Inside the nucleus of every living cell is DNA, the genetic blueprint that makes each living creature unique. In rare cases, a fertilized egg will divide and separate inside the womb, forming two embryos with the same DNA. The resulting identical twins, which make up a scant 0.2 per cent of the world’s population, are nature’s perfect clones.
When we think of artificial cloning, most of us tend to fall back on bad science-fiction movies where teams of evil scientists replicate humans with giant photocopiers or grow armies of clone babies in glass-walled tanks. The reality of artificial cloning is much closer to Mother Nature’s model.
When scientists set out to artificially (or asexually) clone an organism, they take their cues from natural (or sexual) reproduction. In sexual reproduction, the sperm and egg cells each carry a single set of chromosomes containing exactly half of each parent’s genes. When the sperm fertilizes the egg, the two halves of the genetic map combine to form a full set of chromosomes that is unique from either parent.
A fertilized egg is called a zygote. As the zygote divides and develops into an embryo, the same copy of DNA is passed along to every cell in the organism, whether it’s a liver cell, eye cell or brain cell. The cells ultimately look and behave differently because different genes along the DNA strand are expressed to perform different cell functions.
The earliest artificial cloning efforts in the late-Seventies were quite basic. In a procedure called artificial embryo twinning, scientists mimicked the natural twinning process by physically dividing an early embryo into individual cells that had yet to specialize. After a day or two in a petri dish, the developing identical embryos were implanted into a surrogate mother and brought to term.
But the most effective artificial cloning method to date – the one that brought us Dolly the sheep in 1996 – is called somatic cell nuclear transfer. Somatic cells are ‘body’ cells like skin or liver cells that carry a full copy of DNA. To create a clone using this method, scientists extract the nucleus of an adult somatic cell and insert it into an ‘enucleated’ egg cell from a second animal, ie an egg whose nucleus and original genetic material has been destroyed or removed. Lab technicians execute these manoeuvres with pipette needles 2/10,000th of an inch wide.
Scientists then use a precision jolt of electricity or a chemical trigger to stimulate cell division in the transplanted egg. Instead of generating more specialised somatic cells-as you might expect DNA from an ‘adult’ cell to do – the transplanted DNA reboots to its original orders and starts to create an embryo. The resulting fetus, implanted into a surrogate mother, grows to become an exact genetic clone of the animal that donated the somatic cell.
Somatic cell transfer is often used for reproductive cloning – making copies of organisms with highly desirable genetic traits. For example, the same researchers who cloned Dolly have also cloned a genetically modified sheep named Polly whose milk contains a protein that aids blood clotting in haemophiliacs. By producing thousands of clones of Polly, scientists can isolate more and more of the protein to manufacture blood-clotting drugs.
Cattle, sheep, pigs and other animals have been bred by farmers for millennia to produce leaner, protein-rich meat and more flavourful milk. But conventional breeding still allows for a genetic roll of the dice, occasionally resulting in small, sickly offspring. Successful reproductive cloning, some say, would ensure a steady supply of only the fittest animals.
In 2008, the US Food and Drug Administration approved the sale of foods containing milk and meat from cloned animals, although the prohibitive cost of cloning has kept cloned food off the shelves so far. The European Union has made no such approvals, although breeders throughout Europe are already using imported semen and embryos from cloned animals bred in the States.
Scientists have also used reproductive cloning to bring endangered species back from the brink of extinction. In 2001, researchers used somatic cell nuclear transfer to clone an endangered guar ox, but the animal died soon after birth from an infection. Since then, another ox and three African wildcats have been successfully cloned to repopulate their species.
The first cloned cat was born in 2001 and a firm in South Korea produced the first cloned canine in2005- Since then, passionate pet owners have paid as much as GBP600 a year to preserve their beloved cat or dog’s tissue for post-mortem cloning, a process that runs to nearly GBP100,000.
Not all cloning experiments have been for the sake of reproduction, though. With therapeutic cloning, the goal of researchers is to isolate and replicate stem cells in order to study their potentially life-saving applications. Stem cells are undifferentiated cells that carry a full copy of an organism’s DNA, but have yet express any of their genes as skin cells, bone cells, sperm cells or any other adult cell. Researchers can program stem cells to grow specific organ tissue, like healthy brain cells to treat Alzheimer’s sufferers or replacement bone marrow cells for cancer patients.
