The Dangers of Cloning - A Popular Myth?

The world of biology was relatively quiet and untainted, whereas other natural sciences such as physics and chemistry had suffered from some bad reputations. Nuclear physics is now associated with the tragedies of Chernobyl and Fukushima and chemistry has been associated with pesticides, dangerous drugs and horrible toxins. But the view on biology changed in 1996 when Dolly the Sheep was born, the first official clone of a mammal.

Suddenly the press went haywire, drawing scenarios of the doom of humanity. The scientists from Scotland were suddenly accused that they had been interfering with the essentials of life and created a monster. Was this the beginning of Frankenstein come true? Certainly not. The streets are still safe and there is no army of human clones trying to invade us as so beautifully demonstrated in Star Wars. But what is the truth about the myth of cloning? How does it affect our everyday lives and what are the biologists cooking next in their laboratories?

Dolly is dead now. She died from lung cancer in 2003 after enjoying only half the life span of a normal sheep of that breed. Since Dolly, many mammals have been cloned, including bulls and horses and none of those has hit the news as vigorously - or maybe the name Bull 86 just did not quite cut it. Why do we clone animals?

First of all, it is important to understand what is cloning. Cloning is a natural phenomenon, just as is nuclear energy. Many organisms in nature reproduce asexually, for example bacteria, some plants and some insects. By definition, two clones are organisms with exactly the same genetic make-up. If a bacteria divides for example, two clones are formed. There are approximately 40 million bacteria in one gram of soil, often from the same clone, and thus two grams of soil has potentially got more clones than Britain has people.

Cloning is a technique used routinely in laboratories and has been since the dawn of molecular biology. It is a tool absolutely necessary to study the fundamentals of life and study mechanisms in cells that ultimately help us understand many diseases. So, cloning seems to be a good word. But why do we need to clone mammals or potentially even humans, which is still illegal.

Interestingly, Dolly was not even a real clone. There are two pools of DNA in a mammalian cells, one the nucleus which is passed on from the father and the mother, and one in the mitochondria, the "power plants" of a cell, which is only passed on from the mother. The mitochondria were not replaced and thus Dolly was strictly speaking not a clone - neither is any of the other mammals that have been cloned since.

Many plants that we cultivate and finally eat are clones. With the technology available we can also use cloning to genetically modify plants in order to increase crops, yield and even taste. Again there is the question as to whether this is necessary.

Despite heated debates and many laws about stem cells, genetically modified food and cloning, once Pandora's box is open, it usually doesn't get closed again. Cloning is good and has helped our understanding of the mechanisms of the cell and helped guide the development of many drugs which help millions of people. The question is not as to whether one should use cloning, but rather what is the scientist's conscience using such techniques. We live in a society with strict moral codes laid upon us, some of them maybe debatable. As society evolves, the moral code also changes. To me, a scientist conducting experiments is responsible for his/her actions. Politics is responsible for laws that try to lead the conscience of people. It surely is our responsibility to understand the needs of society as a whole and help guide scientists to make good decisions and use the knowledge they generate wisely.

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Chromosome Walking

Chromosome walking is a method used to clone in an orderly fashion the DNA segments along the chromosome starting at any point for which we have a probe. To build up a series of overlapping cloned DNA fragments, it begins with one clone from a library, and then identifies a second clone whose insert overlaps with the insert in the first clone. This was the first method, called chromosome walking, devised for assembly of clone contigs. But there is a limitation to the speed of chromosome walking and that is because of the small size of the fragments that are to be cloned, another limitation is the difficulty of walking through the repeated sequence that are scattered through the gene. A more straightforward approach thus is to use the insert DNA from the starting clone as a hybridization probe to screen all the other clones in the library. Positive hybridization signals that are given by clones, whose inserts overlap with the probe, are used as new probes to continue the walk. There are about 96 clones that a library consists of and each clone contains a different insert. The insert from one of the selected clones is then used as a hybridization probe with all other clones in the library.

A probe may have a genome wide repetition of sequencesand this is a problem that is faced during the walk as then it will not only hybridize to overlapping clones but will also to non overlapping clone whose inserts also contain copies of repeat. This unwanted hybridization can be reduced by blocking the repeat sequence with pre-hybridization with unlabeled genomic DNA. But this isn't that affective solution especially in the case when high capacity vectors such as BACs or YACs are used in the walk. Therefore for chromosome walks with human DNA which have a high rate of repetition, intact inserts are not used in general. Instead the probe is taken from the end of an insert (a short end-fragment) which has a lesser chance of repetition. The walk can also be sped up by using the PCR instead of hybridization to identify the clones with overlapping inserts. Primers, used in attempting PCRs with other clones in the library, are designed from the sequences of the DNA end fragments. Further higher speeds can be achieved by using groups of clones mixed together such that an unambiguous overlapping clone can be made. But care must be taken as ambiguities might arise in this case too.

This procedure is well suited for positional cloning as the objective here is to walk from a mapped mark to an inserting gene. But for the process of assembling clone contigs across entire genome, the procedure proves to be a bad choice. So as an alternative procedure clone finger printing technique is used. The method provides information regarding the physical structure of a cloned DNA fragment and this information (called a fingerprint) can be compared with data of other clones, thus allows identifying the overlaps which might have similarities.

Shilpa verma
https://sites.google.com/site/bioteckarena/

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