Mulitcellular Organisms

Cloning

			There are three types of cloning as depicted in Figures 10-14a, 10-14b, and 10-14c. Media report on cloning in the news usually refers to the type called reproductive cloning. In general, cloning is defined as an asexual process of producing a group of cells or molecular segments or organisms, that are genetically identical descendants of a single parent.
Figure 10-14a DNA Cloning [view large image]	Figure 10-14b Embryo Cloning [view large image]	Figure 10-14c Reproductive Cloning [view large image]

The three types of cloning:


• DNA cloning - As shown in Figure 10-14a the DNA to be cloned is cut into fragments by the restriction enzymes. Such enzymes occur naturally in some bacteria, where they stop viral reproduction by cutting up viral DNA. They restrict the growth of viruses and hence the name. The fragment of DNA is inserted into a vector by DNA ligase, which is another bacterial enzyme that seals any breaks in a DNA molecule. The most common vector is plasmid, which is a small ring of DNA removed from bacteria. Gene splicing is complete when a recombinant DNA (DNA containing fragments from two or more different sources) has been prepared. After the recombined plasmid is taken up by a host cell, cloning is achieved when the host cell and the recombinant DNA of the plasmid reproduce either the cloned gene or a protein product (produced by the gene).
• Embryo cloning - It is also called therapeutic cloning. The goal is to create stem cell for medical purpose. The donor cell is either collected from the blastocyst or oocyte. Blastocyst is extracted from the fertilized egg after it has divided for 5 days, while oocyte is from unfertilized egg (thus by-passing the ethical issue) in the ovaries. These are highly versatile stem cells, which can differentiate into various types of cells. If the nucleus of the stem cell is removed and replaced by the patient's genes, then the various specialized cells produced by the altered stem cell can be transplanted to the patient without the problem of immunological rejection (see Figure 10-14b).
• Reproductive Cloning - The concept behind reproductive cloning seems deceptively simple. By replacing the genetic material of an egg with that of a donated adult cell, it should be possible to create an exact copy of another living creature (see Figure 10-14c). Achieving this goal is another matter. There were lot of failures in mammalian cloning during the 1980s. Cloning of mammals became viable only in early 1990s and was well publicized with the birth of Dolly (the cloned sheep) in 1997. It is found that reproductive cloning is an inefficient and error-prone process. Many cloned embryos are miscarried; others have abnormalities. It is estimated that only 1 to 10 per cent of cloned embryos from adult donor cells develop completely, depending on the species (more complicated organism is harder to clone). It took 277 attempts to clone Dolly the sheep; Carbon Copy, the cloned kitten, came after 86 failures. Only 33 monkey embryos have been created after 738 attempts - and none has produced a live birth.

  Many cloning problems arise because of the key difference between normal fertilization and cloning. A normal fertilized egg contains genetic material from sperm and egg. When united, the genes in them turn on or off in a precise pattern that is in perfect readiness for the development of an embryo. Since clones do not undergo fertilization, this programming does not occur. When the donated adult cell is merged with the egg, the egg tries its best to reprogram - but it doesn't do very well. All the right genes are correctly reprogrammed in only a tiny minority of clones. To succeed in reproductive cloning, scientists need to understand how to reset these genetic instructions.

  Sexual reproduction has evolved for two billion years and is the preferred mode after so many years of natural selection. It increases the rate of adaptive evolution and prevents the accumulation of deleterious (harmful) mutations. Cloning negates these advantages in favour of retaining some special characteristics of an organism. The argument is actually invalid because such characteristics are not determined by the genes alone; they are influenced by external factors such as the developmental environment (for the embryo) as well as subsequent growth and learning (i.e. wiring of the neurons) after birth.

  In the movie "Jurassic Park", dinosaurs are cloned by extracting their DNA stored inside the mosquitoes (preserved in amber), with missing pieces patched up from the modern reptilian version. Scientifically, short segments of DNA has been isolated from 100-million-year-old specimen preserved in ambers - but not in such spectacular scale as presented by Hollywood.

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