Therapeutic cloning also relies on the somatic cell nuclear transfer method. In 2007, researchers harvested stem cells from cloned rhesus monkey embryos. It takes about five days for a cloned embryo to reach the blastocyst phase, when the total cell count has grown to around 100. At this size, stem cells can be extracted and placed in a nutrient-rich culture medium where they grow into complete stem cell ‘lines’.
Artificial embryo twinning
Identical twins are a fluke. When the fertilized egg is only days old, it spontaneously and inexplicably splits into two equal clusters of stem cells, each carrying the exact same genetic blueprint. Researchers can mimic this process in the lab by physically separating individual stem cells during the earliest moments of cell division and implanting the identical embryos into surrogate mothers.
Dolly the sheep
Dolly the sheep was the celebrity of the cloning world. Back in 1996, after 276 failed attempts, researchers at the Roslin Institute in Scotland became the first scientists to successfully clone a mammal. The method was somatic cell nuclear transfer, in which the nucleus of an egg cell is sucked out and replaced with the nucleus of a normal body cell (or ‘somatic’ cell) from an adult sheep. Using electrical impulses, the egg is induced to divide, growing into an embryo that shares the identical genetic material as the somatic cell donor. This requires sensitive manipulations of cellular material at the microscopic level, explaining the low success rate. Dolly’s life in the spotlight was relatively short-lived and she died at age six. One drawback of somatic nuclear transfer is that the age of the donor cell seems to affect the longevity of the cloned organism.
From stem cell to body part
Embryonic stem cells are ‘pluripotent’, meaning they can grow to become any specialised cell in the body. Nature has its own complex mechanisms for differentiating stem cells into brain, muscle or bone, involving both genetic and environmental markers. In the lab, researchers can isolate embryonic stem cells in culture and provide the right chemical triggers to grow fresh skin cells and even organs.
How To Clone Dolly Sheep
1. The donor – Researchers chose a six-year-old Finn Dorset ewe as the donor animal, since its pure white coat and white face would be easily distinguishable from other breeds.
2. Somatic cells – Researchers removed a small amount of somatic cells from the mammary tissue of the ewe. Yes, Dolly the sheep is named after Dolly Parton.
3. Low-nutrient culture – To inhibit cell division, the donor cells were placed in a low-nutrient culture medium until nuclear transfer.
4. Donor egg – Meanwhile, researchers removed a batch of eggs from an adult Scottish Blackface ewe, easily distinguishable by her dark facial markings.
5. Enucleation – Using a miniature drilling pipette mounted on a specialised microscope, the scientists bored through the cell wall and sucked out the egg cell’s nucleus.
6. Electroporation – The enucleated egg cells and somatic cells are placed in together and zapped with an electric field, causing cells to fuse and share a nucleus.
7. Fertilisation – Using another jolt of electricity, researchers induced proteins in the cytoplasm of the egg cell to ‘reprogram’ the genes of the somatic cell to grow an embryo, not more mammary tissue.
8. Embrio formation – Placed in a nutrient-rich culture medium, the egg cell began to divide and grow until it formed a blastocyst.
9. Implantation – The embryo was then implanted into another Scottish Blackface ewe and carried to term in the surrogate mother.
10. Birth of Dolly sheep – Dolly entered the world on 5 July 1996, sharing the identical white coat and white face of the mammary tissue donor and none of the markings of either the egg donor or surrogate mother.
The risks of cloning
Cloning is far from an exact science. Dolly was the 277th cloned embryo and the first to survive. But even Dolly only lived to six years old, half the average life span of her species. Several genetic abnormalities have surfaced in cloned animals, including high birth weight, organ defects and premature ageing.
The culprit, many believe, is the age of the somatic donor cell. Chromosomes get shorter over time as they divide and multiply. At some point, the chromosome becomes too short to divide and the cell dies. It appears that a somatic cell’s chromosomal clock is not fully reset when it’s implanted into an egg, resulting in shorter life spans and sudden organ failure.
Interesting cloning facts
Human-animal hybrids – In 2008, the Human Fertilization and Embryology Authority approved research into ‘cytoplasmic’ hybrids, the transfer of human genetic material into a cow egg cell.
Photocopy clones – There is no exact copy – clone will be younger than the original.
Clones or twins? – If the cloned organisms are born at the same time, then they’re twins.