were removed from sheep and all the DNA from these oocytes extracted to leave cells with the bare minimal structural makeup -cytoskeleton etc.... Subsequently, the cell originating from the specialised tissue in the adult ewe is fused with the oocytes by administering an electric current. These cells thus fuse to give a single cell. This cell is left to grow and divide in culture to provide the blastocyst for subsequent implantation into a surrogate mother sheep. This, as is the case with Dolly, gives rise to a healthy sheep which is in theory a genetic clone of the original adult from which the specialised cell was removed.

Like in so many of these things the story is much more complicated and painstaking than it sounds. For a start Dolly was the only successful clone from 277 udder cell/ oocyte fusions. Many of the unsuccessful attempts involved miscarriage or abnormal development of the sheep. In cases where development didnt even reach the embryonic stage, the Roslin group expalained that it is extremely hard to get the oocytes and the donor cell to coincide exactly in the stages of the cell cycle. Practically, Jonbathan slack from Bath university staed that in these experiments it is extremely possible to obtain False positives since not all the DNA from the oocytes will necessarily be removed. The udder is perhaps not the best tissue to use for this experiment since in fact the udder is rich in stem cells, cells which are less well developed and might therefore be more susceptible to undergo a reversal of the cellular clock(see above). So if another tissue were to be used the success rate, which is already pretty low, might be reduced.

Such research is primarily directed at the agricultural industry where a lot of effort is made to try and obtain prime-breeding animals that will pass on favourable characteristics to subsequent generations. In humans? well, if they ever come into the discussion the implications of human cloning would be scary and very dangerous!!!!!!!!!

I hope that i have provided you with enough infprmation. dont hesitate to contact me if you wish. As i said earlier, the information here was obtained primarily from last weeks issue of the NewScientist. If you want a good read on embryonic development, morphogenetic gradients and differential gene expression i suggest "the making of the fly3 by Lawrence.

Best wishes, Robert

Robert DEYES
UFR des Sciences Pharmaceutiques
Faculté de Pharmacie
74, Route du Rhin, BP24
67401 Illkirch Cedex
France

FAX 00 33 03 88 66 01 90
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E mail deyes@pharma.u-strasbg.fr